May 4, 2020 | 8:40am | Updated May 4, 2020 | 11:00am

As scores of researchers across the world scramble to develop vaccines for the novel coronavirus, a World Health Organization envoy is cautioning that one may never appear at all, according to a new report.

Dr. David Nabarro, who also serves as a professor of global health at Imperial College London, raised the possibility in a new interview with CNN.

“There are some viruses that we still do not have vaccines against,” Nabarro told the outlet. “We can’t make an absolute assumption that a vaccine will appear at all, or if it does appear, whether it will pass all the tests of efficacy and safety.”

In that case, according to the report, treatments may be developed, but the world could still see yearly coronavirus outbreaks — with deaths — and lockdowns will need to be periodically strengthened and loosened.

World Health Organization envoy Dr. David Nabarro
World Health Organization envoy Dr. David NabarroFabrice Coffrini/Getty Images

“It’s absolutely essential that all societies everywhere get themselves into a position where they are able to defend against the coronavirus as a constant threat, and to be able to go about social life and economic activity with the virus in our midst,” he added.

It’s been a challenge to develop vaccines for common rhinoviruses and adenoviruses, which, similar to the coronavirus, can cause cold symptoms, according to Nabarro.

Only one vaccine prevents two strains of adenovirus, and it’s not commercially available, he told CNN.

Vaccine being made in a lab
There are several viruses that still do not have a vaccine and it is, therefore, premature to assume one will be developed for coronavirus. Dr. Nabarro said.REUTERS/Carl Recine

“You have high hopes, and then your hopes are dashed,” Nabarro said. “We’re dealing with biological systems, we’re not dealing with mechanical systems. It really depends so much on how the body reacts.”

Most experts are hopeful that a COVID-19 vaccine will eventually emerge — in part because the deadly bug does not mutate rapidly, according to the report.

During a televised town hall Sunday, President Trump predicted that a vaccine would be ready by the end of 2020.

White House coronavirus task force expert Dr. Anthony Fauci said last month he thought it was within the “realm of possibility” that millions of vaccines could be ready by January. Experts previously estimated that a vaccine could take 18 months, if not longer.

But some others say that bringing a vaccine to fruition in any of those time frames is unheard of.

“We’ve never accelerated a vaccine in a year to 18 months,” Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston, told CNN. “It doesn’t mean it’s impossible, but it will be quite a heroic achievement.”

“We need plan A, and a plan B,” he added.

More than 90 COVID-19 vaccines are underway, in various stages, by research teams in companies and universities worldwide, Nature.com reported last week.

Human trials have begun for an Oxford University-developed vaccine that has protected six rhesus macaque monkeys from “heavy quantities” of the pathogen.

Those trials have also begun for a different vaccine, produced by the Massachusetts biotech firm Moderna.