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Which booster shot should I get? What to know about COVID vaccine mix and match

You can get any of the three COVID-19 vaccines as a booster, the CDC says. Here's what to consider when making your choice.

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Sarah Tew/CNET
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All adults in the US are eligible for COVID-19 boosters now that the US Centers For Disease Control and Prevention recommended them for everyone age 18 and older last week. The US Food and Drug Administration authorized COVID-19 boosters from Moderna and Pfizer for all adults shortly before the recommendation, setting the stage for everyone who got an mRNA vaccine to get another dose of COVID-19 vaccine at least six months after their second dose. Everyone who received Johnson & Johnson's vaccine has been eligible for a booster since last month. 

What's more, no one will be tied to the original COVID-19 vaccine they received. All adults can get any of the three vaccines after the FDA and CDC last month signed off on heterologous boosters, or a "mix and match" approach to boosting for COVID-19.

Politicians in some states and cities, including Colorado, West Virginia, California, New Mexico, Arkansas and New York City, jumped ahead of official guidance by the CDC, having already widened the booster net for all adults who want one, so long as enough time has passed since their last shot. That means the question has been raised for a while: Which COVID-19 booster should you get? 

Mixing COVID-19 vaccine brands has been done in other countries for months, and the US is now on the same page (for boosters, at least). While getting your original vaccination remains the most important thing you can do to protect against severe disease from COVID-19 -- and the availability of third shots hasn't changed the definition of "fully vaccinated" -- here's what you should know before choosing a booster. 

What's the difference between the COVID-19 vaccines? 

While all three vaccines have the same effect -- protection against severe COVID-19 disease -- the way they function is a little different. Pfizer and Moderna are mRNA vaccines, which teach our cells to make a specific protein and build immunity against a virus. Johnson & Johnson is a viral vector vaccine, which uses a harmless virus to activate an immune response and tell our bodies what to fight in future infections. 

Both vaccine types prepare our immune systems for COVID-19 infection, and none of the coronavirus vaccines infects us with the actual coronavirus

What are the benefits of allowing mixing for COVID-19 boosters?

Individual choice in boosters means health care providers can make recommendations based on patients' circumstances. A member of the CDC's advisory panel, which meets prior to recommending a vaccine or booster, pointed out at a meeting about mixed boosters that allowing it could lead to fewer vaccine doses being wasted, if health care providers only have to open one bottle of vaccine for patients in the waiting room, for example. Hopefully, more flexibility with boosters will lead to an easier vaccination process in places that administer many doses at once, such as nursing homes. People may also opt for a different vaccine if they're at higher risk for a rare side effect from a particular vaccine.

The bottom line? Mixing vaccines for a COVID-19 booster may be a great benefit to some people, but it ultimately depends on personal circumstances and what's available.

Which booster shot should I get if I got Pfizer?

Both mRNA vaccines, Pfizer and Moderna, have proven to be effective and continue to protect against severe disease caused by COVID-19. A study published by the CDC in September that compared vaccine effectiveness among adults in the real world found that two doses of Moderna's vaccine were 93% effective at preventing hospitalization, two doses of Pfizer were 88% effective and one dose of Johnson & Johnson was 71% effective. 

Pfizer's booster is the same dose as its original vaccine (30 micrograms), while Moderna's booster (50 micrograms) is half the size of its original vaccine. In a study which examined people's responses to all three vaccines as boosters, people who originally got Pfizer had the strongest antibody response to a Moderna booster. However, that study examined a full dose of Moderna (100 micrograms), rather than the authorized half-dose of the company's booster, which likely minimizes Moderna's edge over Pfizer, the Atlantic reported.

Read more: Pfizer COVID booster gets CDC approval for all adults

For most adults, sticking with another dose of Pfizer makes the most sense, Dr. Robert Wachter, chair of UC San Francisco's Department of Medicine, told the Los Angeles Times.

"I got Pfizer and stuck with getting another Pfizer -- just seemed simpler. Why introduce a new agent into my body for probably zero, or maybe tiny, benefit?" he told the LA Times. "But if your goal were to give yourself any possible advantage in immunity, you can make an argument to switch to Moderna."

If you're in a group that would benefit most from every inch of immunity (which includes the elderly and people with health conditions that make them most susceptible to severe COVID-19), it's worth a quick call to your health care provider to discuss your personal circumstances. This could help you decide whether it's best to stick to Pfizer or try another vaccine, which might also depend on what's available in your area or at a pharmacy that's convenient to you. 

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The definition of "fully vaccinated" hasn't changed. A person is considered fully vaccinated two weeks after their second dose of Pfizer or Moderna, or two weeks after a single dose of Johnson & Johnson. 

Sarah Tew/CNET

What if I got Moderna? 

Most Moderna recipients probably don't have a need to choose a different booster. 

There might be exceptions -- if you had an allergic reaction to Moderna's vaccine, for example, you should consult your doctor and choose a different one for future shots. An early report on Canadian data also suggested Moderna might carry a higher risk of myocarditis, an uncommon side effect of the mRNA vaccines mostly seen in men under 30, compared to Pfizer. With this assumption, a man under 30 who originally received Moderna and decides he needs a booster may ask a health care provider about switching vaccine types. But again, Moderna's booster is a smaller dose than its original vaccine, which could be a factor in what your provider recommends. 

Read more: Moderna COVID booster recommended for all adults. What to know 

Which booster shot should I get if I got J&J? 

Some public health experts have argued that a second shot for people who got Johnson & Johnson is more like completing the series, rather than getting a booster. In a clinical trial shared by Johnson & Johnson, a second dose of J&J two months after the first shot made the vaccine 94% protective against symptomatic COVID-19. However, some experts also argue that Johnson & Johnson recipients are better off choosing either mRNA vaccine while pointing to a different study that showed higher antibody responses following a boost of mRNA vaccine compared to a boost of J&J. 

Women under 50 who originally got J&J should be especially aware of the availability of other vaccines, the CDC says, because they're at an increased risk for the rare but serious blood-clotting disorder associated with J&J's vaccine (which isn't seen with Pfizer's or Moderna's). For example, Dr. Leana Wen, physician and public health professor at the Milken Institute School of Public Health at the George Washington University, detailed her decision to get a Pfizer booster after being vaccinated with Johnson & Johnson in a report for The Washington Post

Older men who seem to be at an increased risk of the rare neurological disorder associated with J&J's vaccine may also choose another type of vaccine. 

In general, people who originally received Johnson & Johnson's vaccine and have a health condition that makes them more susceptible to severe COVID-19 should be especially aware of the benefits of mixing with an mRNA vaccine and the higher immune response it seems to bring. 

"When the vaccines were first introduced, the Johnson & Johnson shot did have its benefits and there wasn't as much supply, so people needed to get whichever vaccine was available," Dr. Mahdee Sobhanie, an infectious diseases physician at The Ohio State University Wexner Medical Center, told NBC News. "But now there's more choice and we need to evaluate the data and make a halftime adjustment."

Read more: Everything to know Johnson & Johnson COVID boosters

Do I have to get a booster? 

When the CDC advisory panel voted unanimously to recommend boosters, they also acknowledged that some need them more than others. People age 50 and older "should" get a booster (a younger age than the previous guidance for everyone age 65 and older), in addition to other groups who "should" get one because of a clearer benefit, including adults living in long-term care facilities and everyone who received Johnson & Johnson's vaccine. 

For all other adults under 50, the need for a booster is less clear. Your decision whether to get another dose will depend on your own circumstances, choice and benefit-risk assessment. 

Importantly, the definition of fully vaccinated hasn't changed. You're considered fully vaccinated two weeks after your second Pfizer or Moderna shot, or two weeks after your Johnson & Johnson shot. 

Is it safe to mix and match COVID-19 shots? 

A study on mixing with different boosters for all three COVID-19 vaccines found no safety concerns and that the mixed boosters elicited a strong antibody response. According to booster data on primary series and booster choice the CDC is collecting, over 2 million Americans so far have opted for a different vaccine brand as a booster. (Most have stuck with their original vaccine.)

There isn't a lot of data on mixing COVID-19 vaccines in the US because it hasn't been allowed until recently. Information from outside the US has been promising, though, as other countries have been officially allowing (or even recommending) people receive two different vaccines, including Germany, Canada, Sweden, France, Spain and Italy.

In a study published in the journal Nature, researchers in Spain found that people who received one dose of AstraZeneca (a similar vaccine to Johnson & Johnson) and then received a dose of Pfizer seem to produce a higher antibody response than people who receive two doses of AstraZeneca. It isn't clear whether this group had a higher immune response than people who received two doses of Pfizer. 

Can I mix and match the first two shots?

No, the CDC's statement on mixing COVID-19 vaccines only applies to boosters. As of now, the FDA has only authorized a mixed-series booster, meaning the first coronavirus vaccine series must be one dose of Johnson & Johnson or two doses of Moderna or Pfizer. 

Once the mixed-series boosters start rolling out to Americans in greater numbers, there will be more data on the safety and effectiveness of a mixed COVID-19 series. Although it might be too early to hope, this might mean that data on mixing for boosters will inform decisions on primary coronavirus vaccine series being used together, making it easier to reach underserved communities, and possibly reducing health care and vaccine inequity. 

The booster rollout has been a controversial one. Officials with the World Health Organization have called on countries such as the US to slow the process of giving booster doses to people who are already vaccinated while much of the world remains unvaccinated against COVID-19. As few as 5.2% of people in low-income countries have had a coronavirus vaccine, according to Our World in Data.

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.