Late last week, after the Food and Drug Administration (FDA) authorized a third dose of the Moderna and Pfizer vaccines for immunocompromised individuals, an expert panel advising the Centers for Disease Control and Prevention (CDC) recommended that such individuals receive these authorized booster shots.

Generally, a booster shot is administered at some point after initial vaccinations to bolster immunity, reignite immunity after it has waned, or protect against new strains of a virus for which someone has already been inoculated. Depending on why a booster is needed, the shot might be identical to the vaccines already administered or slightly tweaked to add specific protection against virus mutations. In this case, the recommended third jab is identical to the first two jabs of Moderna and Pfizer vaccines already received.  So why give them, and to only a select group of individuals?

Why immunocompromised individuals may need third shots

People who are immunocompromised—which includes, according to Jessica Malaty Rivera, MS, Science Communication Lead at the COVID tracking project, organ transplant recipients, people who have advanced or untreated HIV infection, people who are on a number of different types of biologic drugs that make them either immunosuppressive or immunodeficient, and those with other immune system-weakening chronic illnesses—are more vulnerable to serious illness if infected with COVID-19, and their bodies may have less robust responses to the vaccines as well.

So just like two shots were better than one in terms of upping our protection against the worst effects of this virus, three shots may provide increased protection for this specific cohort of extra-vulnerable individuals.

“Now, the category of immunocompromised people is also extremely broad, and not everybody who is considered immunocompromised necessarily needs an additional dose,” says Rivera. “But [this recommendation] is essentially saying that if you fall into these categories, you should have a conversation with the person who’s managing your condition to see if you do have a reduced immune response to the vaccines and if an additional dose will be beneficial.”

Why those with healthy immune systems aren’t yet eligible for third doses, despite the Delta variant

If you don’t fall into the immunocompromised category, you might still be wondering if you need extra protection, stat. After all, breakthrough cases are not unheard of, and we don’t yet have data on how long protection lasts (although Rivera says nine and 12-month data is coming soon). Rivera admits some people will be tempted to fib out of fear to secure a third dose, but if you have a healthy, strong immune system, she says this will not do you much good at this particular point in time, as data shows you are still well protected if you received the shot less than six months ago.

And Rivera, like many in the global health community, objects to administering third doses to healthy individuals when many countries are waiting on first doses. After all, it’s not only arguably unethical, but countries where people are largely unvaccinated impact our safety from COVID-19—the longer the virus circulates in unvaccinated populations, the greater its chances of mutating into something that could completely evade current vaccines.

So, Rivera wants to reiterate that you are well-protected at present by all three of the vaccines. You may worry about the seeming prevalence of breakthrough infections since Delta arrived on our shores. Still, she says those who focus on breakthrough numbers aren’t comparing them to unvaccinated infections—the former are much, much, much lower than the latter. “No vaccines are perfect, but you’re still in a much better place than those who are unvaccinated and those who are immunocompromised,” she says.

When you might expect a booster shot in the future, if you’re not immunocompromised

Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases (NIAID), has said third doses might be needed for the elderly next, and on Monday, news broke that the Biden Administration is planning to recommend boosters for most populations who received mRNA vaccines (Moderna, Pfizer) eight months from the date of their last shot. This decision has not yet been officially announced, but sources familiar with conversations taking place within the administration say that elderly populations will be first in line for these boosters, followed by other vulnerable populations and, finally, otherwise healthy Americans. This new round of shots could roll out as early as mid-September.

These third doses are expected to be identical to the first shots administered, though FDA authorization for these third boosters in populations beyond the immunocompromised is pending. Johnson & Johnson recipients are also expected to require a booster, but data from their two-dose trial is not expected until later this month.

“A booster depends on two factors,” says Timothy Brewer, MD, professor of medicine and epidemiology at UCLA. One, he explains, is whether or not COVID-19 immunity diminishes over time. “We know it does, but the question is how much,” Dr. Brewer says. So far, we’ve seen evidence that immunity lasts as far out as eight months, but we have no public data beyond that yet.

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The second variable, or impetus for a booster, is virus mutations. The reason you get a flu shot every year, Dr. Brewer explains, isn’t because your immunity from last year’s flu shot wore off but rather that the virus has changed enough so that last year’s shot no longer offers adequate protection from it. Fortunately, he says, coronaviruses don’t change as rapidly as influenza viruses.

Existing COVID-19 variants—specifically, the Delta B.1.617 variant, the South African B.1351 variant, the Brazilian P.1 variant, and the two Californian B.1.427 and B.1.429 variants—render current vaccines somewhat less effective, to varying degrees. So while existing vaccines still prevent serious disease, hospitalization, and death in those infected by these strains, they don’t necessarily protect as well against these infections, full-stop. And the Biden Administration is citing Delta specifically as an impetus for the proposed eight-month boosters.

Still, drug companies are proactively preparing booster shots to roll out. Pfizer’s CEO Albert Bourla said it’s “likely” people will need a booster within the next 12 months, and Moderna has announced that its booster will be ready by the fall. But the decision as to whether or not populations need to be reinoculated with a booster shot isn’t the drug companies’ to make, says Dr. Fauci. That call will instead come from the CDC. Dr. Fauci currently projects that experts will know enough by the end of this summer or the start of fall to determine booster rollout.

In short, there’s no official word on when boosters will be widely distributed. And while such unknowns can be frustrating or even scary, keep in mind that if you’ve been vaccinated, you are currently well protected from serious illness and likely will be long enough for the booster question to be answered.

When we do need them, the relative success of our current vaccination program bodes well for boosters, too. In April, Andy Slavitt, senior advisor to President Biden’s COVID response team, told reporters that the administration is considering the potential need for boosters in its planning efforts. “Requiring additional shots in the future is obviously a foreseeable potential event,” he said. “I can assure you that when we do our planning, when the president orders purchases of additional vaccines as he has done, and when we focus on all the production expansion opportunities that we talk about in here, we very much have scenarios like that in mind.”