So you’ve got your COVID shots, but with the delta variant surging across the country you are now understandably concerned about whether that is enough. Will you need a booster shot?

Health and infectious-disease experts are still dissecting that topic. Last week, members of the Advisory Committee on Immunization Practices, which helps shape federal COVID-19 vaccination policy, were presented with evidence that a third dose of the Moderna or Pfizer vaccine could help immunocompromised people.

Dr. Anthony Fauci, the nation’s top infectious-disease expert, said over the weekend that the committee’s ongoing study of the issue “might push us in that direction.” But as of right now, that’s not an official recommendation.

What is a booster shot? And why might two shots of Pfizer or Moderna, or a single-shot of the J&J vaccine, not be enough?

Essentially, booster shots are another dose of the same (or slightly tweaked) vaccine that is aimed at amplifying the immune system’s response. Viruses mutate and evolve over time – some more quickly than others – to get around the immunization roadblocks that prevent them from replicating and jumping from host to host.

Also, the body’s immune response can decrease over time. For instance, doctors recommend tetanus boosters every 10 years to make sure your body can still fight off the bacterial infection that can lead to “lockjaw” after, say, stepping on a rusty nail.

The COVID-19 vaccines are so new that manufacturers and independent medical researchers still aren’t sure how long that immune response will last. The influenza viruses that cause seasonal flu mutate so rapidly that drug manufacturers typically release new vaccines annually, targeting what they anticipate will be the predominant strains – with varying effectiveness.

“Vaccines vary quite a bit so we have to admit that we don’t know” the duration of the COVID vaccines, said Dr. Robert Finberg, a professor at the University of Massachusetts Medical School involved in clinical trials for the Pfizer vaccine.

The emergence of the delta variant – a mutant believed to be twice as transmissible as the original COVID-19 strain – is further complicating matters.

So what is the official policy on booster shots?

“At this time, the U.S. CDC and the U.S. FDA are not recommending boosters for individuals who are otherwise healthy and who have gotten whatever vaccine they got so far because of the continued effectiveness of all three of these vaccines against even the delta variant,” Dr. Nirav Shah, director of the Maine CDC, said on Wednesday.

The federal Advisory Committee on Immunization Practices, which makes recommendations to the U.S. Centers for Disease Control and Prevention, is still reviewing the evidence on whether immunocompromised people should get a third shot of Pfizer or Moderna. That group includes cancer patients, people who received organ transplants, individuals taking immunosuppressant drugs or people with other conditions that suppress the body’s natural defense mechanisms.

“Eventually, there might be a need for an additional dose … particularly in certainly groups that may not have responded well to vaccine,” said Dr. Gabriela Andujar Vazquez, an infectious disease physician and associate hospital epidemiologist at Tufts Medical Center in Boston. “As of today, we don’t see the need for a booster in all three of the authorized vaccines … but that may change. It’s hard to predict.”

Andujar Vazquez said all three of the vaccines in use in the U.S. are very effective. And while that effectiveness may be somewhat lower against the delta variant, the number of vaccinated people contracting the disease compared to unvaccinated people is “way far less – and they are not getting nearly as sick, which is what we want from the vaccine.”

“I don’t foresee a need for (boosters) in the near future, in the next couple of months,” Andujar Vazquez said. “But the caveat is we do have a big population of people who are not vaccinated and that may change what we see over the next few months into the fall or winter.”

Who will ultimately decide on boosters?

The U.S. CDC sets the policy, based on guidance from the advisory committee. And that committee makes its recommendations based on studies being done by researchers around the globe as well as the vaccine manufacturers.

One researcher involved in Pfizer’s clinical trials on booster shots is Finberg, the UMass professor of medicine who also serves on Gov. Charlie Baker’s COVID-19 advisory group. Finberg recruited 130 volunteers to participate in Pfizer’s initial clinical trials, and roughly half of those people are now returning for the booster trials.

“It’s very effective and prevents disease in essentially everyone that is fully vaccinated and prevents spread as well,” Finberg said of the vaccine. “The issue is how long that protection is going to last. And we don’t know the answer to that. It seems pretty good now after about six months, but we don’t know … and that’s why we are doing the trial.”

Half of the returning volunteers will receive a third Pfizer shot and half will receive a placebo. Because it is a “double-blind study,” neither Finberg’s team nor the volunteers will know who got a booster and who received the placebo, nor will other researchers in Pfizer’s roughly 10,000-person trial.

As part of the study, Finberg’s group will monitor whether the volunteers contract COVID-19 – so-called “breakthrough” cases – how long and severe the illness, and any changes in levels of antibodies, which are proteins that help prevent viruses from entering cells. The clinical trials also will examine the safety of those third shots in all recipients.

“We know that if you give additional shots you will see more antibody levels,” Finberg said. “What we are looking to see is will it prevent disease, does it prevent asymptomatic disease and then, most importantly, is it safe to give three shots?”

Pfizer’s clinical trial is only looking at third shots of the same vaccine. Other researchers are looking at the efficacy of boosters using vaccines that use different mechanisms to trigger that immune response. The Pfizer and Moderna vaccines use mRNA to teach the body how to fight the virus while the J&J shot and the AstraZeneca vaccine widely used in Europe and Canada use a different, inactivated virus as a vector.

Public health officials in Canada, the U.K, Germany and other countries have already endorsed receiving a mix of doses, when necessary, to achieve full vaccination amid supply constraints. Now researchers are looking at mixing and matching vaccines for booster shots.

Can I get a booster anyway?

You probably can, somewhere. But you’d be doing so before the science is complete about the efficacy – or potential side effects – of getting more shots than the current regimen.

“We have no safety data on that and we … have always wanted to be transparent and make decisions based on safety,” said Andujar Vazquez of Tufts.

“I would really urge everyone to have a little faith and trust in that we and the CDC, the FDA and the (advisory committee) are following the data very closely,” she said. “And if there is any indication that we may need an additional dose, that will be implemented right away.”

If boosters are eventually recommended, where will I be able to get one?

It’s still too early to say whether we could see a revival of mass-vaccination clinics if booster shots are recommended or if the current network of pharmacies, medical clinics and doctor’s offices will be tasked with administering the additional jabs.

A lot will depend on what, exactly, is recommended.

Boosters only for immunocompromised or particularly vulnerable individuals would be a comparatively small number of people. But if all vaccinated people are urged to come in for another shot, hospital networks and public health officials might organize larger vaccination clinics or events.

Of course, a rush for booster shots could lead to a supply crunch. Right now, the line for lobster rolls is longer than the one for COVID shots in many parts of Maine.

The good news is that, more than eight months after the vaccination campaign began, the inoculation infrastructure is now in place in Maine.

What else can I do?

Well, if you’re not vaccinated, getting a shot is the most effective and easiest way to avoid catching the coronavirus and potentially spreading it to others. They’re also free. And if you still happen to contract COVID-19, being vaccinated dramatically reduces your chances of developing a severe and potentially life-ending illness.

“Get the vaccine,” Finberg said. “That’s the most important thing.”

The vaccinated and unvaccinated, alike, can also dramatically reduce their likelihood of contracting and then potentially spreading COVID-19 by taking precautions that had become almost second nature for many people before rules were relaxed this summer. That means wearing a mask into stores or public spaces, keeping your distance, sanitizing often and avoiding crowds, whether indoors or outdoors.

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