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

Truth is, all of the three authorized COVID-19 vaccines do a good job, and you can decide to boost with whichever vaccine 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 making – particularly for older adults, who may choose to prefer the mRNA boosters, especially Moderna's, based on the available data.

Generally speaking, Moderna's vaccine tends to prompt slightly stronger immunity, and it also 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 eke out 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, saying he doesn'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, who have been advised to get a booster at any time past two months after their initial shot.

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

Johnson & Johnson takers, however, seem to have a slight preference for Moderna boosts. 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).

Data until December 1, 2021 for which booster was chosen. (CDC)

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 inside. (Half-dose boosters are common for many vaccines, and they work well.)

Pfizer's booster, in contrast, is exactly the same size as the first two shots administered from that manufacturer, with 30 micrograms of mRNA in it.

Like Moderna's vaccine, Pfizer's provides a strong jolt to 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 afterwards. (How long the extra protection lasts and exactly how good it is is something 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 showed that from January to May 2021 (before the emergence of the Delta variant) Moderna's vaccine was associated with a 21 percent lower risk of confirmed infection, and a 41 percent lower risk of hospitalization than Pfizer's.

And 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 elderly 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 Pfizer or Moderna.

The one issue people in Moderna's clinical trial tended to complain of more often after their booster shot 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, tend to report very similar, albeit slightly milder side effects for a second shot, compared to their first.

But all of 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 what booster you get, it's all the same

With time, over the next several months and years, it will become clearer what the very best COVID-19 vaccination strategy truly is. In the meantime, infectious disease experts stress, don't overthink it too much.

Regardless of which booster shot people choose, they are better protected against the virus than people with just one (J&J) or two (Pfizer, 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."

Bottom line: It's generally agreed (especially with the Omicron variant circulating) as reasonable for all adults to go ahead and get boosted with whichever shot they may choose to, ahead of the holiday gathering season.

This article was originally published by Business Insider.

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