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How to choose a booster shot: What to know about mixing vaccines and the omicron variant

With the omicron coronavirus variant now in the US, health officials are urging people to get boosted.

vaccine syringes
Sarah Tew/CNET
For the most up-to-date news and information about the coronavirus pandemic, visit the WHO and CDC websites.

On Wednesday, the US confirmed its first case of the omicron variant in a person in San Francisco. The person, who is fully vaccinated with Moderna but hadn't yet received a booster, returned from South Africa and tested positive for COVID-19 on Nov. 22. The person is experienced mild symptoms that are improving, and none of the person's close contacts have tested positive so far, according to the US Centers for Disease Control and Prevention

With the expectation that the omicron variant would eventually be detected in the US, the CDC strengthened its recommendation Monday that all US adults "should" get a booster. Though all adults have been eligible since mid-November, the CDC previously split hairs over who "should" get one and who "may" get a booster if they choose. As a new variant begins circulating, health officials are calling on adults to be as protected as possible through the same health measures that've been used throughout the pandemic to curb COVID-19.

"We know what we need to do to protect people," President Joe Biden's chief medical advisor, Dr. Anthony Fauci, said at a briefing Wednesday. "Get vaccinated if you're not already vaccinated. Get boosted if you've been vaccinated for more than six months with an mRNA (Pfizer or Moderna) or two months with J&J (Johnson & Johnson)."

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 6% of people in low-income countries have had a coronavirus vaccine, according to Our World in Data. In South Africa, the country which first reported the omicron variant to the WHO, 24.7% of people are fully vaccinated against COVID-19, according to Johns Hopkins data. The Biden administration has said the US can continue vaccine donations to other countries while also providing boosters to Americans. 

All adults can get any of the three available COVID-19 vaccines in the US as a booster shot -- a "mix and match" approach to COVID-19 vaccines. Currently, the CDC doesn't have a "product preference" on one booster over the other. The agency says that while it's generally recommended to stick to the same brand for primary and second vaccine series, people can choose whichever booster they'd like. As more people start seeking out extra doses, or wondering which one they should get, here's what to know about mixing COVID-19 vaccines and what to consider when choosing yours. 

Are the boosters effective against omicron?

On Friday, the World Health Organization classified omicron as a "variant of concern" -- the same label as the highly transmissible delta variant. This means that the new coronavirus variant has mutations that suggest it might be more contagious, may change the way the disease presents (its symptoms) or that it may potentially decrease the effectiveness of public health measures, including vaccines. 

In an interview with NBC, Fauci said that even in the face of a variant of concern, you do well against it if you have the increased protection of a booster. "It may not be as good in protecting against initial infection, but it has a very important impact on diminishing the likelihood that you're gonna get a severe outcome from it," he said, adding that boosting gives an "extraordinary increase in protection."

Read moreDelta vs. omicron: How the new COVID variant is different 

What's the difference among 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 'mixing and matching' 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. Johnson & Johnson's booster is also the same size as its original vaccine. In a study that 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 who were vaccinated with Pfizer, 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 might be worth a quick call to your health care provider to discuss your personal circumstances. However, the urgency of a new variant might also influence your decision, which includes what's available to you the soonest. 

covid-19-masks-n95-cnet-2021-012

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? 

Similar to Pfizer, most Moderna recipients probably don't have a need to choose a different booster, unless how quickly they can get an appointment is an issue.

There might be exceptions -- if you had an allergic reaction to Moderna's vaccine, for example, you should consult a doctor and choose a different vaccine 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 but is concerned about myocarditis may ask a health care provider about switching to Johnson & Johnson or Pfizer. But again, Moderna's booster is a smaller dose than its original vaccine, which could be a factor in your decision. 

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 with a boost of J&J. (Antibody response is not the only element of immunity, however.)

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 might 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. 

Read more: Everything to know Johnson & Johnson COVID boosters

Do I have to get a booster? 

As of this week, the CDC's recommendation is that all adults in the US "should" get a booster. This is because even if the omicron variant lowers the efficacy of the vaccines slightly, like delta, scientists expect COVID-19 vaccines to remain protective against severe disease.

Earlier this month, the CDC advisory panel voted unanimously to recommend boosters for all adults, but they also acknowledged that some people need them more than others. The guidance was that 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, the choice was more dependent on personal circumstances or preference.

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

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 so far.)

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 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.