Clock is ticking for delivery of updated vaccines. Roseann Renouf, 77 has grown tired of the current generation of coronavirus shots.

Having ''never been one for a lot of vaccination,'' she decided to forgo the latest round of boosters after watching vaccinated friends contract Covid-19, even though the doses offer a critical extra layer of protection.

''It's just taking another same booster,'' said Ms. Renouf, a retired nurse anesthetist from Fort Worth. ''They haven't done anything different with them to cover new variants.''

Vaccine updates are becoming more urgent by the click day, many scientists said. The most evasive forms of Omicron yet, known as B.A.4 and B.A.5, appear to be driving a surge of cases across the United States, and the world.

The same subvariants have led to increased hospital admissions in Britain, France, Portugal, Belgium and Israel.

''We're hoping that we can convince people to get that booster,'' said Dr. Peter Marks, who oversees the vaccines office at the Food and Drug administration,'' and help mature their immune response and help prevent another wave.''

Many scientists believe that updated boosters will be critical for diversifying people's immune defenses as subvariants reduce the protection offered by vaccines. Catching up with a virus that has been so rapidly mutating may be impossible, they said. But it was better to be only a few months, rather than a couple of years, behind the pathogen.

''Omicron is different that,  to me, it seems pretty clear that we're starting to run out ground in terms how well these vaccines protect against symptomatic infections,'' said Ceepta Bhattacharya, an immunologist at the University of Arizona. ''It's very important that we update the shots.''

The question is whether these modified boosters will arrive in time. In an effort to match the latest form of  the virus, the F.D.A. asked vaccine manufacturers to tailor their new shots to the B.A.4 and B.A.5 subvariants, rather than the original version of the Omicron from last winter.

Virologists said that a subvariant vaccine would generate not only the strongest immune defenses against current versions of the virus, but also the type of antibody responses that will help protect against whatever form of the virus emerges.

But building a full booster campaign around vaccines at the forefront of the virus's evolution could also come at a cost. Pfizer and Moderna said that they could deliver subvariant vaccine doses no earlier than October.

Some F.D.A. advisers warned in a public meeting last week that the timeline could be slowed even further by routine delays.

In contrast, a vaccine targeting the original version of Omicron is closer at hand : Moderna and Pfizer have already making doses tailored to the original form Omicron, and Moderna said that it could start supplying them this summer.

Whether the benefits of a newer vaccine outweigh the drawbacks of having to wait longer depends on when it arrives and the impact of the virus before then, scientists said.

They said that some form of an updated vaccine by the fall would be crucial.

The original Covid vaccines had to withstand slow and laborious testing : Volunteers took the shots and then went  about their lives while researchers tracked who got sick. But there is now ample evidence that the shots are safe.

And any tweaks to the recipe could be wasted if scientists were to spend the better part of a year testing them.

Instead, vaccine manufacturers have been studying volunteers' blood samples in the lab to gauge their immune responses to a booster that is tailored to the first version of Omicron.

The subvariant boosters have so far been through lighter testing : Pfizer has studied only how they have affected antibody responses in mice.

The F.D.A. said  that it would not require clinical trial data for the subvariant boosters before authorization and would rely instead on studies of boosters targeting the original version of Omicron.

Some scientists said it was essential to authorize modified vaccines without time consuming human studies.

The Publishing continues. The World Students Society thanks author Benjamin Mueller.


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