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The Omicron Booster: Your Questions Answered

BY CARRIE MACMILLAN January 26, 2023

Yale Medicine experts discuss what we know—and don't know—about the new COVID-19 booster shots.

[Originally published: Sept. 23, 2022. Updated: Jan. 26, 2023.]

Note: Information in this article was accurate at the time of original publication. Because information about COVID-19 changes rapidly, we encourage you to visit the websites of the Centers for Disease Control & Prevention (CDC), World Health Organization (WHO), and your state and local government for the latest information.

When the mRNA COVID-19 vaccines were first unveiled in December 2020, medical experts touted the benefits of this new technology, saying formulations could easily be tweaked someday to match a quickly changing virus. That day has finally come.

In the fall of 2022, the Food and Drug Administration (FDA) authorized—and the Centers for Disease Control and Prevention (CDC) recommended—two updated boosters. The boosters target two Omicron subvariants, BA.4 and BA.5.

While the original mRNA coronavirus vaccines have proven effective at preventing death and severe disease from COVID-19, breakthrough infections and reinfections have become more common in the face of an evolving virus.

Since the updated boosters (one from Pfizer-BioNTech and the other from Moderna) were granted an FDA emergency use authorization (EUA), the FDA and CDC have approved them for children, starting at age 6 months.

We asked Yale Medicine infectious diseases experts to answer common questions about these new, reformulated boosters.

How is this booster different?

The new booster is a bivalent vaccine, which means it contains two messenger RNA (mRNA) components of the coronavirus. Half of the vaccine targets the original strain, and the other half targets the BA.4 and BA.5 Omicron subvariant lineages, which are predicted to continue circulating this fall and winter.

How do we know the booster is safe?

The vaccines were authorized by regulators based on safety and effectiveness data from the original COVID-19 mRNA vaccines, as well as trials of the new formulation in mice. Regulators also took into account data from human trials by Pfizer and Moderna of a similar reformulation, aimed at a previous version of Omicron, BA.1.

In November, Pfizer released updated clinical trial data showing that the bivalent booster’s safety and tolerability in human adults remained favorable and similar to its original COVID-19 vaccine. Likewise, Moderna reported no new safety concerns in its human trials compared to its monovalent vaccine.

How effective are the new boosters?

In a study published in late January in The New England Journal of Medicine (NEJM), the bivalent vaccine was 58.7% effective against hospitalization compared to 25% for the original, monovalent vaccine. The bivalent was also 61.8% effective against infection versus 24.9% for the monovalent vaccine. The study, which drew data from people 12 and over who were included in North Carolina’s vaccine registry, was done during a time when Omicron BQ.1 and BQ.1.1 were circulating. The bivalent vaccine was developed before those subvariants began circulating.

Meanwhile, a study also in January from the CDC examined how effective the bivalent vaccine was against XBB and XBB.1.5, the more recent Omicron subvariants. The study found that in people who had previously received two to four monovalent vaccines doses, the bivalent vaccine was similar to how well it worked against BA.5 for at least the first three months after vaccination.

When should you get the new booster?

The CDC recommends the new vaccine as a single booster dose at least two months following your most recent COVID-19 vaccine (whether it was completing two doses of a primary series or a booster). 

Should you wait longer than two months between boosters?

The FDA set the minimum wait time at two months. But some advisers to the CDC said it may be better to wait longer. Some health experts have suggested that more time between boosters—up to six months—might be preferred. 

That’s because someone who recently got a booster already has more virus-fighting antibodies in their bloodstream. Antibodies gradually wane over time, and another shot too soon won’t offer much extra benefit.

Should you get a booster if you recently had COVID?

According to the CDC, if you recently had COVID-19, you may consider delaying your next vaccine dose by three months from the date your symptoms started—or, if you had no symptoms, when you first received a positive test. Studies have shown that increased time between infection and vaccination may improve your immune response.  

Reinfection, although possible, is also less likely in the weeks to months after infection. However, certain factors, such as personal risk of severe disease and local COVID-19 community level, could be reasons to get a vaccine sooner rather than later.

Can you get the new COVID booster at the same time as the flu vaccine?

Yes, it’s fine to get the flu vaccine—as well as any other vaccine—and the new booster at the same time, Dr. Murray says. 

“Historically, we try to time the flu vaccine for October to maximize immunity when the flu peaks in the winter,” Dr. Roberts says. “But the flu season has been changing in the past few years, which means predictions will be much harder this year.”

Dr. Murray agrees. “Unfortunately for the flu shot, you never know the best time to get vaccinated until after flu season,” he says. “But influenza is circulating now in the community. So, it’s a good idea not to wait too long.” 

Can children get the booster?

Yes, both bivalent vaccines are now authorized for children six months of age and up.

Should you even get a booster?

It’s really a personal decision, say the doctors.

“If you haven’t been boosted in the last couple of months, this is a great opportunity to be better protected,” Dr. Murray says. “Deciding on timing has a lot to do with your own health profile as well as that of those around you. If you're with people who could become quite sick if you transmitted it to them, even if you had mild disease, that is something to consider.”

Lifestyle also plays a role, Dr. Murray adds. “Some people wear masks; others do not. If you’re going on a cruise, I would recommend seriously thinking about getting that booster two weeks before you go,” Dr. Murray says. “If you don’t expose yourself to many large crowds or don’t go out to eat a lot, then you may choose to wait. But if it's been more than 90 days since you've had COVID-19 or the booster, the benefits of getting this booster would outweigh any benefits from waiting.”

Information provided in Yale Medicine articles is for general informational purposes only. No content in the articles should ever be used as a substitute for medical advice from your doctor or other qualified clinician. Always seek the individual advice of your health care provider with any questions you have regarding a medical condition.