All adults are now eligible to receive avaccine booster shot, after the Centers for Disease Control and Prevention published greenlighting another dose for all recipients of Pfizer, Moderna and Johnson & Johnson's vaccines.
The new recommendations come as public health officials are warning of a potential new wave of cases this winter, and as some states are already seeing spikes in hospitalizations.
The FDA is also allowing Americans to mix-and-match their booster shots, which means you can choose to receive an additional dose from any of the three brands that are available.
Though children as young as 5 are eligible to be vaccinated against COVID-19, they are not recommended to receive booster shots.
Here's what we know so far:
Pfizer and Moderna recipients
The CDC recommends that all adults 50 and older "should" receive a booster dose six months after they completed their first two shots of Pfizer or Moderna. Adult residents in long-term care settings, like nursing homes or psychiatric facilities, are also urged to get a booster shot.
All other adults 18 and older who were vaccinated with Pfizer or Moderna "may" receive a booster dose after six months, the CDC says in its latest guidance.
The CDC's outside vaccine advisers stopped short of a full-throated recommendation for all adults to receive a third dose. Some members of the Advisory Committee on Immunization Practices cited unanswered questions that could affect the benefits and risks Americans should weigh before getting another shot.
The committee considered real-world data suggesting the protection offered by the vaccines against infection declines over time, increasing the risk of breakthrough infections as states are facing new surges of cases.
Pfizer said data from its clinical trial showed more than against symptomatic COVID-19 after the booster shot, compared to vaccinated people who did not receive a third dose. Moderna has yet to report results from its booster shot trial, but said preliminary data showed their third dose increased antibody levels to "consistently high" levels.
"While we know that protection against asymptomatic infection may not be permanent, even temporary protection may factor into the benefit-risk balance, especially as we approach the winter and holiday switch increased traveling and indoor gatherings," the CDC's Dr. Sara Oliver told the advisers.
However, the panel acknowledged protection appears to remain high from the vaccines against severe disease and hospitalization for many younger, healthy adults.
Health authorities are also continuing to investigate the risk after a booster shot of myocarditis — a "rarely" reported heart inflammation side effect that has been linked to the first two shots in mostly younger men, who are often hospitalized but typically recover quickly.
FDA officials said that data from Israel, where Pfizer's booster shots were rolled out nationwide earlier this year, suggests the risk of myocarditis after the third dose is "much lower" than after the second.
Moderna's move to halve the size of its booster to 50 micrograms eased the agency's concerns for that shot, though FDA Acting Commissioner Dr. Janet Woodcock acknowledged the regulator does not "have full data" on rates of myocarditis after Moderna's booster.
"We still have things to learn and we will be doing surveillance and have required the companies to also do surveillance to look at this side effect," Woodcock told a recent webinar hosted by Stanford University.
Johnson & Johnson recipients
The CDC recommends a booster shot for all adults who were vaccinated with Janssen COVID-19 vaccine at least two months ago.
That comes after the FDA concluded that even the highest estimates of the single-shot vaccine's protection were "consistently less" than the mRNA-based vaccines from Pfizer or Moderna.
Data presented to the FDA's outside vaccine advisers by Johnson & Johnson says vaccine effectiveness against symptomatic COVID-19 climbed to 94% in the U.S. after a booster shot.
A group of the CDC's vaccine advisers also concluded that the risks of rare but serious side effects following a second dose of the vaccine were "unlikely to be greater" than after the first dose.
People vaccinated abroad with another brand
People who got vaccinated abroad with shots greenlighted by the World Health Organization, but not authorized in the U.S. by the Food and Drug Administration — such as the AstraZeneca, Sinovac or Sinopharm vaccines — can now also get a booster dose of Pfizer's vaccine under recently published CDC guidance.
These people only need to wait four weeks before they are able to get an extra dose.
To allow third Pfizer doses in these cases, the CDC relied on rarely used "emergency use instructions" that have previously been used to greenlight drugs in the case of an anthrax outbreak.
A CDC spokesperson said the agency was only able to allow use of Pfizer's vaccine for these additional doses because the shots by Moderna and Johnson & Johnson have yet to receive the FDA's full approval.
The FDA's decision to allow for mixing-and-matching booster shots was based in part on early data from trials led by the National Institutes of Health which suggest all combinations of so-called "heterologous" boosting are safe and effective at increasing protection.
Federal health officials also pointed to situations that made requirements to match booster shot brands tricky, like when people do not remember which shots they were first vaccinated with, had a bad reaction to their initial vaccinations, or do not have access to the same brand at their nearby pharmacies.
For now, the FDA's scientists concluded there is not enough data from the NIH's trial to pick a "preferred strategy" for boosting. The study's scientists had cautioned that their trial was not designed to "directly compare" the immune response triggered by different booster combinations.
Data presented to the CDC's advisers suggest that mixing-and-matching booster shots have turned up no new safety concerns. Reactions from Moderna's boosters were more commonly reported than after Pfizer's boosters.
"We will not articulate a preference. My understanding is that most people will have done largely well with the initial vaccine that they got and may express a preference, very much, for the original vaccine series they got, having done very well," CDC Director Rochelle Walensky told reporters.
The majority of Pfizer and Moderna recipients have so far stuck with the same brand for their booster shot so far, according to the CDC. Only a quarter of Americans first vaccinated with Johnson & Johnson have opted to stay with the brand for their second shot.
"There may be some people who might prefer another vaccine over the one that they received, and the current CDC recommendations now make that possible."
Fourth doses and special cases
Beyond the broad recommendations governing booster shots for most Americans, the CDC has also updated its extensive guidance over the past months detailing specific situations that might require more doses of vaccine.
For example, a few million fourth "booster" shot, six months after they received their third shot.Americans who were allowed to receive a third "additional" dose of the Pfizer or Moderna vaccines will be able to receive a
Others who received certain kinds of cancer treatments may be entirely revaccinated after completing stem or T-cell therapies.