Mutations in the new coronavirus could reduce the effectiveness of vaccines against it. But vaccines themselves can also drive viral mutations, depending on exactly how the shots are deployed and how effective they are.

So far, vaccines still appear to work against the new strains – though scientists are warily watching a variant that first appeared in South Africa since it seems to reduce vaccine effectiveness. And evolution isn't standing still, so scientists realize they may need to update vaccines to keep them working reliably.

What's going on here is somewhat similar to a larger, and more concerning problem in medicine: Many bacteria have gradually evolved the ability to survive even when walloped by a large dose of antibiotics. That problem has created new strains of deadly, drug-resistant germs.

Viruses also evolve, but the process is different and the result is usually much less severe when it comes to vaccines. When a virus such as the coronavirus infects someone, that person's immune system mounts a response. Viruses produce slight variations when they multiply, and if any of these variants can evade a person's immune response, those variants are more likely to survive and possibly to spread to other people.

So far, the concerning coronavirus strains have appeared in individuals who have not been vaccinated. But this evolution can happen in vaccinated people, as well.

Paul Bieniasz, a Howard Hughes investigator at the Rockefeller University, is particularly concerned this could happen between the time of an initial vaccination and a second shot to maximize the immune response.

"They might serve as a sort of a breeding ground for the virus to acquire new mutations," he says.

This issue is part of a debate over the best timing of vaccine doses. Some scientists have argued that it would be better to use the scarce vaccines to give first doses to as many people as possible, so the maximum number of people have at least partial immunity. That could help slow the spread of the virus.

Bieniasz worries that would also hasten the evolution of new strains of virus.

Scientists simply don't know how this will play out. For one thing, it's unclear whether the first shot of a vaccine is strong enough to prevent the virus from multiplying inside someone and being abundant enough to spread to somebody else. If the virus can't spread, how it has evolved in an individual becomes irrelevant.

It's clear that the vaccines reduce the risk of illness and death, but it's not known to what extent they prevent the virus from infecting an individual, or spreading from one person to another. Does this happen after the first dose? The second?

"There are really too many unknowns to really be definitive and positive about what the best way forward is, what the most effective way to use the available vaccine doses is," Bieniasz says.

Dr. Anthony Fauci, the government's top infectious disease specialist, says the vaccines used in the United States are 95% effective when used as intended, and there simply are no data supporting any other approach.

Fauci also says that a fully vaccinated person is apparently better able to fend off virus variants, so it makes sense to get people the maximum protection as quickly as possible.

The flip side, though, is that the virus – including mutant strains – can spread through the population faster if fewer people are vaccinated.

Extending the time between the first and second dose of a vaccine "does run the risk of promoting evolution," says Andrew Read, an evolutionary microbiologist at Penn State University. But he adds, "I must say, at the moment, that seems like a second-order issue compared to just reducing the transmission through the population as a whole."

When it comes to new vaccine-induced variants, "I know everybody's worried about it," he says, but history shows that viruses that have mutated generally don't render a vaccine useless. "It's often got strong anti-disease properties, so you get less sick," he says.

And even a fully vaccinated person can still play host to an evolving virus, in situations where the vaccine prevents illness but still allows a virus to replicate. That appears to happen even with the most effective COVID-19 vaccines. So, viral evolution doesn't just occur in the time between shots.

"I think there are a lot of options here for dealing with evolution, should it occur," Read says. For example, it helps that there are already more than half a dozen COVID-19 vaccines in use globally, and many others in development.

"One of the great things about having a lot of vaccine options is we might end up with a population which is heterogeneously vaccinated," Read says. Different people will have different vaccines, each stimulating a different immune response. "That will really help hinder the spread of mutants that are good at [diminishing] any one of those."

Also, a virus that has picked up a trait to evade one person's immune system will encounter a different set of defenses in the next individual. "If you and I have a different response, that really helps," Read says, "because anything that gets out of me might be killed by you."

Drugmakers are also keeping a close eye on mutants, and are already formulating new vaccines that would be more effective, if it turns out the original vaccines lose too much potency with the new variants.

So, this isn't a crisis.

"We're not going to fall off a cliff tomorrow in terms of vaccine efficacy," says Bieniasz at Rockefeller. "What we're likely to see is a slow, steady erosion of efficacy over perhaps quite a long period of time."

To slow this evolutionary process as much as possible, he says, the best strategy is to slow the spread of the virus right now, using masks and social distancing, so people who get vaccinated are at lower risk for getting infected in the first place.

You can contact NPR Science Correspondent Richard Harris at rharris@npr.org.

Copyright NPR 2021.