5 Things to Know About COVID Booster Shots, According to an Infectious Disease Doctor

This month marks two years since COVID-19 first emerged, and though the world has changed in many ways, the development of lifesaving vaccines (with upwards of 94 percent protection against the initial strains of the disease) have offered us a chance to get back to some semblance of normalcy.

However, new variants including delta and omicron mean that there's still a ways to go in eradicating the disease entirely — but luckily, doctors agree, there are more weapons in our arsenal than before.

"The way I think about it is, a lot of people thought they were running a sprint," says Dr. Jaimie Meyer, an infectious disease physician at Yale medicine, and an associate professor of medicine in public health at Yale University. "It turns out, no, it's an ultra marathon."

One of the most promising tools to help reach the finish line (once you're fully vaccinated) is a booster shot. A booster shot is an additional dose of vaccine you get once the protection from the initial shot or series of shots starts to wane. The CDC recently expanded booster recommendations to anyone 16 and older who got their initial series of the vaccine done at least six months ago.

If you qualify and are considering a booster, Meyer answered some of to some of the most pressing booster shot questions.

Do I need a booster shot?

Studies show that, while the vaccines are highly effective at preventing serious cases of COVID, that effectiveness wanes over time.

"The farther out people got from their initial [vaccination] series, that's really when we're seeing these breakthrough cases," notes Meyer. (She also underscores that the point of the vaccine is not to prevent infection, but to prevent severe disease.)

Data, including from the CDC, and from a large-scale Israeli study, suggests that when people are boosted, severe breakthrough infections are incredibly rare. "It's just one more layer of protection," she says.

When should I plan to get a booster?

The CDC recommends getting your booster at least six months after completing your primary COVID vaccination series. For the severely immune-compromised, though — including people undergoing chemotherapy, or who have had a transplant — the recommendation is to get a booster at least one month after your primary series is done.

Meyer notes that people who stand to benefit from boosters are those who are at highest risk of severe disease (e.g., those with pre-existing conditions, and adults 65 years and older), as well as people who are at a higher risk of exposure because of where they live or work (like a nursing home or assisted living facility, or hospital).

"But honestly, as we head into respiratory virus season (otherwise known as the holidays) we really want everyone to be boosted," she adds.

What side effects should I expect after getting my booster?

The reactions to the booster that have been reported have been similar to the second dose of the primary vaccine series, according to the CDC. Typically, these include mild to moderate fever, body aches, headaches, and fatigue.

"In the clinical trials of the booster, they're very well tolerated," Meyer says. "I actually have seen that people tolerate the booster even better — whatever side effects or symptoms they have seem to be relatively mild."

Are you able to mix and match booster shots?

Great news for anyone trying to decide which booster to get: a recent study suggests there's strong protection, regardless of which you get. British researchers compared boosters from seven different vaccine brands, and found that every one elicited a strong immune response.

(The study found that mRNA shots from Pfizer-BioNTech and Moderna prompted the strongest responses; the latter saw antibodies and T cell levels — T cells are immune cells that attack and kill coronavirus-infected cells — increase at least 1,000 percent above the control group.)

The CDC's data also showed that people got robust immune responses whether they mix and match brands, or stick with what they started with. (The exception is the Johnson & Johnson vaccine. People who got the single dose do better with a booster of mRNA vaccine — either Moderna or Pfizer-BioNTech — after their initial shot.)

Now that the study has been published, Meyer says, "the data is even clearer for that. Get boosted, get what's most convenient for you, and what's most accessible for you. That's what matters."

Does the omicron variant impact the current booster guidelines?

According to Meyer, the emergence of the omicron variant isn't changing the booster guidelines right now, but it does strengthen the push to get people boosted.

"We know now that boosters are so protective against severe disease, and we want people to be as protected as possible." she says. "Omicron is here, and data is emerging, but it does seem to be much more highly transmissible than earlier variants, including delta." It's all the more reason, as we head into the holiday season, to schedule your shot. "People are going to be gathering with friends and family, and that's all the more urgency behind getting boosted."

Breakthrough cases — COVID-19 infections that occur in people who have been fully vaccinated against the virus — are possible and expected, as the vaccines are not 100% effective in preventing infections. Still, vaccinated people who test positive will likely be asymptomatic or experience a far milder illness than if they were not vaccinated. The majority of deaths from COVID-19 — around 98 to 99% — are in unvaccinated people.