April 14, 2022 10:56 AM EDT

At this point, COVID-19 vaccines have been available for well over a year in the U.S. But the shots weren’t designed to prevent infection with SARS-CoV-2, and the immunity generated by the vaccines wanes, so the virus continues to mutate and become even more transmissible—and infect even the vaccinated. Boosting with another dose of the vaccine can increase that immunity again, so public health officials authorized an initial booster shot for most people last year. But as new variants of the virus continue to emerge, it’s time to consider another booster dose for the wider population. Already, that second booster is recommended for certain people at high risk of COVID-19 disease who could benefit from the enhanced protection provided by the additional shot.

Experts, however, aren’t comfortable with a strategy of simply adding booster after booster of the same vaccine. So they have launched studies to see if there is a better way to optimize the vaccines, and whether the current versions of the shots are the best ones to rely on in the face of a still-mutating virus. Here’s what we know so far.

Who can get a second booster shot?

For most adults in the U.S., being up to date with your COVID-19 vaccines right now means getting vaccinated, and then receiving a booster dose a few months later. For either Pfizer-BioNTech’s or Moderna’s shots, that entails two primary doses and a booster; if you were immunized with Johnson&Johnson-Janssen’s shot, it’s one initial dose and a booster.

Children aged 12 to 17 years old can get boosted, but only with the Pfizer-BioNtech shot; Moderna’s and Johnson&Johnson’s boosters have not been authorized yet for this age group.

For certain people, such as those over age 50 and people with health conditions that make it harder for them to mount a strong immune response, health officials have already recommended a second booster dose four months after the first. That, in part, is based on data from Israel, which showed that adding a second booster in January for people over age 60 and for health care workers, reduced the risk of death from severe COVID-19

At a recent Food and Drug Administration (FDA) meeting of independent advisory committee members, the experts reviewed studies looking into the potential benefits of adding another booster dose. For example, they looked at research showing that the immunity generated by the first booster wanes after a few months—studies from Israel show that rates of severe disease among boosted people are starting to rise, although they still remain lower than rates among non-boosted people. They also considered lab-based studies from the U.S. confirming that levels of virus-fighting antibodies produced after a booster dose start to wane after several months, dropping by about six-fold during that time.

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Additionally, any incremental benefit of a second booster has to be weighed against the potential side effects, which include inflammation of heart tissue, especially for younger people receiving the Pfizer-BioNTech or Moderna vaccines, and the risk of blood clots for people getting the J&J shot. That means continuing to boost every four months or so with the same vaccines isn’t a sustainable way to confront COVID-19, especially if the virus continues to mutate.

After considering all of the studies available so far, the committee concluded that there aren’t yet enough data to support recommending a second booster for the general population. While a second booster can raise the levels of a person’s antibodies against COVID-19, and protect them from more serious disease, that protection doesn’t last as long as health officials would like. So for the vast majority of the U.S. population, that second booster isn’t necessary—at least not yet.

If I’m authorized, when should I get a second booster?

For now, the people who are eligible to receive a second booster dose includes anyone over age 50 and certain groups of people with weakened their immune systems. If you are immunocompromised and received the Pfizer-BioNTech vaccine as your primary vaccination (meaning three doses), and a booster dose, you can now receive a second booster, or fifth dose, four months after the first booster if you are 12 years or older. If you are immunocompromised and received the Moderna vaccine as your primary vaccination and a first booster, you can receive a second booster dose if you are 18 years or older.

A second booster won’t necessarily protect you from getting infected with SARS-CoV-2, but if you do get infected, it can reduce your chance of getting severely ill and even going to the hospital for COVID-19-related symptoms.

Read More: How Wearing a Mask Can Help Protect You Even If No One Else Wears One

When will the rest of the population be able to get a second booster?

When it comes to the rest of the population, the situation isn’t as clear. There is no doubt that booster shots do increase antibody levels. It’s also well-established that antibodies are an important, although not the only, contributor to a strong immune defense against SARS-CoV-2. Studies show that each dose of the currently-approved COVID-19 vaccines build up a person’s T-cell response—a group of immune system cells which tend to remember specific viral infections and remain at the ready the next time they encounter them. In fact, research suggests T-cells are primarily what are protecting vaccinated and boosted people from getting severely ill if they get infected. Understanding how well a second booster might continue to enhance this T-cell response could help health officials decide if that additional dose is necessary for people who aren’t at highest risk of severe disease—the T-cell response is, indeed, currently under scientific investigation.

The questions scientists are trying to answer now involving figuring out how strong vaccine-induced immune response is, how long the protection lasts, and whether the current vaccine is the best way to generate it. At the moment, the shots being used as a second booster dose are the same as the original vaccine, and don’t seem to be pushing protective antibody levels any higher than they were after the first booster.

Should I wait for a variant-specific booster shot?

Up to this point, the boosters have consisted of another dose of the original vaccine. But with new variants like Omicron and its subtype BA.2 now responsible for almost all recent infections, health experts are considering changing the COVID-19 vaccine strain, similar to the way they switch influenza strains each year for the flu shot.

The drug manufacturers behind the most successful vaccines to date have already developed and are testing new versions for the Beta, Delta, and Omicron strains. These companies are also testing combinations of these shots to identify a combination that could be used as a booster to generate a longer-lasting and broader immune response. But the results from these studies won’t be available until summer at the earliest, so it’s unlikely health officials will recommend additional boosters for the bulk of the U.S. population until after those results are analyzed.

For now, only certain groups are eligible to get a second booster, because their risk of developing serious illness is higher if they get infected, even if they are vaccinated and boosted. For the rest of us who are vaccinated and boosted, making individual decisions about continuing to wear masks in crowded indoor settings, and avoiding environments where we might be in close contact with a lot of people, continue to be the best ways to limit our exposure and risk of COVID-19—at least until more data become available.

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