The Centers for Disease Control and Prevention recommended Thursday that teenagers and adults get updated booster shots from Pfizer or Moderna. The shots — also known as bivalent vaccines —are designed to target both the original coronavirus strain and the currently circulating omicron subvariants BA.4 and BA.5.
The decision follows a similar recommendation from a panel of independent advisers to the CDC, which voted in favor of the shots Thursday.
The CDC’s recommendation means the shots can now be administered to the public. But a spokesperson for the Department of Health and Human Services said people likely won’t start getting updated boosters until after Labor Day.
After that, appointment availability is expected to ramp up over several days, with appointments becoming more broadly available in a few weeks, a senior administration official said. People will be able to search for the closest sites offering updated boosters at Vaccines.gov.
Here’s what to know about the updated shots.
Are there enough doses for everyone?
White House officials said vaccine supply should meet demand this fall. The administration has purchased 171 million updated booster doses — 105 million from Pfizer and 66 million from Moderna — thus far, with the option to procure up to 429 million more.
Distribution of the doses began after the Food and Drug Administration authorized the shots Wednesday, with shipments to tens of thousands of locations, including pharmacies. Before that, pharmacies, community health centers and rural health clinics could pre-order the shots from the federal government.
A CVS spokesperson said its pharmacies expect to get updated booster doses on a rolling basis over the next few days. People can make appointments as usual on CVS’ website or its app.
Walgreens similarly said people can make appointments to get updated boosters through its website or its app or over the phone.
For now, the shots remain free.
How are these boosters different?
Whereas the initial COVID vaccine boosters targeted only the original strain of the coronavirus, the updated boosters are designed to add protection against omicron subvariants. For that reason, the modified shots will be the only boosters available for teens and adults moving forward.
The newly authorized shots target the BA.4 and BA.5 subvariants. As of Tuesday, BA.5 accounted for at least 87% of new U.S. cases. BA.4 and a similar sublineage, BA.4.6, made up around 11%.
“The updated COVID-19 boosters are formulated to better protect against the most recently circulating COVID-19 variant. They can help restore protection that has waned since previous vaccination and were designed to provide broader protection against newer variants,” CDC Director Dr. Rochelle Walensky said in a statement on Thursday.
Pfizer’s and Moderna’s trials of their bivalent vaccines in people studied a formulation that targeted the original omicron strain. The updated version, however, was tested in laboratory studies, which found that the boosters generated strong antibody responses against BA.4 and BA.5.
Laboratory tests “so far have been a very good predictor of how well the vaccines protect against infection, as well as protecting against severe disease and hospitalization and death,” said David Montefiori, a professor at the Human Vaccine Institute at Duke University Medical Center.
Who should get a booster?
The FDA authorized Pfizer’s shot for people ages 12 and up and Moderna’s for ages 18 and up. For those who are up to date on their COVID vaccinations, the updated booster constitutes a fourth, fifth, or sixth shot, depending on one’s age and health status.
But some vaccine experts wonder whether the shots are necessary yet for young, healthy people, given the lack of clinical trial data to demonstrate how well they work against the newer omicron subvariants.
Nonetheless, Pablo Penaloza-MacMaster, an assistant professor of microbiology-immunology at Northwestern University, said the potential benefits seem to outweigh the risks.
“The way that I look at it right now is that it seems like there’s not much to lose,” he said.
When is the ideal timing for this booster?
The CDC suggests that people wait at least two months since their most recent COVID shots to get the latest shots.
People who are elderly or immunocompromised should get boosted as soon as they meet those qualifications, Montefiori said. But he suggested that there’s likely to be more wiggle room in the timing for young, healthy people.
“The longer you wait to get the boost, the more potent of a boosting effect it’s going to have,” he said. But for those who hold off, he added, “there’s that trade-off between waiting to get boosted so that you have a stronger boosting effect and the risk of getting infected while you’re waiting to get the boost.”
Montefiori, who is 68, said he got his fifth shot three weeks ago and plans to wait three months for his bivalent booster.
The CDC advises that people who recently had COVID consider delaying their boosters until three months after their symptoms started or, if they were asymptomatic, since their positive COVID tests.
Penaloza-MacMaster said his research suggests that healthy people of all ages could even wait six months between shots or following COVID infections.
But Montefiori said it’s hard to know how long immune protection lasts after a COVID infection.
“The best advice that I would give people is to get the bivalent boost as soon as they’re eligible to, regardless of whether or not they’ve been infected, because of the uncertain nature of how much that infection really boosted your immunity,” he said.
This story originally appeared on NBCNews.com.