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Why COVID Boosters Weren’t Changed to Better Match Variants | Covid-19


More COVID-19 booster shots may be on the way – but when it’s your turn, you’ll receive an additional dose of the original vaccine, not an update to better match the extra-contagious delta variant.

And this is leading some experts to wonder if the recall campaign is not a missed opportunity to target delta and its likely descendants.

“Don’t we want to match the new strains most likely to circulate as closely as possible? Food and Drug Administration adviser Dr. Cody Meissner of Tufts Medical Center recently challenged scientists at Pfizer.

“I’m not quite sure why it’s not delta because that’s what we’re up against right now,” fellow counselor Dr Patrick Moore of the University of Pittsburgh said last week, while government experts wondered if it was time for Moderna boosters. He wondered if such a switch would be particularly useful in blocking a mild infection.

The simple answer: Last month, the FDA accepted additional doses of Pfizer’s original recipe after studies showed it still worked fairly well against delta – and those doses could be rolled out immediately. Now the FDA is evaluating the evidence for recalls of the original Moderna and Johnson & Johnson vaccines.

“It’s less churn and burn on the make” to only change formulas when it’s really necessary, said FDA chief vaccine officer Dr. Peter Marks.

But Pfizer and Moderna are hedging their bets. They’re already testing personalized experimental doses for delta and another variant, learning how to quickly change the formula in case a change is ever needed – for today’s mutants or a brand new one. The most difficult question for regulators is how they would decide if and when to order such a switch.

What we know so far:


Vaccines used in the United States remain highly effective against hospitalizations and deaths from COVID-19, even after the delta variant has taken over, but authorities hope to strengthen the declining protection against less serious infections and for populations at high risk. Studies show that an extra dose of the original formulas increases anti-virus antibodies that repel infection, including antibodies that target delta.


The vaccines target the spike protein that covers the coronavirus. Mutations in this protein made the delta more contagious, but to the immune system it doesn’t look all that different, said virus expert Richard Webby of St. Jude Children’s Research Hospital.

That means there is no guarantee that a delta-specific booster would protect better, said University of Pennsylvania immunologist John Wherry. Waiting for studies to resolve this issue – and if necessary, prepare updated doses – would have delayed the deployment of boosters for people deemed to need them now.

Yet because delta is now the dominant version of the virus around the world, it will almost certainly be a common ancestor for anything that evolves next in a mostly unvaccinated world, said Trevor Bedford, biologist and genetics expert at Fred. Hutchinson Cancer Research Center.

An updated delta vaccine “would help provide a buffer against these additional mutations,” he said. Bedford is paid for by the Howard Hughes Medical Institute, which also supports the Associated Press’s Department of Health and Science.


The Pfizer and Moderna vaccines are made with a piece of genetic code called messenger RNA that tells the body to make harmless copies of the spike protein so that it can be trained to recognize the virus. Updating the formula simply requires exchanging the original genetic code with mRNA for a mutated spike protein.

The two companies first experimented with modified doses against a mutant that emerged in South Africa, the beta variant, which has been the most resistant to vaccines to date, more than the delta variant. Laboratory tests showed that the updated injections produced strong antibodies. But the beta variant has not spread widely.

Now the companies have studies underway of fully vaccinated people who have agreed to test a booster dose adjusted to match delta. Moderna’s studies also include injections that combine protection against multiple versions of the coronavirus – just as current flu vaccines work against multiple strains of the flu.

MRNA vaccines are considered the easiest type to modify, but some other vaccine makers are also exploring how to modify their recipes if necessary.


Moderna’s Dr. Jacqueline Miller told an FDA advisory group last week that the company is now studying variant-specific boosters to see if they provide any benefits and to be prepared if they’re needed.

And Penn’s Wherry said it is essential to carefully analyze how the body responds to updated injections because the immune system tends to “imprint” a stronger memory of the first strain of virus it encounters. . This raises questions as to whether a subtly different booster would cause a temporary jump in the antibodies the body has made before – or the more important goal, a larger, longer-lasting response that might even be better positioned for future mutations. future.


“What’s the trigger point? Asked Webby, who is part of a World Health Organization network that tracks the progress of the flu. “Much of what will have to go into this decision making will unfortunately only be learned through experience. “

Bedford said now is the time to decide what drop in vaccine effectiveness will trigger a change in formula, as happens every year with flu shots.

This is important, not just if a considerably worse variant suddenly develops. Like many scientists, Bedford expects the coronavirus to eventually evolve from a global crisis to a regular threat every winter – which could mean more regular boosters, perhaps even every year in combination with the vaccine against the flu.

The timing between shots matters too, Wherry noted.

“Your boostability can actually improve with longer intervals between stimulations,” he said. While scientists have learned a lot about the coronavirus, “the story is not over yet and we don’t know what the final chapters say.”