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The latest pronouncement from the US Centers for Disease Control and Prevention on COVID-19 booster shots cuts to the chase: if you're an adult and it's been at least six months since you got your Pfizer-BioNTech or Moderna vaccine, you're eligible for a booster. It's up to you which booster to get: Pfizer, Moderna, or Johnson & Johnson (J&J)/Janssen.

The guidance follows the US Food and Drug Administration's (FDA) decision to authorize a booster dose of either Pfizer or Moderna for anyone 18 and older after their primary vaccination.

Adults who received the J&J COVID vaccine were already eligible for a booster dose two months after their initial shot. And, again, those who got the one-shot J&J vaccine may choose to have a J&J, Moderna, or Pfizer booster.

The expanded approval for all adults—paired with the FDA's recent approval of mixing-and-matching COVID-19 vaccines and boosters—has most people wondering (and planning) which booster shot they should get, based on their original COVID-19 vaccine.

Unfortunately, there's not a clear answer to that yet for anyone. "We're still learning about what is the best strategy," Abinash Virk, MD, an infectious disease expert at the Mayo Clinic, tells Health. But there may be certain factors that can help inform your decision. Here's what we know so far, regarding which booster shot you might want to choose.

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Which Booster Should You Get? Top view of bunch of sterile syringes with vaccine arranged on pink background. Set of syringes with medication . In other words, if you want to stick with a booster of the same brand of vaccine you received initially, you can do that.

There is some data, however, on which booster combinations may elicit the highest antibody levels in the body. In October, a small preprint study funded by the National Institute of Allergy and Infectious Diseases (NIAID) found that all possible booster combinations were able to stimulate a neutralizing antibody response that should—at least in theory—help protect you from COVID-19.

But going deeper, the study, which was not peer-reviewed, found that those antibody responses had varying levels. Study participants who got the two-dose Moderna vaccine and a full-dose Moderna booster had the highest antibody levels, followed by those who had two doses of Pfizer plus a Moderna booster, and then two doses of Moderna again with a Pfizer booster. On the flip side, those who got the J&J vaccine with a J&J booster had the lowest antibody levels; though a J&J vaccine followed by an mRNA booster elicited better responses.

According to the study authors, however, this research "was not designed to directly compare responses between different booster regimens." The Moderna booster used in that study is also not the one currently in circulation: Researchers from the NIAID study used a full dose of the Moderna vaccine at 100 micrograms for research purposes, but Moderna boosters are only being offered in a half-dose, 50-microgram regimen in the real world.

There's also a big question mark surrounding antibody levels and COVID-19—namely just how much protection they offer from the virus. "There is some correlation that antibodies do protect against infection," Dr. Virk says. But experts still don't know exactly how much antibody protection someone needs to keep them from getting sick, or how antibody levels in studies translate to real-world protection, since other factors (like the body's T-cells, which attack cells that have been infected with a virus) also play into immunity. "We still do not know clinically if [higher antibody levels result] in decreased infections, transmission, or severe disease."

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Does your demographic matter, regarding which COVID-19 booster you should get?

Beyond expanding booster eligibility to all adults, the CDC notes that some people "should" get a booster, while others "may" get a booster. Here's how that advice breaks out:

People who got the Pfizer or Moderna vaccine should get a booster at least six months after their primary vaccination if they are:

• 50 and older
• 18 or older and live in a long-term care setting

People who got the Pfizer or Moderna vaccine may get a booster at least six months after their primary vaccination if they are:

• 18 and older

People who got the J&J vaccine should get a booster at least two months after their COVID vaccine if they are:

• 18 and older

RELATED: Why You Might Want to Drink Water Before You Get Your COVID-19 Vaccine or Booster Shot

What do experts say about which booster to get?

Past that, the CDC doesn't specifically list which boosters may or may not work better—but, based on expert opinions, there are some factors that you might want to consider to get the best result possible.

First (and arguably most important): All of the experts interviewed for this piece say anyone 65 or older will benefit from any COVID-19 booster and should get any dose that is available to them. "If you're over 65, there is clear data that you will benefit from a third dose," Paul Offit, MD, director of the Vaccine Education Center and professor of pediatrics in the Division of Infectious Diseases at Children's Hospital of Philadelphia, tells Health. That same advice goes for anyone else who is currently approved to receive a booster.

From there, you can begin to refine your booster choice, if you so choose. Regarding those who got the J&J single-dose vaccine specifically, it may be a good idea to get an mRNA booster. "There may be slightly higher antibody responses, particularly for those that got the Johnson & Johnson/Janssen COVID-19 vaccine upfront and get an mRNA booster," Dr. Virk says. Still, if you want to get the J&J booster along with your J&J vaccine, you can feel comfortable doing that. "I think of Johnson & Johnson as a two-dose vaccine," Dr. Offit says, citing data from the company's two-dose trial, which showed 100% protection against severe COVID-19 after a second dose, with a 94% effectiveness at protecting Americans against moderate to severe symptoms of COVID-19.

Here's where it gets even more specific: According to our experts, there are some differences in reactions to each vaccine between male and female recipients that may cause you to choose your booster a bit more carefully.

• Women under the age of 50 may prefer an mRNA (Pfizer or Moderna) booster: Dr. Offit references an "extraordinarily rare risk of blood clotting"—now known as thrombosis with thrombocytopenia syndrome (TTS)—that has been linked to the J&J vaccine. The side effect was seen in 15 women who received the J&J shot, as of April 21, 2021, and the majority occurred in those ages 18–49, according to CDC data. The CDC advises that women in that age group "be made aware of the increased risk for TTS and the availability of mRNA COVID-19 vaccines for use as a booster dose." It further states anyone who developed TTS after their initial J&J vaccine should not receive the J&J booster. "It will only affect a handful of women in about five football stadiums worth of people, but you don't have to get the vaccine that puts you at risk—you can choose an mRNA booster," says Dr. Offit.
• Men under the age of 30 may prefer the J&J booster: According to the CDC, instances of myocarditis (inflammation of the heart muscle) were seen most often after the second dose of an mRNA vaccine, mainly in male adolescents or young adults. Dr. Offit says that if someone has had myocarditis after a first or second dose of an mRNA vaccine, they may want to get the J&J vaccine. (However, CDC points out that such cases are rare and, when they did occur, patients who received care felt better quickly.)
• Men over the age of 50 may prefer an mRNA (Pfizer or Moderna) booster: The CDC says men ages 50–64 are at the highest risk for developing a condition called Guillain-Barré Syndrome (GBS) following their primary dose of a J&J vaccine. While there is currently no data on how the J&J booster will affect people who had GBS following their J&J vaccine, the CDC says people should be made aware of the risk and given the option to receive an mRNA vaccine.

If you're still confused about which booster you can and should get, you're in good company with the general public. Basically, "what the FDA and CDC are trying to say is that it doesn't matter which booster you get—all of them work," Dr. Offit says. "So if you go to the site where you received your first vaccination, and they don't have the booster that matches what you've previously received, anything available will do."

While you're welcome to consult your doctor to find out which booster, if any, is right for you, just keep this in mind: Experts agree that widespread adaptation of boosters isn't going to put COVID-19 behinds us. "The goal of vaccination to prevent severe illness—the kind that drives you to seek medical care or go to a hospital," Dr. Offit says, adding that the vaccinations themselves are incredibly effective for this purpose. "To end the pandemic, the goal is not to boost the vaccinated; it's to vaccinate the unvaccinated."

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