Experts agree that the Covid-19 vaccines will need to be updated to provide greater protection against future variants—but many "don't think the entire population is going to need annual vaccines," Katherine Wu writes for The Atlantic.

Your top resources on the Covid-19 vaccines

When will vaccine updates be necessary?

To vaccinate properly against a specific Covid-19 variant, scientists must first detect it by tracking the coronavirus and identifying how and where it hides and transforms, Wu writes.

Radio Advisory episode: The unanswered vaccine questions you should be tracking

According to Wu, influenza provides a template for this type of "viral voyeurism." For decades, scientists have maintained a global surveillance network—made up of around 150 laboratories—that collects millions of samples from sick individuals every year and then identifies the genetic sequences of the viruses in each sample. Then, that information is sent to the World Health Organization (WHO), which determines which strains should be included in next season's vaccine.

A similar "watchdog system" for SARS-CoV-2 could mirror the one for influenza. According to Richard Webby, director of the WHO Collaborating Center for Studies on the Ecology of Influenza in Animals and Birds, hospitals are "already collecting those samples."

Based on influenza's model, scientists would likely call for a vaccine revision if a variant meets the following three criteria:

• A variant is full of unusual mutations
• It is recognized poorly by antibodies
• It spreads at least somewhat rapidly from person to person

However, Soumya Swaminathan, WHO's chief scientist, said the most clear-cut case for updating the Covid-19 vaccines would be if a variant is so heavily modified that it "overcomes our immunity enough" to make healthy, vaccinated individuals severely ill.

Will we need annual booster shots?

In September, WHO formed a new technical advisory group tasked with recommending ingredient adjustments to Covid-19 vaccines.

While Swaminathan said she envisions the committee mirroring WHO's influenza committee, she noted that, over time, the conditions that demand a revised version of the Covid-19 vaccines may not arise very often. Right now, "we don't understand SARS-CoV-2 as well as we do flu viruses," writes Wu. This means that there is less predictability as to when we’ll need boosters.

"I don't think the entire population is going to need annual vaccines," Swaminathan said. However, she noted a few important exceptions, including vulnerable populations such as immunocompromised people and older adults.

"The hope is that we head toward seasonality and stability," said Helen Chu, a flu-vaccine researcher at the University of Washington.

"But there's no telling how long that transition will take, or how bumpy it will be, or if it will occur at all," Wu writes.

In addition, Chu said she worries we still don't have the proper infrastructure to identify variants that emerge in places where they can mutate unusually fast.

"For at least the short term, our Covid-vaccine-update process is likely to remain a bit plodding; variants will crop up, and our shots will pursue them," Wu writes.

However, Raina MacIntyre, a member of the WHO's technical advisory group on Covid-19 vaccines and a biosecurity expert at the University of New South Wales, argued that the "whack-a-mole approach" of chasing single variants must end.

"Ideally, future vaccines should protect, with a single injection, against multiple variants at once," Wu writes. (Wu, The Atlantic, 2/24)