COVID boosters: CDC recommends second bivalent vaccine shot targeting …

Many seniors can now get another COVID-19 vaccine booster, after the Centers for Disease Control and Prevention greenlighted a broad swath of changes on Wednesday that were first authorized by the Food and Drug Administration the day before.

The CDC's move clears the final hurdle for eligible Americans to be able to get another round of the booster shots that were rolled out last year from Moderna and Pfizer-BioNTech, which had been updated to include a "bivalent" recipe for the BA.4 and BA.5 variants of Omicron.

Most Americans who have already gotten one of those updated boosters last year will not need another one until this fall and winter, officials said.

But older adults, ages 65 and older, can now get another bivalent shot as long as it has been at least four months after their last dose. People with compromised immune systems can get more doses as early as two months after their last bivalent shot.

The new round of shots is part of a sweeping set of revisions by the Biden administration to streamline the myriad of immunization schedules that had been laid out for Moderna and Pfizer-BioNTech, moving them close to the annual seasonal flu shot.

"At this stage of the pandemic, data support simplifying the use of the authorized mRNA bivalent COVID-19 vaccines and the agency believes that this approach will help encourage future vaccination," the FDA's top vaccines official Dr. Peter Marks said in a release Tuesday announcing their authorization.

However, Marks cautioned that the FDA is still waiting for a June meeting of its outside advisers to decide whether COVID-19 vaccine booster shots will be authorized annually for future seasons.

"Is it possible that this will be something that will happen on a regular basis? It is, but that is something for discussion at the advisory committee," Marks told reporters Tuesday.

Around 20% of adults and 43% of seniors have gotten their first bivalent vaccine dose so far, according to the CDC's figures.

Additional shots have already been cleared in some countries abroad. The United Kingdom began offering second doses of bivalent vaccine this month to some vulnerable residents, like those 75 and older.

Asked why the U.S. took longer to greenlight additional shots, Marks said the FDA needed additional time to complete "a very major action of consolidation" across all mRNA shots as well as to vet data on the second boosters.

"That time gave us a good chance to look at data, some of which have only recently come out in the public, to be able to feel comfortable that this is a reasonable thing to do," said Marks.

The CDC's decision came after an hours-long meeting of the agency's outside advisers, the Advisory Committee on Immunization Practices (ACIP), to discuss the FDA's changes. While the agency did not ask the ACIP to formally vote on the new doses, as it has before some previous changes, it said that the meeting was "critical in order for the rationale for these changes to be conveyed to the public in a timely and efficient manner."

"Although there was no vote at this meeting, ACIP members expressed their support for these recommendations," the CDC said in a statement.

Under the plan first backed by a panel of the FDA's outside vaccine advisers back in January, still-unvaccinated Americans will now be able to bypass the two original "monovalent" shots designed to fend off the original strain of the virus, and start with shots of the bivalent vaccine.

The Biden administration has been working to phase out supply of those earlier monovalent COVID vaccines, which are no longer being produced and are expiring.

Instead, most Americans would only get one bivalent shot a year to stay "up to date" with their shots, regardless of what they got in previous seasons.

It is unclear how this will impact holdovers of requirements to be "fully vaccinated" from earlier in the pandemic, like for health care providers. A CMS official told the CDC meeting that they were still "looking into this, with these changes in recommendations" to the regimen.

Young children who were never vaccinated before would still need two doses of Moderna or three doses of Pfizer-BioNTech for their initial shots.

"Evidence is now available that most of the U.S. population 5 years of age and older has antibodies to SARS-CoV-2, the virus that causes COVID-19, either from vaccination or infection that can serve as a foundation for the protection provided by the bivalent vaccines," Marks said.

Unvaccinated adults will still also be able to elect to get their initial shots from the original Novavax or Johnson & Johnson supplies for now, although the CDC warned jurisdictions earlier this year that all remaining stocks of Johnson & Johnson's vaccine are set to expire next month.

Novavax's vaccines had also been due to expire, although the Biden administration shored up supply with more new doses in February. This week's changes do not affect availability or recommendations around those doses.

The FDA and CDC's moves come ahead of key decisions expected next month over what variants the bivalent vaccines should be updated to target next season. That would give companies just enough time to scale up production of revisions before the fall and winter.

A panel of the World Health Organization's scientists have already announced plans to weigh revisions next month as well, not unlike the annual process for coordinating updates to seasonal flu shots around the globe.

Most infections in the U.S. right now are still being driven by XBB.1.5 variant, a descendant of two Omicron strains, the CDC estimates. XBB.1.5 was dominant over the winter and did not appear to drive more severe disease or worse vaccine effectiveness relative to earlier Omicron variants.

A handful of new siblings are being tracked climbing around the country. XBB.1.16 is the largest, at 7.2% of new infections nationwide. That strain has a mutation that could lead to some worse infectivity and disease, but authorities think it is overall largely "very similar" to earlier variants.

"It's not clear that there's something new besides what we have circulating currently and what's dominant to pick," Marks said April 4 at a session hosted by the World Vaccine Congress.

He acknowledged it was possible SARS-CoV-2 could again evolve unpredictably, long after officials have already locked in their picks for shots in the fall.

"We may have a variant that will come out, that will emerge over the summertime, that may become dominant, that might not be as well neutralized by whatever we pick," Marks said.

CBS News reporter covering public health and the pandemic.

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