First documented coronavirus reinfection reported in Hong Kong

A judge struck down a state order requiring most Florida schools to open for in-person instruction.

A Florida judge ruled on Monday that the state’s requirement that public schools open their classrooms for in-person instruction violates the Florida constitution because it “arbitrarily disregards safety” and denies local school boards the ability to decide when students can safely return.

The ruling was a victory for the American Federation of Teachers, the nation’s second-largest teachers’ union, and one of its affiliates, the Florida Education Association. The unions sued Gov. Ron DeSantis and Richard Corcoran, the education commissioner, last month in the first lawsuit of its kind in the country.

The state’s order required that school districts give students the option to go back to school in person by Aug. 31 or risk losing crucial state funding. An exception was made only for Miami-Dade, Broward and Palm Beach counties, which have been the hardest hit by the coronavirus and plan to start the school year online.

“The districts have no meaningful alternative,” Judge Charles W. Dodson of the Leon County Circuit Court wrote of the rest of the state’s schools. “If an individual school district chooses safety, that is, delaying the start of schools until it individually determines it is safe to do so for its county, it risks losing state funding, even though every student is being taught.”

Later Monday, the state filed an appeal to the ruling, prompting an immediate stay.

“This fight has been, and will continue to be, about giving every parent, every teacher and every student a choice, regardless of what educational option they choose,” Mr. Corcoran said in a statement.

In Tampa, the state’s reopening order prevented the Hillsborough County school district from starting the school year with four weeks of online-only instruction, as the school board wanted to do. The Hillsborough board is scheduled to meet on Tuesday, although no vote is expected, a district spokeswoman said. The superintendent, Addison Davis, said in a statement after the ruling that the school system continued to plan to start classes on Aug. 31 with a choice of in-person or online instruction.

During a three-day hearing last week, the unions presented testimony from public health experts and teachers concerned about risking their health. One teacher said he would quit to avoid exposure to the virus. Another, who is quadriplegic, said he could not afford to leave his job, though his doctor had warned him that Covid-19 would threaten his life.

At a news conference on Sunday, President Trump announced the emergency authorization of the use of blood plasma for treatment of hospitalized Covid-19 patients. The president and two of his top health officials — Alex M. Azar II, the health and human services secretary; and Dr. Stephen M. Hahn, the commissioner of the Food and Drug Administration — highlighted the same statistic: that the treatment reduced Covid-19 deaths by 35 percent.

But scientists and experts, including one researcher who worked on the study cited by the officials, say that the framing and use of the statistic, which refers to a subset of a Mayo Clinic study, are misleading.

“For the first time ever, I feel like official people in communications and people at the F.D.A. grossly misrepresented data about a therapy,” said Dr. Walid Gellad, who leads the Center for Pharmaceutical Policy and Prescribing at the University of Pittsburgh.

The statistic in question was not mentioned in the official letter authorizing the treatment, the 17-page memo written by F.D.A. scientists about the treatment or in the Mayo Clinic’s analysis.

Some fear that the process of approving treatments and vaccines for the coronavirus has been politicized, and as data emerges from vaccine clinical trials, the safety of potentially millions of people will rely on the scientific judgment of the F.D.A. “That’s a problem if they’re starting to exaggerate data,” Dr. Gellad said.

Plasma has been touted by Mr. Trump as a promising cure for the coronavirus, with his administration funneling $48 million into a program with the Mayo Clinic to test infusions. Although there have been some positive signs that it can reduce deaths in Covid-19 patients, no randomized trials have shown that it works.

Dr. Hahn’s claim that 35 out of 100 sick Covid-19 patients would have been saved by receiving plasma appeared to be an overstatement, statisticians and scientists said.

Dr. Eric Topol, a professor of molecular medicine at Scripps Research in La Jolla, Calif., said that convalescent plasma has not yet shown the benefit that Dr. Hahn described — and that he should issue a correction.

On Monday night, after The New York Times published an article questioning the statistic, Dr. Hahn said on Twitter that the “criticism is entirely justified,” and clarified that his earlier statements imprecisely suggested an absolute reduction in risk, instead of the relative risk of a certain group of patients compared with another.

In a surprise speech, Mr. Trump accused his opponents of “using Covid to steal the election,” repeating claims that voting by mail was part of a plot to bring about his defeat in November.

Mr. Trump also criticized Roy Cooper, the Democratic governor of North Carolina, telling the crowd in Charlotte that Mr. Cooper and other Democratic governors had enacted virus restrictions simply to hurt his re-election chances and would end them after Election Day.

“You have a governor who is in a total shutdown mood,” he said. “I guarantee you on November 4, it will all open up.”

Multiple speakers, including the president, occasionally referred to the pandemic in the past tense, as the convention undertook a significant rewriting of the Trump administration’s pandemic response. There was a particular focus on Mr. Trump’s decision to ban travel from China in January.

“Before Communist China gave us the coronavirus, we were breaking economic records left and right,” Ms. Haley said. “The pandemic has set us back but not for long.”

Mr. Trump’s son said that the virus had hit the United States “courtesy of the Chinese Communist Party.” He praised his father for restricting travel from China, and for having “rallied the mighty American private sector to tackle this new challenge.”

“There’s more work to do but there is a light at the end of the tunnel,” the younger Mr. Trump said.

Outside the convention hall, public health officials in Mecklenburg County, which includes Charlotte, continue to fight to contain the virus, with an average of 1,100 new cases a day over the past week, according to a New York Times database.

Just six representatives from each state and territory were in the room where the speeches were given, masked and seated at a distance from one another. Vice President Mike Pence also spoke to the delegates, and Mr. Trump plans to appear every night during the convention.

Despite the precautions in place inside the convention hall, photographs of crowds gathered by the stage while Mr. Trump spoke showed people failing to practice social distancing. Some wore masks and some did not.

More than 40,350 new cases and more than 500 new deaths were reported in the United States on Monday, according to a New York Times database.

EDUCATION ROUNDUP

Zoom fixes partial outages that disrupted the first day of virtual classes for many U.S. students.

The video call service Zoom reported partial outages on Monday morning, causing problems on the first day of remote classes for many schools in the United States.

Zoom said it began receiving reports of users being unable to start or join meetings at about 8:50 a.m. on the East Coast, as working and school hours began. About two hours later, the company said that it was “deploying a fix across our cloud,” and at about 12:45 p.m. it said “everything should be working properly now.”

As the pandemic has kept students out of classrooms and workers out of offices, Zoom has quickly become critical infrastructure for many school districts, companies and local governments. The partial disruption in service, which lasted approximately four hours in some areas, adds another element to the contentious debate over how to safely and effectively resume learning this fall.

The Atlanta school district, which serves about 50,000 students, was among those affected by the outage. And students and professors at Penn State University reported widespread problems on campus on Monday morning, as did Michigan’s Supreme Court, which has conducted hearings online since the pandemic began.

Another online learning platform, Canvas, also experienced technical issues on Monday. Cory Edwards, a spokesman for the company, said the system had slowed down for about 75 percent of its U.S. customers for about a half-hour on Monday morning. The problem probably resulted from heavy usage as many students returned to school this week, he said.

The website DownDetector, which tracks outages at social media companies and tech companies, showed significant Zoom outages in major cities around the country, including New York, Washington, Atlanta, Chicago, St. Louis and San Francisco. The site reported more than 15,000 outages by about 10 a.m. Eastern time.

Many courthouses also rely on Zoom to conduct hearings, city councils govern through virtual meetings, and the police face reporters in video news conferences.

Here are other key education developments:

• The University of Alabama said that 531 cases had been identified among students, faculty and staff on its Tuscaloosa campus since classes resumed there on Aug. 19. It said the total number of cumulative cases over that period, including infections on other campuses in the university’s system, was 566.

• The University of Southern California said it had identified 43 new cases in the past week, all of them related to “off-campus living environments.” The university called it “an alarming increase” and said that more than 100 students were under two-week quarantine because of exposure to the virus. It warned students that “every surface, every interaction where you share close contact or remove your face covering, can pose a risk to yourself and your friends.”

• Following months of pressure to set up outdoor classrooms in New York City, Mayor Bill de Blasio said Monday that principals can apply by this Friday to create outdoor classes in their schoolyards. The city’s public school system, the nation’s largest, is scheduled to reopen in just under three weeks in a hybrid model, leaving schools little time to move classroom infrastructure outdoors. The city will prioritize 27 neighborhoods badly hit by the virus with schools that do not have usable outdoor space. The mayor said that outdoor learning “won’t work every day” because of bad weather, but that it was still a good alternative for many schools.

• The University of Illinois, Urbana-Champaign, became the latest college to find a significant number of students testing positive for coronavirus upon their return to campus, university and county health officials said on Monday. The university said it had recorded 326 positive results since Aug. 15. On its website, the university said that it has conducted more than 87,000 tests since early July, with an average positive rate of .74 percent over the past five days — considered quite low. The college began modified in-person instruction on Monday.

• More than 730 American colleges and universities have announced at least one case on campus among students, faculty or staff since the beginning of the pandemic, according to a New York Times database. Among the latest: Millikin University in Decatur, Ill., which reported its first case on Monday, the first day of fall classes.

• The University of Kansas, where fall classes began Monday in Lawrence, issued 14-day public health bans to two fraternities on Sunday for violating university policies on mask wearing and social distancing. The university’s chancellor said in a statement that Kappa Sigma and Phi Kappa Psi were ordered not to host any event without approval from the university.

• The University of Nebraska in Lincoln announced Sunday that students at the Kappa Alpha Theta sorority have been placed under quarantine after five cases were identified.

GLOBAL ROUNDUP

Quarantine rules are expanding around the world, but not in America.

As many countries maintain or even tighten their quarantine requirements for international arrivals, the United States is moving in the opposite direction.

The U.S. Centers for Disease Control and Prevention revised its guidance for travelers on Friday, dropping its recommendation that anyone coming from overseas or a state with a high infection rate should self-quarantine for 14 days. Travelers are now advised only to “follow state, territorial, tribal and local recommendations or requirements.”

A number of states have their own quarantine policies in place, but with the exception of a few places like Hawaii, they are rarely enforced.

U.S. quarantine policies stand in stark contrast to those of many other countries and regions — including Australia, Canada, China, Hong Kong, New Zealand, Singapore, South Africa and South Korea — that have strictly monitored international arrivals for months. Travelers are sometimes required to stay in designated hotels or government quarantine facilities, wear electronic tracking devices or report their temperatures daily, with violations punishable by fines or imprisonment.

Many of those rules were implemented after some travelers carrying the virus contributed to fresh outbreaks in places where it had been mostly under control. Some countries, including Australia, South Africa and Spain, have even restricted or banned travel between states and provinces for the same reason.

European countries that had loosened their travel and entry restrictions earlier this summer are now experiencing a surge in cases and reinstituting quarantine requirements for people coming from certain countries. Britain recently added Belgium, France, the Netherlands and other countries to its quarantine list, while France is planning to impose a reciprocal quarantine requirement for visitors from Britain.

The C.D.C. also updated its testing guidelines on Sunday to say that people who have been in contact with someone who tested positive for the coronavirus for at least 15 minutes, been in a high transmission area or attended a large gathering “do not necessarily need a test.” Studies indicate that the virus can spread from a seemingly healthy person, and that a person may spread the virus before showing symptoms.

In other global news:

• As Hong Kong on Tuesday announced plans to begin easing its social distancing rules, the city’s leader, Carrie Lam, said that criticism by health experts of a new, Beijing-backed coronavirus testing program was a “politically calculated” effort to smear the Chinese government. Some of those experts say the plan is a waste of resources, while activists fear it could lead to the harvesting of DNA samples for China’s surveillance apparatus — accusations that local officials deny.

• A tsunami of job cuts is about to hit Europe as companies prepare to carry out sweeping downsizing plans to offset a collapse in business. Government-backed furlough programs that have helped keep about a third of Europe’s work force financially secure are set to unwind in the coming months. As many as 59 million jobs are at risk of cuts in hours or pay, temporary furloughs or permanent layoffs, especially in industries like transportation and retail, according to a study by McKinsey & Company.

• For 40 days, millions of people in Urumqi, the capital of the Xinjiang region in western China, have been unable to leave their homes because of a sweeping lockdown to fight a virus resurgence. Now, with the outbreak seemingly under control but the restrictions still largely in place, many residents say they are being confined to their homes unnecessarily and denied access to critical services like health care. The ruling Communist Party has been widely criticized in recent years for a harsh crackdown on the region’s Muslim minority.

• The United Nations said Monday that up to 100 million jobs directly reliant on international tourism are at risk because of the pandemic and that revenue generated by the global tourist industry could fall by as much as $1.2 trillion this year. U.N. officials, who have called the pandemic the biggest challenge in the organization’s 75-year existence, also said in the report that some of the smallest countries are particularly vulnerable, as tourism represents a large chunk of their economic output.

• Bali, Indonesia’s leading tourist destination, has abandoned its plan to allow foreign tourists starting Sept. 11, Gov. I Wayan Koster announced, and will wait at least until the end of the year before opening to them. Bali’s economy contracted 11 percent during the second quarter, with about 2,700 tourism workers laid off and another 74,000 on unpaid leave, the governor said.

• Prime Minister Jacinda Ardern of New Zealand extended a lockdown in Auckland, New Zealand’s largest city, until Sunday night. The restrictions had been set to expire on Wednesday, but Ms. Ardern said the extra time was necessary to ensure that a virus cluster had been brought under control. Eight new confirmed or probable cases were announced on Monday, bringing the total to 101.

• The first volunteer was inoculated with a “made in Italy” vaccine on Monday at Spallanzani hospital in Rome, which specializes in infectious diseases. The vaccine is produced by ReiThera, a biotechnology company based near Rome but headquartered in Switzerland.

• Health authorities in France said a virus outbreak at a nudist camp in the southern resort town of Le Cap d’Agde was “very worrying.” More than 140 people have tested positive in the town, the Agence Régionale de Santé (ARS), France’s health agency, said on Sunday, and 310 more are awaiting results

Authorities in the blockaded Gaza Strip announced the first coronavirus cases transmitted through the community on Monday, raising concerns that the pandemic could spread widely in the densely populated and impoverished coastal enclave.

Before Monday’s announcement, authorities had found infections only at quarantine facilities, where all returning travelers were required to quarantine for three weeks and pass two tests before being permitted to leave.

Ashraf al-Qidra, a spokesman for the Hamas-run Health Ministry, told a news conference that four people in the Maghazi refugee camp in central Gaza tested positive for Covid-19, while noting officials were carrying out epidemiological investigations.

Mr. Qidra said that authorities tested the four individuals after learning they had been in contact with a resident of Gaza who tested positive for the disease at the Augusta Victoria Hospital in East Jerusalem.

Salama Maroof, the head of the Hamas-operated government media office, said that the entire territory would be placed under curfew for 48 hours.

“We call on everyone to exercise the greatest degree of carefulness, stay in their homes and follow the health measures,” he said at the news conference.

Early Tuesday morning, police cars were seen driving around Gaza using loudspeakers to call on residents to remain in their homes.

Experts have warned that Gaza’s health sector, already devastated by years of war and conflict, lacked the resources to deal with a widespread outbreak.

Gerald Rockenschaub, the head of the World Health Organization’s mission, said medical institutions carry only about 100 adult ventilators, most of which are already in use.

As of early Tuesday, 113 virus cases had been recorded in Gaza and only a single fatality. But only about 17,000 tests have been conducted during the pandemic, according to the Hamas-run Health Ministry, meaning some cases could have gone undetected.

The small number of cases relative to the tens of thousands in Israel and the West Bank has largely been seen as a result of the coastal enclave’s isolation and Hamas’s strict quarantine policy for returning travelers.

Reluctant to vacation abroad, Britons are pitching tents and sticking close to home.

“For the first time in the U.K., owning a caravan is kind of cool,” said Gareth Mills, a 38-year-old father who lives on the English seaside, referring to big, boxy campers or motor homes. “Some of my parents’ friends who are caravan club enthusiasts, they are very smug at the moment.”

Hotels have largely reopened in England, but many of them are at 30 to 40 percent occupancy, with popular areas such as Cornwall and elsewhere in the southwest faring better, said Patricia Yates, a director at the tourism organization VisitBritain.

Finding a spot for a caravan or tent may be more competitive, as demand has surged. During a recent weekend, Pitchup.com, a booking site for camping spots, recorded 6,100 bookings, almost double the amount from the same weekend in 2019.

Camping has deep roots in Britain. The man considered the father of modern camping, Thomas Hiram Holding, was a traveling London tailor whose 1908 how-to, “The Camper’s Handbook,” documents the joys of self-reliance and getting away from it all, inspiring generations. About the same time, the Boy Scouts were started in Britain, followed by the Girl Guides a couple of years later.

Caravan parks across Britain have been flooded with bookings for the traditional summer period and into the fall, according to the National Caravan Council, an industry group. Parkdean Resorts, which operates 67 parks across the country, reported a 140 percent rise from last year at its parks in Devon.

Huw Pendleton, the managing director of Celtic Holiday Parks in Wales, said he hadn’t seen anything like it in his two decades in the industry.

“We’re sold out pretty much through to September, with little or no availability now this season for the top-end lodges and glamping with hot tubs,” he said.

U.S. ROUNDUP

A U.S. measure protecting 12 million tenants from eviction ends, as requests for help in housing court spike.

With a federal eviction moratorium coming to an end in the United States, legal aid lawyers say they are preparing to defend renters in housing court.

The fourth-month moratorium followed by a 30-day notice period protected about 12 million tenants living in qualifying properties. Local moratoriums in some states have protected others not covered by the federal law.

For tenants, especially those with limited means, having a lawyer can be the difference between being evicted and being able to stay, but tenants in housing courts rarely have legal representation. Surveys in several big cities over the years have found that at least 80 percent of landlords, but fewer than 10 percent of tenants, tend to have lawyers.

The president’s recent executive order on assistance to renters doesn’t offer much immediate hope for people facing eviction; it merely directs federal agencies to consider what they could do using existing authority and budgets.

“Tenants are not equipped to represent themselves, and eviction court places them on an uneven playing field,” said Ellie Pepper of the National Housing Resource Center.

Demand for legal assistance with housing issues is on the rise in states where local moratoriums have ended. “Our caseloads haven’t yet exploded, because the courts just started hearing cases that were pending before the pandemic struck,” said Lindsey Siegel, a lawyer with Atlanta Legal Aid. “But it’s coming.”

Elsewhere in the United States:

• Louisiana shut down its coronavirus testing sites on Monday as the state braced for two tropical storms, Marco and Laura, in quick succession. Hospitals and urgent care facilities can still perform tests, said Kevin Litten, a spokesman for the Louisiana Department of Health. The shutdown of the state sites, and any power outages the storms cause, will probably lead to “some kind of disruption in data collection,” Mr. Litten said, followed by a jump in cases when testing resumes afterward. Similar effects were seen after Tropical Storm Isaias, which disrupted testing in Florida and the Carolinas early this month. Coastal Louisiana is among the hardest-hit areas of a state that has recorded at least 143,000 coronavirus cases and nearly 4,750 deaths, according to a New York Times database.

• Joseph R. Biden Jr. and Senator Kamala Harris will be tested regularly for Covid-19 as Election Day approaches, the Biden campaign said on Monday, a day after a senior Biden official said Mr. Biden had not yet been tested. The Biden team said that “with the potential of additional events” over the remainder of the campaign, it had increased its health protocols. Staff members who interact with Mr. Biden and Ms. Harris will also be tested regularly, and the campaign said it would announce publicly if either candidate ever has a confirmed case of coronavirus.

• The Trump administration tied billions of dollars in badly needed coronavirus medical funding this spring to hospitals’ cooperation with a private vendor collecting data for a new Covid-19 database that bypassed the Centers for Disease Control and Prevention. The office of the health secretary, Alex M. Azar II, laid out the requirement in an April 21 email obtained by The New York Times that instructed hospitals to make a one-time report of their Covid-19 admissions and intensive care unit beds to TeleTracking Technologies, a company in Pittsburgh whose $10.2 million, five-month government contract has drawn scrutiny on Capitol Hill.

• Louis DeJoy, the postmaster general, defended his record on Monday, as he testified before the House Oversight Committee on Monday. He said he told some of Mr. Trump’s advisers that the president’s repeated attacks on mail-in voting were “not helpful.” Watch the hearing live as lawmakers raise concerns about postal changes that could complicate mail-in voting.

• Representative Jenniffer González-Colón, Puerto Rico’s nonvoting delegate to Congress, said on Monday that she had tested positive for the coronavirus, a week after the island’s Aug. 16 primary drew politicians to many indoor events. “I think it was a mistake on my part to be in a closed environment,” she said on Facebook Live. Ms. González-Colón, a member of the New Progressive Party, which supports Puerto Rican statehood, was not the only party member to test positive. Among the others were the House speaker, the Senate majority leader and two top aides to the party’s nominee for governor.

• With the 2020 census into its final stage, more than one in three people hired as census takers have quit or failed to show up. And with 38 million households still uncounted, state and local officials are raising concerns that many poor and minority households will be left out of the count. The coronavirus and rising mistrust of the government on the part of hard-to-reach groups like immigrants and Latinos already have made this census challenging. But another issue has upended it: an order last month to finish the count a month early, guaranteeing that population figures will be delivered to the White House while President Trump is still in office.

• After a cripplingly slow vote count in New York’s June primary, marred by thousands of disqualified ballots, Gov. Andrew M. Cuomo said on Monday that he would sign a series of executive orders to make it easier for voters to cast valid absentee ballots in November. The orders will require local officials to “take steps to be ready to start counting votes ASAP,” after the Nov. 3 election. The governor also ordered a redesign of ballot-return envelopes to make it clear where they should be signed, addressing a common reason for disqualification.

In an attempt to limit a resurgence of the coronavirus, Italy has banned dancing in nightclubs and outdoor dance halls.

As in other countries, new cases in Italy are being driven by young people, with several clusters traced to nightclubs crowded with maskless patrons. Yet the new rules aimed at stopping young people from gathering en masse have also swept up older Italians for whom an evening at the dance hall is a cherished part of life.

The Italian government’s decree on dancing, issued on Aug. 16, made no distinction between packed, sweaty clubs blaring reggaeton and sedate community centers where people swirl in pairs to accordion-driven waltzes.

Many regulars at Caribe, an outdoor dance hall in Legnago that caters to an older clientele, said they understood that the government was trying to protect the country — and people their age in particular. But they didn’t understand why they could no longer hold their partners on the dance floor while bars, beaches, amateur soccer courts and gyms stayed open.

This spring, the idea that the overwhelming majority of domestic soccer leagues in Europe might be able to finish their seasons — and that a new European champion might be crowned — seemed a distant, fanciful one.

On Sunday night in Lisbon, though, Bayern Munich won the Champions League, bringing the curtain down on the 2019-20 campaign. European soccer made it through.

That it did is not only a testament to the progress their countries made against the virus, but also to the willingness of thousands of players to observe some of the toughest controls imposed on any individuals in any industry.

The Bundesliga — the first major sports league to return — blazed the trail. Before resuming play in May, the German league issued each of its players a handbook containing precise instructions on “private hygiene,” guidance that in some cases went above and beyond the advice issued by the government to the public.

The Bundesliga’s rules were as stringent, and comprehensive, as possible, and governed almost every aspect of how players lived. Hand towels were to be used once only, for example, and to be washed at 140 degrees Fahrenheit as soon as they were damp.

In other sports news:

• Usain Bolt, the Jamaican sprinter who won eight gold medals over the course of three Olympics, has tested positive and is isolating at his home in Jamaica. He celebrated turning 34 on Friday at a surprise party attended by, among others, his girlfriend, his newborn daughter and the prominent soccer players Raheem Sterling and Leon Bailey. Videos posted by the music news outlet Urban Islandz showed attendees dancing near one another without wearing masks. Jamaica has recently had a spike in cases.

• When 11 National Football League teams were notified over the weekend that a total of 77 people, including players and staff members, had apparently tested positive, they scrambled to respond, holding players out of practice and rescheduling training sessions. But the results were all false positives.

• In New York, school-sponsored sports that are considered “lower risk,” including tennis, soccer, cross country, field hockey and swimming, may practice and play with limits starting Sept. 21 statewide, the governor said Monday. Teams may not travel to play outside of the school’s region or contiguous regions or counties until Oct. 19. Sports with more physical contact that are considered “higher risk,” including football, wrestling, rugby and hockey, may begin practicing with limits but cannot play until a later date or Dec. 31.

A 33-year-old man was infected a second time with the coronavirus more than four months after his first bout, the first documented case of so-called reinfection, researchers in Hong Kong reported Monday.

The finding was not unexpected, especially given the millions of people who have been infected worldwide, experts said. And the man had no symptoms the second time, suggesting that even though the prior exposure did not prevent the reinfection, his immune system kept the virus somewhat in check.

“The second infection was completely asymptomatic — his immune response prevented the disease from getting worse,” said Akiko Iwasaki, an immunologist at Yale University who was not involved with the work but reviewed the report at The New York Times’s request. “It’s kind of a textbook example of how immunity should work.”

People who do not have symptoms may still spread the virus to others, however, underscoring the importance of vaccines, Dr. Iwasaki said. In the man’s case, she added, “natural infection created immunity that prevented disease but not reinfection.”

“In order to provide herd immunity, a potent vaccine is needed to induce immunity that prevents both reinfection and disease,” Dr. Iwasaki said.

Doctors have reported several cases of presumed reinfection in the United States and elsewhere, but none of those cases have been confirmed with rigorous testing. Recovered people are known to carry viral fragments for weeks, which can lead to positive test results in the absence of live virus.

But the Hong Kong researchers sequenced the virus from both of the man’s infections and found significant differences, suggesting that the patient had been infected a second time.

The study is to be published in the journal Clinical Infectious Diseases. The Times obtained the manuscript from the University of Hong Kong.

The man’s first case was diagnosed on March 26, and he had only mild symptoms. He later tested negative for the virus twice and had no detectable antibodies after that first bout. He was positive again for the coronavirus on a saliva test on Aug. 15 after a trip to Spain via the United Kingdom. The man had picked up a strain that was circulating in Europe in July and August, the researchers said.

His infections were clearly caused by different versions of the coronavirus, Dr. Kelvin Kai-Wang To, a clinical microbiologist at the University of Hong Kong, said: “Our results prove that his second infection is caused by a new virus that he acquired recently, rather than prolonged viral shedding.”

Common cold coronaviruses are known to cause reinfections in less than a year, but experts had hoped that the new coronavirus might behave more like its cousins SARS and MERS, which seemed to produce protection lasting a few years.

When 11 National Football League teams were notified over the weekend that a total of 77 people, including players and staff members, had apparently tested positive, they scrambled to respond, holding players out of practice and rescheduling training sessions.

Then on Monday came word from the testing lab: Never mind.

The results were all false positives, BioReference Laboratories said in a news release on Monday, citing “isolated contamination during test preparation” at one of its facilities in New Jersey.

“All individuals impacted have been confirmed negative and informed,” Dr. Jon R. Cohen, BioReference’s executive chairman, said in the news release.

N.F.L. officials said on Sunday that the affected clubs were following contact tracing, isolation and rescheduling protocols that were outlined by the league and players’ association. Among the 11 affected teams were the Minnesota Vikings, Chicago Bears and Buffalo Bills.

Eight Vikings athletes with false positives watched team meetings virtually on Sunday, unable to attend practice. The New York Jets, Cleveland Browns and Bears all rescheduled training sessions before getting the all clear; the Pittsburgh Steelers, Philadelphia Eagles and Detroit Lions held out players who had falsely tested positive.

The league’s regular season is expected to start Sept. 10.

A 40-day lockdown in western China prompts anger and anxiety.

For 40 days, millions of people in the western Chinese city of Urumqi have been unable to leave their homes after the authorities put in place a sweeping lockdown to fight a resurgence of the coronavirus.

Now, with the outbreak seemingly under control but the restrictions still largely in place, many residents of Urumqi, the capital of the Xinjiang region, are lashing out at the government. They say they are being unnecessarily confined to their homes and denied access to critical services like health care.

“Is this a prison or cage?” one user wrote on Weibo, a popular social media site. “Is this prevention or suppression?”

The mounting anger poses a challenge for the ruling Communist Party, which is trying to hold up its handling of the epidemic as a model for the world. The party has long taken a harsh approach in Xinjiang, and in recent years has been widely criticized for leading a draconian crackdown on the region’s Muslim minority.

The lockdown in Urumqi, employing many of the same extreme measures used in Wuhan, began in mid-July as dozens of people fell ill with the virus. In recent weeks, locally transmitted cases have dwindled; there have been no such cases for eight days, officials say.

As anger mounted online, the authorities in Urumqi, a city of 3.5 million, on Monday said they would ease restrictions in some districts, allowing residents to leave their homes and walk inside their apartment complexes, according to Chinese news reports. Officials did not say when the full lockdown would be lifted.

Why are the numbers of U.S. cases decreasing? Because restrictions are working, experts say.

After cases surged in June and July, the number of new reported U.S. cases began to level off, then drop, though the infection rate remains one of the world’s highest.

Of the states that are driving the decrease, all have at least some local mask mandates. And most have paused or reversed statewide reopening policies, again closing bars, gyms and theaters.

Many of the states with the biggest decreases per million people also had some of the country’s worst outbreaks in July.

Experts said that the drop in reported cases could not be attributed to the recent drop in testing volume. They explained that decreased hospitalizations and a lower share of positive tests indicated that the spread had most likely slowed.

A July surge in Florida affected young people in particular. Statewide bar closures following earlier reopenings and local mask mandates are among the policies that have helped reverse the trend, said Mary Jo Trepka, the chair of the Florida International University epidemiology department. Deaths were greater in July for residents under 65 than for those over 90.

And though Florida is doing better now, the state did surpass 600,000 cases on Sunday.

Arizona and Louisiana have also seen cases drop after taking mask mandates and other measures came into force.

KEY DATA OF THE DAY

Residents in Danbury, Conn., are urged to to stay home and limit gatherings after a rise in cases.

Officials in Connecticut have issued a public health warning for the city of Danbury, urging residents to stay home when possible and limit gatherings after new cases jumped sharply there in the first 20 days of August.

Danbury, a city of about 84,000 people near the New York border, reported 178 new cases in that time, the state said, more than quadruple the figure for the prior two weeks.

The state’s public health department now recommends that residents not attend large church services or outdoor gatherings, or any gathering indoors with people other than those they live with.

“It does worry us that the number has gone up quite a bit,” Gov. Ned Lamont said at a news conference on Monday afternoon, referring to the share of positive test results in Danbury. He also urged people to self-quarantine, wear face coverings, social distance and get tested.

In a statement on Friday, officials said that many new cases in Danbury appeared linked to recent domestic and international travel. Connecticut currently requires travelers from dozens of states and two territories to quarantine for 14 days upon arrival.

Danbury was among the state’s hardest-hit places earlier this year; Connecticut’s first confirmed case worked at Danbury Hospital.

Danbury’s public schools will start the year with distance learning because of the outbreak, the superintendent said in a letter posted to Facebook on Monday.

Bali postpones its plan to begin allowing foreign tourists.

The Indonesian island of Bali, which has seen a steady increase in the number of its coronavirus cases, has abandoned its plan to allow tourists from other countries starting Sept. 11, the governor announced.

The island, Indonesia’s most important tourist destination, will wait at least until the end of the year before opening to foreign visitors, said the governor, I Wayan Koster, in a statement released on Saturday.

“The situation in Indonesia is not conducive to allowing international tourists to visit Indonesia, including to visit Bali,” he said.

In explaining his reversal, Mr. Koster noted that many countries are not allowing their citizens to travel overseas, including Australia, which has long been a major source of visitors to Bali.

The governor said Bali’s economy contracted 11 percent during the second quarter of the year. About 2,700 tourism workers have been laid off and another 74,000 are on unpaid leave, he said. Many others have had their hours reduced and are working part time, tourism operators say.

With Bali’s tourism economy devastated, many hotel workers have returned to their home villages where they can help their families grow food. Some also fish or collect clams and shrimp. But workers from neighboring islands, who don’t have access to farmland in Bali, are struggling to feed themselves and rely in part on assistance from aid groups.

Bali has been trying to attract domestic tourists to compensate for the loss of foreign visitors. But many Indonesian tourists would be likely to come from virus hot spots like Jakarta, the capital, and its neighboring province East Java, compounding Bali’s health problems.

Bali had reported 4,576 cases and 52 deaths as of Monday. Indonesia has recorded at least 153,000 cases and almost 7,000 deaths, according to a New York Times database. Some experts, citing Indonesia’s low testing rate, say that the actual number of cases is much higher.

In addition, Mr. Koster has been among the Indonesian officials who have promoted quack cures and misinformation about the virus. His recommended remedy: inhaling the steam from boiled arak, a traditional alcohol made from coconuts.

Reporting was contributed by Geneva Abdul, Iyad Abuheweila, Liz Alderman, Maggie Astor, Gillian R. Brassil, Chelsea Brasted, Emma Bubola, Marie Fazio, Sheri Fink, Claire Fu, Christoph Fuhrmans, Matthew Goldstein, Maggie Haberman, Ethan Hauser, Javier C. Hernández, Mike Ives, Jennifer Jett, Annie Karni, Andrew E. Kramer, Sharon LaFraniere, Théophile Larcher, Lauren Leatherby, Apoorva Mandavilli, Patricia Mazzei, Jesse McKinley, Claire Moses, Richard C. Paddock, Tariq Panja, Elisabetta Povoledo, Adam Rasgon, Frances Robles, Amanda Rosa, Eliza Shapiro, Dera Menra Sijabat, Rory Smith, Sheryl Gay Stolberg, Eileen Sullivan, Katie Thomas, Elaine Yu, Alan Yuhas and Albee Zhang.