How to choose which booster shot is right for you, between Pfizer and Moderna

• Most who've received boosters in the US have stuck with the brand of vaccine they got initially.
• It's fine to switch it up. Some early studies suggest Moderna's jab is a bit stronger than Pfizer's.

Now that free booster shots are being offered to all adults in the US, many are wondering which is the best one to get.

The truth is, all three authorized COVID-19 vaccines do a good job, and you can decide to boost with whichever one you want. All three have been deemed safe and effective.

But there are some small differences in the vaccines that could be a factor in your decision — particularly for older adults, who may prefer the mRNA boosters, especially Moderna's, based on the available data.

Moderna's vaccine tends to prompt slightly stronger immunity, and it seems to elicit slightly higher antibody levels than the other two vaccines. That doesn't mean it's a dramatically better vaccine in the long run, but the extra protection it provides in the immediate term does seem to pass that of Pfizer, at least a little bit.

"To be honest, I'm not sure there's really a big difference between those two," Dr. William Moss, who directs the International Vaccine Access Center at Johns Hopkins University, said of Pfizer and Moderna, adding that he didn't see any strong argument for picking one mRNA vaccine over the other. "I would probably say, you know, whichever is more convenient to get."

Most US adults have stuck with their original brand — except for J&J users

All adults are advised to get a booster shot six months after their initial series — except for those who've gotten Johnson & Johnson's single-shot vaccine. They have been advised to get a booster at any time two months after their initial shot.

Federal data on booster shots suggests most people who got Moderna or Pfizer end up sticking with their initial brand when they get boosted.

Johnson & Johnson takers seem to have a slight preference for Moderna boosters. There is evidence to suggest that J&J users might benefit from this "heterologous" boosting strategy — i.e., switching vaccine platforms — meaning they would add an mRNA vaccine (Pfizer or Moderna) on top of their adenovirus vaccine (J&J).

Moderna's shot is a bigger dose than Pfizer's

Moderna's booster is a half dose of the original vaccine, with 50 micrograms of mRNA in it. (Half-dose boosters are common for many vaccines, and they work well.)

Pfizer's booster is the same size as the first two shots, with 30 micrograms of mRNA in it.

Like Moderna's vaccine, Pfizer's ups antibody levels, meaning that people who get boosted with it reap the benefit of some increased protection against infection for at least a few months afterward. (How long the extra protection lasts and how good it is are two things scientists are still figuring out.)

Moderna's vaccine may be slightly more powerful than the others

There is some emerging clinical data that suggests Moderna's vaccine performs slightly better at keeping older people out of the hospital.

One Veterans Affairs study published on December 1 found that from January to May (before the emergence of the Delta variant) Moderna's vaccine was associated with a 21% lower risk of confirmed infection and a 41% lower risk of hospitalization than Pfizer's. In a more recent federally funded mix-and-match COVID-19-vaccine trial of more than 450 adults, Moderna's vaccine elicited the most robust antibody responses post-boost, while Pfizer's was marginally lower and Johnson & Johnson took third place.

That's why some experts say it might be a better booster for older adults, who are generally more vulnerable to severe outcomes and need more frequent and potent boosters of all kinds.

How the side effects differ

Moderna boosters may be the most powerful, but they're also the most intense.

Because Moderna's vaccine tends to be more reactogenic than Pfizer's, people who switch to Moderna for their booster may notice the severity of their side effects is a little higher than with their second shot. The most widespread complaint people have after either booster shot is some pain at the injection site. Headaches, fatigue, and muscle aches are also common.

For those who don't switch brands, getting a booster shot feels a lot like getting a second shot, whether it's from Pfizer or Moderna.

The one issue people in Moderna's clinical trial tended to complain of more often after their booster shots was some armpit swelling or tenderness. Pfizer trial recipients also reported a bit more swelling (lymphadenopathy) after their boosts.

People who got Johnson & Johnson's shot and stuck with the same brand for boosters tended to report very similar, albeit slightly milder, side effects for a second shot compared with their first.

But all these adverse effects of booster shots are mild to moderate and temporary, usually not lasting for more than a day or two after vaccination.

Many experts say it doesn't matter which booster you get

With time, over the next several months and years, it will become clearer which COVID-19-vaccination strategy is best. In the meantime, infectious-disease experts say not to overthink it. Regardless of which booster shot people choose, they are better protected against the virus than people with just one (J&J) or two (Pfizer or Moderna) shots.

"Right now, don't make it complicated," Dr. Anthony Fauci, the nation's top infectious-diseases expert, recently told Insider. "The effect of boost is very, very favorable to preventing people from getting infected."

Th bottom line is: It's generally agreed (especially with the Omicron variant circulating) that it's reasonable for all adults to go ahead and get boosted with whichever shot they may choose to ahead of the holiday season.