Federal regulators have approved a second booster shot of the COVID vaccine, for certain groups, so who is now eligible and what can you expect with the fourth dose?
Here's a breakdown of what we know so far.
Who is eligible for a second COVID booster shot?
Americans 50 and older can get a second COVID-19 booster if it’s been at least four months since their last vaccination.
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The Food and Drug Administration on Tuesday authorized an extra dose of the Pfizer or Moderna vaccine for that age group and for certain younger people with severely weakened immune systems.
The Centers for Disease Control and Prevention later recommended the extra shot as an option but stopped short of urging that those eligible rush out and get it right away. That decision expands the additional booster to millions more Americans.
For the more than 4 million who got Moderna or Pfizer as their second shot, the CDC says an additional booster is only necessary if they meet the newest criteria — a severely weakened immune system or are 50 or older.
According to a press release, certain immunocompromised individuals and residents over the age of 50 who have received their COVID vaccine booster dose more than four months ago will now be eligible for another booster thanks to the new recommendations.
Any adults who received Johnson & Johnson single-shot vaccines and a booster shot will also be recommended to receive another booster at this time.
Until now, the FDA had allowed a fourth vaccine dose only for the immune-compromised as young as 12. Only the Pfizer vaccine can be used in those as young as 12; Moderna's is for adults.
What about those who got the Johnson & Johnson vaccine?
What about people who got Johnson & Johnson’s single-dose shot?
They already were eligible for one booster of any kind. Of the 1.3 million who got a second J&J shot, the CDC said now they may choose a third dose — either Moderna or Pfizer.
That's because a CDC study that tracked which boosters J&J recipients initially chose concluded a Moderna or Pfizer second shot was superior to a second J&J dose.
When can eligible people get their second COVID booster?
Those now eligible for a second booster should wait until at least 4 months after they received their first booster dose of any authorized or approved COVID-19 vaccine, officials said.
What about the rest of the population?
The federal government is expected to evaluate whether to recommend fourth vaccine doses for all Americans, and could choose to issue those recommendations during the fall when more Americans are indoors as the weather cools.
Next week, the government will hold a public meeting to debate if everyone eventually needs a fourth dose, possibly in the fall, of the original vaccine or an updated shot.
Even if higher-risk Americans get boosted now, Marks said they may need yet another dose in the fall if regulators decide to tweak the vaccine.
For that effort, studies in people — of omicron-targeted shots alone or in combination with the original vaccine — are underway. The National Institutes of Health recently tested monkeys and found “no significant advantage” to using a booster that targets just omicron.
Why do people need a second booster?
Dr. Rochelle Walensky, CDC's director, said it was especially important for older Americans — those 65 and older — and the 50-somethings with chronic illnesses such as heart disease or diabetes to consider another shot.
“They are the most likely to benefit from receiving an additional booster dose at this time,” Walensky said.
There's evidence protection can wane particularly in higher-risk groups, and for them another booster “will help save lives,” FDA vaccine chief Dr. Peter Marks said.
The move toward additional boosters comes at a time of great uncertainty, with limited evidence to tell how much benefit an extra dose right now could offer. COVID-19 cases have dropped to low levels in the U.S., but all vaccines are less powerful against newer mutants than earlier versions of the virus — and health officials are warily watching an omicron sibling that's causing worrisome jumps in infections in other countries.
"Specifically for the BA.2 variant, we know that we need to maintain a higher level of immunity and antibodies," Cook County Health's Dr. Pamela Vergara-Rodriguez told NBC Chicago. "We understand now, the science is teaching us, that over time our immunity begins to come down. So, this additional booster is going to help us prevent people who do get infected with the BA.2 variant end up in the hospital and sick and, of course, prevent death."
Pfizer had asked the FDA to clear a fourth shot for people 65 and older, while Moderna requested another dose for all adults “to provide flexibility” for the government to decide who really needs one.
FDA's Marks said regulators set the age at 50 because that's when chronic conditions that increase the risks from COVID-19 become more common.
“There might be a reason to top off the tanks a little bit” for older people and those with other health conditions, said University of Pennsylvania immunologist E. John Wherry, who wasn’t involved in the government’s decision.
While protection against milder infections naturally wanes over time, the immune system builds multiple layers of defense and the type that prevents severe illness and death is holding up.
During the U.S. omicron wave, two doses were nearly 80% effective against needing a ventilator or death — and a booster pushed that protection to 94%, the CDC recently reported. Vaccine effectiveness was lowest — 74% — in immune-compromised people, the vast majority of whom hadn’t gotten a third dose.
To evaluate an extra booster, U.S. officials looked to Israel, which opened a fourth dose to people 60 and older during the omicron surge. The FDA said no new safety concerns emerged in a review of 700,000 fourth doses administered.
Preliminary data posted online last week suggested some benefit: Israeli researchers counted 92 deaths among more than 328,000 people who got the extra shot, compared to 232 deaths among 234,000 people who skipped the fourth dose.
What’s far from clear is how long any extra benefit from another booster would last, and thus when to get it.
“The ‘when’ is a really difficult part. Ideally we would time booster doses right before surges but we don’t always know when that’s going to be,” said Dr. William Moss, a vaccine expert at the Johns Hopkins Bloomberg School of Public Health.
Plus, a longer interval between shots helps the immune system mount a stronger, more cross-reactive defense.
“If you get a booster too close together, it’s not doing any harm — you’re just not going to get much benefit from it,” said Wherry.
What are the side effects of a second booster dose?
Vergara-Rodriguez said the side effects likely won't differ from the original doses and initial booster dose.
"We are expecting that the adverse side effects that we're all aware of that can occur after we get any vaccine can continue to occur," she said.
That includes things like fatigue, feeling ill, a fever or pain at the infection site, she said.
She noted, however, that some people may experience different side effects with each dose.
"It is very possible for people to have different side effects after either a booster vaccine or alone the course of a full vaccination," she said. "So we're aware that when we vaccinate over time - that might include a series of vaccines to reach complete vaccination - that a person can have a different type of reaction... that is very possible."
She added that's likely because while vaccinations tell the body to create antibodies, but boosters remind the body to do so.
"That reaction is actually protecting us," she said. "So even though we might be fearful that we'll feel a little sick, or maybe sick for a couple of days after a booster, it's actually normal and it's something that will protect our body."
The CDC stated that side effects with booster shots "were similar to that of the two-dose series."
The most common symptoms include fatigue and pain at the injection site, but "most symptoms were mild to moderate."
As with previous doses of the vaccine, the CDC notes that, "serious side effects are rare, but may occur."