The wait is over: Updated versions of Covid-19 booster shots have received a green light from the Food and Drug Administration, and the Centers for Disease Control and Prevention now recommends them for all Americans age 12 and older.

This push for new, updated boosters is coming at a good time. Health officials are warning of another likely spike in Covid-19 transmission this fall as people head back indoors once again.

The reformulated booster doses of the mRNA vaccines from Pfizer/BioNTech and Moderna are “bivalent.” That means they target the original version of SARS-CoV-2 as well as the newer omicron variant. The hope is that when administered as boosters, these new shots will increase protection against the latest mutations in the virus, and head off a rise in hospitalizations and deaths.

With new vaccines come new questions about what makes them better, who’s eligible, and when to get one. To help people understand when, why, and how to get their booster doses, we’re answering some of those questions here.

Let’s get into it.

Who’s eligible for the new coronavirus boosters?

According to the CDC’s recommendations, everyone 12 and older should get an updated booster shot. People 18 and over can get either a Pfizer or Moderna shot, while teens between 12 and 18 should get a Pfizer shot, as the FDA hasn’t reviewed Moderna data for younger teens yet. The CDC’s guidance suggests that getting either booster, regardless of what prior vaccines you received, is fine, although there’s no hard data yet on mixing and matching.

The CDC’s Advisory Committee on Immunization Practices did not recommend updated boosters for kids younger than 12 because manufacturers haven’t yet submitted data on these age groups for the FDA’s review. That will likely happen later this fall. Meanwhile, younger kids (between 6 months and 12 years) can get the original formulation of the vaccines.

How will I know if my booster is updated for the omicron variant?

As the bivalent boosters roll out, the older, monovalent boosters will become unavailable, so you won’t have to decide whether to get one or the other.

For people 12 and older, it’s all bivalent boosters for now — at least, until next-generation vaccines (like universal coronavirus vaccines and intranasal vaccines) become available in the US, and that’s unlikely to happen in the next few months.

How long after my last vaccine (or a recent Covid-19 infection) should I get a new booster shot?

Covid-19 infection and vaccination both provide some level of protection against future infections, which can last for a few months. During those months, your memory B cells — your immune system’s antibody factories — don’t respond as robustly to Covid-19 booster shots, perhaps because they’re busy fine-tuning the quality of the antibodies they’re producing in response to the original infection or vaccine.

Whatever the reason, this means that if you were recently infected with or vaccinated for Covid-19, you should take a beat before getting your bivalent booster shot.

How many months? Regardless of whether you were recently boosted or recently got Covid, the guidance is similar. The data supporting this guidance is pretty scant, though, so while we’ve summarized it below, it’s worth knowing there’s no perfect time interval.

If you’ve recently had Covid-19, you’ll probably want to wait at least three months to get a booster shot (even though you technically can get a booster shot as soon as your symptoms are gone). A recent preprint (i.e., non-peer-reviewed) study suggested that receiving a booster within two months of infection doesn’t really add much protection, and CDC guidance suggests waiting even longer to get vaccinated — three months after symptom onset — to improve the immune system’s response to the booster shot.

“A Covid infection in a vaccinated person — essentially that functions as a booster,” said Andrew Pekosz, a professor of microbiology and immunology at Johns Hopkins University, in a late-July interview.

If you’ve recently been boosted, the CDC offers a tool to help people determine when they are eligible for a booster. The timeline of your eligibility can change slightly based on age and individual risk factors, with people at higher risk being prioritized. But note: The CDC’s tool doesn’t include a question about recent infection. You can think of a recent infection as basically equivalent to a vaccine for the purposes of using the tool, or you can talk to your health care provider if you’re not sure what to do.

Generally, epidemiologist Katelyn Jetelina suggested in a recent issue of her newsletter, healthy people should wait four to six months after an infection or vaccination to get a booster — a bit longer than the CDC’s recommendations. She also suggests shortening that wait to three to four months for people at high risk for severe infection or who plan to attend an event where the infection risk is high (like a wedding), and stresses that, broadly speaking, there’s some uncertainty about the ideal interval between doses.

Bottom line: Although there are no firm rules about how soon to get a bivalent booster after infection or vaccination, you’re likely to get the most bang for your vaccine buck if you give it at least three months.

What makes these booster shots different?

You might remember that mRNA vaccines are different from conventional vaccines: Rather than delivering a whole inert virus or a fragment of it, mRNA vaccines give human cells the genetic instructions for making pieces of the virus.

In the case of the Covid-19 vaccines, the vaccines serve as an assembly manual for the spike protein of the virus. The bivalent vaccines from both Moderna and Pfizer/BioNTech that have been authorized for use in the US contain mRNA instructions for making the spike protein of the original version of SARS-CoV-2 and the spike protein common to the BA.4 and BA.5 omicron subvariants.

After you get a vaccine, your immune system revs up and starts making antibodies, which are proteins that bind to the virus and can stop it from causing an infection. If you have high levels of antibodies that can neutralize a virus, this usually means you’re well protected against infection. Antibody production, however, tapers off over time, so a vaccinated individual may be vulnerable to an infection after a few months. A booster shot ramps antibody production back up.

But antibodies attach best to very particular sites on the virus. If those sites mutate, as they have with the recent SARS-CoV-2 variants, antibodies become less effective at blocking infection. The bivalent vaccines restore some of this protection by training the immune system on both a newer version and an older version of the spike.

How effective are these boosters?

Although clinical trial data suggests the bivalent boosters offer some improvement over the older monovalent boosters, the size of the step up isn’t as big as some researchers would like it to be.

(A caveat here: These studies reported findings on a vaccine designed to target the BA.1 omicron variant. But the US has decided to go ahead with a vaccine that targets the more current BA.4 and BA.5 variants in the hope that it will provide more protection. European and Canadian authorities are using vaccines aimed at the spike protein found in the BA.1 subvariant. It’s a trade-off between having more human data, as with the BA.1 vaccine, versus a more timely match to the current variant, as with the BA.4/5 vaccine.)

The new vaccines raise antibody levels less robustly than the original boosters did — twofold compared to 25-fold — and it’s not yet clear they offer much improvement in preventing severe disease or death compared with immune system memory cells — B cells and T cells — trained on an earlier version of the virus.

Additionally, there are multiple effective treatments for Covid-19 now available. The risks of the worst forms of Covid-19 disease are in decline. Some experts have argued the money spent on bivalent vaccines would be better spent on other investments, like next-generation vaccines.

“We’ve spent $3 billion on these bivalent vaccines. Is that really how you’re going to best spend your money, given how uncomfortably scant those data were?” said Paul Offit, director of vaccine education at the Children’s Hospital of Philadelphia. “There are, I think, other strategies out there,” including developing intranasal vaccines that could help prevent infection.

But many feel the bivalent vaccines are a great step forward. For starters, the BA.4/5-targeted bivalent boosters being rolled out in the US will likely reduce the risk of transmission in the people who receive them, although that benefit is theoretical until human trial data on these vaccines becomes available — probably not for a few more weeks.

During an Infectious Diseases Society of America briefing on Wednesday, Kathryn Edwards, scientific director of the Vanderbilt Vaccine Research Program, expressed hope that surveillance systems for detecting new viral variants and determining changes in vaccine effectiveness would simplify further iterations of booster shots. “I’m optimistic that we have a technology that allows us to rapidly change and make vaccines for different kinds of viruses,” she said. The goal is for the vaccines to follow the model of annual flu shots, with scientists making adjustments aimed at protecting people from the viral variant most likely to be circulating at any given moment.

How often will we need boosters?

At a White House briefing on Tuesday, chief medical adviser Anthony Fauci said Covid-19 boosters would likely go from being needed every four to six months to becoming an annual shot, much like the flu shot. Whether that actually happens depends on how things go in the coming months.

The indicators experts will be watching include signs that in healthy adults, protection from severe disease wanes sooner than a year after getting a booster shot. They’ll also be watching for signs that the virus is evolving unpredictably in dangerous ways. A “variant curveball” could change the plan for annual vaccination, said Ashish Jha, the White House Covid-19 response coordinator, in the same White House briefing.

“We’ll have to see how long the booster persists,” said Edwards during Wednesday’s briefing. “As more people throughout the world get vaccinated, we may find that there’s fewer changes to the virus,” making it easier to predict the most effective formula for future booster shots, she said.

Even if next year’s Covid-19 trajectory turns out to support an annual vaccination plan, severely immunocompromised people may need more frequent booster shots because their immune systems mount a weaker response to vaccines.

Can I get a Covid-19 booster and a flu shot at the same time?

Yes. The CDC says there’s no difference in vaccine effectiveness or side effects whether a person gets a Covid-19 vaccine alone or with another vaccine.

Earlier on, before there was much “real-world” experience with Covid-19 boosters, it made sense to allow some time between these boosters and other vaccinations. That way, if people experienced adverse events after one of the vaccinations, providers could more easily identify which of the vaccines had caused the problems. But now, there’s more than enough experience with Covid-19 boosters to make the wait unnecessary.

Where do I get one of the new bivalent booster shots?

The US government has already purchased more than 170 million doses of bivalent Covid-19 vaccines. Many of the same places that offered original booster shots will simply be replacing them with the bivalent boosters, but where those places are depends on where you live. The vaccines.gov website is a great way to find a location nearest you.

State and local health departments have vaccination sites, and many retail pharmacies, health centers, and doctor’s offices are also offering Covid-19 boosters. This morning, my local CVS had bivalent booster appointments open as soon as this Friday.

What happens if too few people get boosted?

For booster shots to be effective at reducing both transmission and illness, as many people as possible need to get them. So far, just 49 percent of people eligible for a first booster in the US have gotten one. If that trend continues with the reformulated shots, boosted individuals will likely be protected, but the virus will continue spreading. That will give it more opportunities to mutate in dangerous ways.

However, vaccines are not just a tool to protect individuals, but a way to protect the population at large, since they lower rates of transmission and relieve burdens on the health system. This extends beyond the US. As the pandemic has demonstrated over and over, problems in other countries don’t stay in other countries.

“We have to think of Covid-19 as a global disease [and] really have to make an even stronger effort here in the US to get vaccines out into the world,” said Pekosz.

Otherwise, we risk repeating the same patterns of new variants causing renewed surges in cases, hospitalizations, and deaths.

Does getting a booster come at the expense of someone in a low-income country?

Given the need to vaccinate the world and the ongoing vaccine inequities leaving many of the most vulnerable unprotected against Covid-19, it’s reasonable to ask whether there’s an opportunity cost to getting a fourth shot when nearly one-third of humanity has yet to receive their first.

But experts say that your booster shot isn’t the main problem. Closing international vaccination gaps requires strategy and action from the government, not individuals.

“Once a vaccine gets into your local pharmacy, it’s really not going to be pulled back and sent someplace else,” Pekosz said. “The US government needs to realistically assess what the needs are for vaccines here and send surplus vaccines directly to other countries as opposed to stockpiling them here in the US.”