(NEXSTAR) – Americans looking to get a booster shot against the coronavirus this fall don’t just have one new option – they’ve got two. Both Moderna and Pfizer have a new FDA-authorized COVID-19 vaccine, specially formulated to ward off the omicron variant of the virus.

Both vaccines are bivalent, meaning they contain parts of the original COVID-19 strain and the omicron strain that’s grown dominant in 2022.

Both companies’ new shots are very similar. Here’s what you need to know if you’re trying to choose between the two.

Who is eligible for each type of booster?

This is the key difference between Pfizer’s and Moderna’s bivalent booster: Moderna’s is authorized for adults, 18 and over, while Pfizer’s is cleared for anyone 12 and older, per the Food and Drug Administration.

Both boosters are recommended by the Centers for Disease Control and Prevention at least two months after your last dose.

Is there anything different about the makeup of the Pfizer and Moderna shots?

Both bivalent boosters are mRNA shots, created much like the original vaccines released nearly two years ago. They also both contain the original COVID-19 strain and the omicron strain, designed to give you maximum protection from the variants circulating now.

Pfizer and its partner BioNTech say their new booster contains 15 micrograms each of encoding for the spike protein of the original COVID variant and of the BA.4/BA.5 subvariants of omicron. Moderna’s contains a bit more: 25 micrograms of each type.

A single dose of Moderna’s booster comes out to 0.5 milliliters, while a single dose of Pfizer’s booster is 0.3 milliliters.

Can you mix and match? Should you?

Adults are allowed to mix and match brands, meaning even if you’ve gotten Pfizer all along, you can switch to Moderna now (and vice versa). People between 12 and 17 years old still have to stick with Pfizer for now.

But just because you can mix and match, is there any benefit? When we asked Dr. Bob Wachter, chair of the University of California San Francisco’s medicine department, about the prospects of mixing and matching last year, he said switching things up probably wouldn’t greatly boost your immunity.

However, if you had a bad reaction to one of the brands with a past dose, he said, you should talk to your doctor about switching vaccine types.

Do Pfizer and Moderna have different side effects?

It’s too early to have robust studies on whether these two brands of booster shots have different side effects from each other. However, evidence from both clinical trials shows the side effects of the bivalent shots are similar to the side effects of the last booster shot and the original vaccine.

“Most often it’s redness at the site of inoculation, some soreness, feeling tired for a day or two afterwards — all the same side effects we’re seeing at relatively the same rates with the bivalent booster,” said Andrew Pekosz, a virologist with the Johns Hopkins Bloomberg School of Public Health, in a recent media briefing. 

Other common side effects include headaches, fever, chills and nausea, according to the CDC.