New research published in the journal Frontiers in Public Health suggests that SARS-CoV-2 is likely to be affected by the changing seasons in a way similar to other human coronaviruses and influenza.
In temperate regions, this would mean reduced infections in the summer and peaks in the winter. However, this seasonality is only likely to occur once a vaccine is developed and greater herd immunity is achieved.
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The sudden emergence and rapid spread of SARS-CoV-2 and the disease it causes, COVID-19, have left scientists urgently attempting to develop vaccines to combat the virus and treatments for its disease.
Another key area of research is how the virus is transmitted from one person to another.
Understanding how the virus spreads is crucial, as it allows governments to enact policies that effectively limit viral transmission.
While policies have varied from country to country, they have generally involved maintaining social distance, washing the hands regularly, and wearing face masks.
This is because the virus can be transmitted on surfaces, through direct human contact, and via droplets expelled when a person sneezes, coughs, or talks.
In addition to transmitting through droplets from the respiratory tract, the virus may also spread through aerosols: very small droplets that are expelled alongside larger ones or that form when larger droplets evaporate.
Determining precisely how the virus transmits requires time and research. However, given the lethality of COVID-19, policy decisions need to be made urgently, based on the best evidence currently available. Making the best suggestions requires scientists to analyze emerging research on COVID-19 and past studies that have looked at similar viruses.
Doing so may also allow researchers to better predict how the virus will react in the future.
In the present study, the team pooled the latest research on COVID-19 and compared it with information about other viruses that affect the respiratory tract.
They did this to predict whether the novel coronavirus is likely to become seasonal — particularly severe in the winter in temperate regions — or whether it will circulate throughout the year.
The researchers noted that many other human coronaviruses are more prevalent in winter than in summer, as is the influenza virus thought to react to temperature similarly to SARS-CoV-2.
They argue that this seasonal pattern will likely develop in SARS-CoV-2, due to the effects of the climate on the virus and on humans.
First, the researchers point out, the climate can affect the stability of the virus. Previous research has suggested that enveloped viruses, including SARS-CoV-2, become more stable in cold weather. This means that they are able to survive for longer periods between hosts.
Cold weather may also allow the virus to travel through the air more easily, while higher levels of ultraviolet radiation in the summer may be more likely to kill the virus.
Second, cold weather may affect our physiology, making it easier for the virus to infect us. People also generally get less vitamin D in the winter, when sunlight is less intense, which has been linked to a weakened immune response to respiratory infections.
In addition, people are more likely to stay indoors during the winter months, increasing the risk of viral transmission at home, work, and school, for example.
While cold weather may increase the rate of transmission of SARS-CoV-2, the prevalence of the virus in countries with significant heat and high levels of moisture suggest that climatic conditions alone are not enough to make the virus seasonal.
Instead, the researchers argue that seasonality is only likely once an effective vaccine has been developed and deployed, and once a greater level of herd immunity comes about as more people develop the infection.
That means that, in the meantime, emergency measures remain crucial for limiting the spread of the virus — no matter the time of year.
As study co-author Hadi Yassine, an assistant professor of infectious diseases at Qatar University, in Doha, notes: “The highest global COVID-19 infection rate per capita was recorded in the Gulf states, regardless of the hot summer season. Although this is majorly attributed to the rapid virus spread in closed communities, it affirms the need for rigorous control measures to limit virus spread until herd immunity is achieved.”
As senior study author Hassan Zaraket, an assistant professor of virology at the American University of Beirut, in Lebanon, observes, “COVID-19 is here to stay, and it will continue to cause outbreaks year-round until herd immunity is achieved.”
“Therefore, the public will need to learn to live with it and continue practicing the best prevention measures, including wearing of masks, physical distancing, hand hygiene, and avoidance of gatherings,” he adds.
The authors stress that their study is a “best guess” at how SARS-CoV-2 may react to changing weather conditions. Although it can behave similarly to previous viruses, the new virus is unique and may react in unexpected ways.
“This remains a novel virus, and despite the fast-growing body of science about it, there are still things that are unknown. Whether our predictions hold true or not remains to be seen in the future. But we think it’s highly likely, given what we know so far, [that] COVID-19 will eventually become seasonal, like other coronaviruses.”
– Hassan Zaraket, Ph.D.
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