Editor’s Note: This article is outdated and refers to the bivalent omicron-updated COVID-19 vaccines first authorized in fall 2022. With the approval and authorization of newer monovalent shots for the 2023-2024 season, the bivalent vaccines are no longer authorized or available.
In September 2022, the U.S. began administering the first COVID-19 booster vaccines that were updated to better match the latest circulating coronavirus strains.
Many scientists expect the revised boosters will be more effective than their predecessors, but whether that’s the case and to what degree remains unknown.
The new vaccines, from mRNA vaccine makers Pfizer/BioNTech and Moderna, are bivalent, meaning that along with the original version of the coronavirus, or SARS-CoV-2, they also specifically target the BA.4 and BA.5 omicron subvariants. At the end of August, the Centers for Disease Control and Prevention estimated that together the two subvariants accounted for more than 90% of new COVID-19 cases in the U.S.
The Food and Drug Administration authorized the retooled boosters on Aug. 31, and the CDC signed off on the shots the following day, after the agency’s vaccine advisory committee voted 13 to 1 to recommend both boosters.
The authorizations marked a shift in American COVID-19 vaccination policy. In what was called a “fall booster ‘reset,’” people no longer had to count the number of vaccine doses they received. Instead, the guidelines were simple: If you’ve had your primary series (one dose of J&J or two doses Pfizer/BioNTech, Moderna or Novavax) and it’s been at least two months since your last dose, then you’re eligible for one omicron-updated booster.
On April 18, 2023, the FDA allowed some people to receive a second omicron-updated booster. The agency also ended the use of the original mRNA vaccines. The CDC agreed with the changes the following day.
We’ll explain how the new vaccines are different and what experts are saying about them.
How are the updated shots different from the original ones?
The revised boosters are essentially identical to the original ones, except for a tweak to some of the mRNA included in the shots.
For both Pfizer/BioNTech and Moderna, half of the mRNA in the vaccine includes the instructions used in the earlier vaccines for cells to make the spike protein of the original coronavirus strain. The other half includes the instructions for making the spike protein of the BA.4 and BA.5 strains, which is the same in the two subvariants. The spike proteins trigger a protective immune response in the body.
The dual components are why the boosters are referred to as “bivalent.” For the same reason, you may hear the original vaccines being called “monovalent.”
As before, the new Pfizer/BioNTech booster contains a total of 30 micrograms of mRNA, matching the dosage of the primary series shots, while the revamped Moderna booster contains a total of 50 micrograms, or half of the dosage of the primary series.
Who is eligible to get an updated vaccine?
People 12 years of age and older are eligible to receive Pfizer/BioNTech’s updated vaccine and adults 18 years or older are eligible to receive Moderna’s. (Children are also eligible for updated vaccines, see our stories on those vaccines for more information.)
Initially, the updated vaccines were authorized only as a single booster, two or more months after a previous COVID-19 vaccination. But on April 18, the FDA ended the use of the original, monovalent vaccines for primary vaccination and also allowed additional bivalent boosters for some people. As a result, all mRNA doses are now bivalent.
Now, people who have not yet been vaccinated will receive a single bivalent dose. Individuals who have not yet received a bivalent vaccine are eligible for a single bivalent dose, as before. But immunocompromised people and those 65 years or older are eligible to receive a second bivalent dose.
The second bivalent doses are authorized for use at least two months after a previous dose for those who are immunocompromised and at least four months after a previous dose for people 65 years old and above. Additional bivalent doses are allowed at the discretion of health care providers for people who are immunocompromised.
What evidence supports the use of these omicron-updated boosters?
To authorize the updated boosters, the FDA borrowed its approach for influenza vaccines, which each year are modified to match the flu strains that are expected to circulate that season. Because the changes are only tweaks — and because it would be impractical if not impossible to test the vaccines in people prior to the flu season — flu vaccines are approved without doing clinical studies each year.
The omicron-updated boosters are similar in that at the time of authorization, they had not yet been evaluated in people, although there was other supporting evidence to suggest that they will work. For one, there was clinical data on a slightly different omicron-specific bivalent booster that targets the BA.1 subvariant that was dominant earlier in 2022.
Moderna tested that booster in about 600 adults who had received two primary doses and one original booster, and at least three months later were given a second original booster or a BA.1 bivalent booster. In blood tests, there was a stronger antibody response a month out in those who had received the BA.1 booster against both BA.1, BA.4/5 and the original virus, as well as against a variety of other variants.
The Pfizer/BioNTech BA.1 bivalent booster was tested in a similar way, in about 600 people over the age of 55, with a median of about six months in between the booster doses. At one month, the antibody responses to BA.1 were better in those who had received the BA.1 booster than the original, and the antibody responses to the original virus were similar in the two groups.
Although similar studies were being done in people for the BA.4/5 bivalent booster, those results weren’t in yet. However, experiments from both companies showed that mice previously vaccinated and then boosted with the bivalent BA.4/5 vaccines had higher BA.5 neutralizing antibody responses than those boosted with the original vaccine.
Moderna also challenged, or intentionally infected, vaccinated and boosted mice with BA.5 and found animals who received the bivalent BA.4/5 booster were better protected in their lungs than those boosted with the original vaccine.
An unpublished study posted as a preprint following authorization of the new boosters also found that mice given Moderna’s BA.4/5 booster had a stronger and broader antibody response compared with those given the original booster, which appeared to translate into better protection against BA.5 in the lung.
Fox News host Tucker Carlson and some posts on social media have highlighted the lack of clinical data on the BA.4/5 boosters, noting that at the time of authorization, the specific vaccines had only been given to eight mice. That’s mostly true (Pfizer lists eight, while Moderna shows 10), but it’s misleading to suggest that’s the only information scientists had on these shots.
As we said, the revised boosters are not entirely new vaccines. Regulators considered the abundance of data indicating the original shots are safe and effective, particularly against severe disease, and they had the clinical data from the highly similar BA.1 bivalent boosters along with the animal data.
Moreover, as was pointed out during the CDC’s advisory meeting, the companies now have several years of experience with the animal models and have consistently seen that they correlate well with human immune responses.
Still, some experts were wary of moving forward with a new COVID-19 booster without human data on these specific vaccines. Dr. Pablo Sanchez, a professor of pediatrics at the Ohio State University and a member of the CDC’s vaccine advisory committee, ultimately voted against recommending the new boosters because of that concern.
“I voted no because I really feel that we need the human data,” he said during the meeting. “There’s a lot of vaccine hesitancy already. We need the human data.” Nonetheless, Sanchez said that, given his age, he was “almost sure” that he would take it.
The rest of the committee, though, felt comfortable enough, given the precedent with flu, to recommend the boosters.
Numerous other countries have opted to go with updated BA.1 bivalent vaccines, which do have clinical data (although the European Medicines Agency, which recommended BA.1-adapted boosters in early September, also recommended the BA.4/5 boosters on Sept. 12). The FDA, however, decided that the best strategy would be to target BA.4/5, since BA.1 is already no longer circulating.
The agency could have waited for the clinical data on BA.4/5, but thought that doing so would sacrifice too many lives. Estimates from the COVID-19 Scenario Modeling Hub suggest that delaying the booster campaign by a month would result in 137,000 more hospitalizations and 9,700 more deaths.
“We have to be a step ahead, or at least we have to try,” FDA chief Dr. Robert Califf explained in a press conference following the authorization of the new boosters. “Because if we waited for all the proof to come in, the wave will have already passed us by and the damage will have been done.”
E. John Wherry, an immunologist at the University of Pennsylvania, said the difference between BA.1- versus BA.4/5-targeted boosters was likely to be small, but given the choice, he would also opt for BA.5.
“This will not be the last version of the vaccine that we see,” he said. “Going with what’s here currently makes a lot of sense to me.”
As for when the human data on the bivalent BA.4/5 booster will be available, Dr. Peter Marks, the head of vaccines at the FDA, said on Aug. 31 that it would probably be at least until the end of September or October, “because of the time it takes to actually dose and then do the assays.”
On Oct. 13, Pfizer announced early clinical data from trial recipients who received the BA.4/5 bivalent booster. One week after vaccination, the company said, those recipients showed “a substantial increase” in BA.4/5 neutralizing antibodies, with a “safety profile similar to that of the original vaccine.” The company added that the antibody boost observed with its original booster was “more limited” than with its BA.4/5 bivalent booster.
How effective are the new boosters?
Early estimates of vaccine effectiveness from the CDC show the updated boosters provide additional protection against symptomatic COVID-19 and hospitalization, relative to those who received two or more doses of the original vaccines.
In theory, the omicron-updated boosters should be better than the original boosters in protecting against disease because they will more specifically target the coronavirus strains currently circulating — and at the time the shots were authorized, some data suggested that would be the case.
It’s also possible the updated boosters will prevent more infections, although that protection will be short-lived.
The real question is not whether the booster will increase protection — they will, scientists told us — but whether and to what degree the updated booster will be better than the original boosters.
At the time they were authorized, Wherry said he thought the new boosters would be better, but the difference would likely be modest.
“I think it’s important to not give the false sense of hope that this new bivalent vaccine is going to be a magic bullet that stops all omicron viruses,” he said. “We should be expecting that they keep the most vulnerable people out of the hospital, but we should not be expecting them to completely protect from, say, mild disease.”
Dr. Paul A. Offit, a vaccine expert and pediatrician at the Children’s Hospital of Philadelphia, was not convinced that the updated boosters would be an improvement.
“I think they’re all going to be of value, I don’t think they’re necessarily going to be of any greater value than just boosting with the ancestral strain,” he said in September. “What worries me in all this is … that it’s sort of being oversold, being overpromised,” Offit added. “I just fear that people might be disappointed.”
How safe are the new boosters?
At the time of authorization, the exact formulation found in the new boosters had not yet been tested in people, but the revised vaccines are very similar to the original vaccines that have now been given hundreds of millions of times in the U.S. alone and have been shown to only very rarely result in serious side effects.
In addition, both companies tested the slightly different BA.1 omicron-specific bivalent booster in people, and found no new safety concerns. Vaccine recipients reported experiencing the same expected and temporary side effects as the original shots, including pain, redness and swelling at the injection site; fatigue; headache; muscle pain; and fever.
Given the similarity in design and manufacturing process as the original vaccines, the FDA felt very confident authorizing the boosters. Other experts also told us there is no reason to think that the revamped boosters will pose any new safety hazards. Offit, for example, said it was “extremely unlikely” that the new boosters would be any different in terms of safety.
Notably, Sanchez, the sole dissenter on the CDC’s advisory committee, explained after his “no” vote that while he felt there needed to be clinical data to be able to recommend the boosters, he was not that worried about safety.
“I am comfortable that the vaccine will likely be safe like the others,” he said, adding that he would almost certainly get the new booster himself.
Like the original vaccines, scientists do expect the updated shots will carry a small increased risk of myocarditis and pericarditis, or inflammation of the heart muscle and its surrounding tissue, particularly in young males. Most people who are affected by the rare side effect and are treated, the CDC says, feel better quickly.
Another issue being monitored by scientists is whether boosting could hurt a person’s ability to respond to a future variant, as we have written. But Wherry, who has been following this topic, said there is no indication that is a current risk. “From the data that exists, I see no concern about that whatsoever,” he said. Some animal studies suggest that giving an omicron-only vaccine as a first vaccine dose in animals could be detrimental, he added, but that’s not what is being given to people.
What do experts say about who should get the updated shots, and when?
There is broad agreement that older people and those at higher risk of developing severe COVID-19 should get the new boosters. But experts differ on whether young, healthy people should get another dose.
“I don’t think that a healthy young person who has already received three doses frankly needs another dose, because I think they are protected against serious illness,” Offit said. “After about six months after their last dose, they’re not going to be as protected against mild illness, but that’s true of all infections like this one, meaning short incubation period, mucosal infections.”
Offit still recommends that people over 75 years old, those with significant underlying health problems and those who are immunocompromised seek out the shots. Those are the groups, he said, that have benefited from the previous boosters.
But others think it’s reasonable to give the shots more widely, and that younger people should at least consider them.
“Most young, healthy people are protected from severe disease even after three doses and that protection is pretty durable,” Wherry said. The updated booster, he said, is primarily meant to protect the most vulnerable and to perhaps offer a little bit better protection from mild or moderate disease.
“If you’re over 65, everyone should get boosted,” he said. “If you have comorbidities or are immunosuppressed, absolutely get boosted.”
For younger people, Wherry said boosters are still a good idea, but there can be more individual choice — and that people shouldn’t think that the booster will make them impervious to infection.
“I would encourage people to think about their own behavior and when the right timing for the boost would be,” he said. “If you last got a dose of vaccine or were last infected three months ago and you’re a middle-aged, otherwise healthy individual, you may consider waiting a month or two and time your next dose closer to the holidays or closer to when there’s going to be more indoor activities so that peak level of antibodies coincides with when you’re going to be attending family gatherings or be inside more.”
Although two months is the minimum amount of time to wait since the last COVID-19 dose before getting the updated booster, many experts, including Offit and Wherry, suggest that people wait longer than that since last being vaccinated or being infected with SARS-CoV-2.
“The science really says for a young, healthy individual, you’re going to get the best boosting if you wait four to six months,” Wherry explained. “That allows for your memory B cells and memory T cells to mature a little bit, antibody levels to come down from their sort of max peak level after infection or vaccination.”
But for people who are older or have health problems, Wherry suggested consulting a doctor, because those individuals might need to get their doses sooner.
Is this the start of a shift to an annual COVID-19 vaccine?
Maybe. The Biden administration has presented this fall’s booster as the first of a once-a-year shot for COVID-19, similar to an annual seasonal flu vaccine. But while that could be a reasonable approach, it’s too early to truly know.
Some federal health officials have said as much, noting that new variants might disrupt those plans and that older or high-risk people might need more than one vaccine a year. Some experts, including Offit, are critical of the administration for getting ahead of itself on this question. Offit told us it was reasonable to target high-risk groups, but that young, healthy people may not need an annual boost.
Others are more comfortable with the concept. Wherry, for instance, said that given data that protection against severe disease begins to wane a little around nine months, the one-year time frame makes sense as “a benchmark for the average person” — and also is practical.
“A bit of aligning to traditional health care is an important part of this because it’s actually going to help with vaccine compliance, keeping up with what’s new,” he said, adding that there would remain flexibility for higher-risk people, who likely already interact more with the health care system, to get additional doses if needed.
Where can I get an updated booster?
You can find where the new boosters are available in your area by visiting Vaccines.gov. As with other COVID-19 vaccines, the reformulated boosters will be available at pharmacies, community health centers, and some clinics and doctor’s offices. But a lack of funding means they are less likely than the earlier shots to be available in various public health outreach efforts.
In some places, the boosters may be difficult to find at first. Certain locales have reported shortages of the Moderna vaccine in particular in the first weeks of the rollout, in part due to the delayed release of 10 million doses from a packaging plant in Indiana.
Are the shots still free?
Yes. The U.S. government has purchased more than 170 million doses of the updated boosters. All COVID-19 vaccinations will remain available to the public free of charge, regardless of immigration or insurance status, until those vaccines run out.
Eventually, the COVID-19 vaccines will transition to the commercial market. When that occurs, the vaccines will likely be covered for most people with health insurance, similar to flu and other vaccines. People without insurance, however, might need to pay out of pocket (some governmental programs may be able to offer free or discounted vaccines).
Can I get my updated booster along with my flu shot?
Yes. Health officials are suggesting this pairing be offered to you this fall, since they know that getting both in one go is more convenient and increases the likelihood that you’ll get both vaccinations.
After one nearly nonexistent and one mild flu season — likely thanks to COVID-19 mitigation measures — some experts are concerned this flu season might be worse than normal. The relative lack of flu for two years running likely means there is less immunity in the population and more people will be susceptible. Clues from the Southern Hemisphere, which often presage flu severity in the North, have been mixed. Australia has had a bad flu season in terms of the number of cases, raising concerns — but other countries have not had particularly active seasons.
Regardless of how severe the flu season turns out to be, vaccination is still recommended. Several studies, along with surveillance data, indicate that getting a COVID-19 shot at the same time as a flu shot is safe and doesn’t reduce your immune response to either virus. The temporary, expected side effects of vaccination are usually on par with or only slightly worse in people getting both shots compared with those just getting a COVID-19 dose.
For some individuals, getting the shots together may make sense, but for others, the timing might not be ideal. The CDC recommends getting the flu shot in September or October, but many experts recommend October or later because vaccine protection against flu wanes and may not last the entire season if given too early. Still, getting the flu vaccine a bit early is better than not getting it at all.
If you opt for dual vaccination, you should get the two shots in different arms or in the same arm with the injections at least an inch apart.
This year, for the first time, the CDC is preferentially recommending that people over 65 years of age get a high dose or adjuvanted flu vaccine instead of a standard flu vaccine, given evidence that those shots may work better for this group.
Editor’s note: SciCheck’s COVID-19/Vaccination Project is made possible by a grant from the Robert Wood Johnson Foundation. The foundation has no control over FactCheck.org’s editorial decisions, and the views expressed in our articles do not necessarily reflect the views of the foundation. The goal of the project is to increase exposure to accurate information about COVID-19 and vaccines, while decreasing the impact of misinformation.