If you're thinking of getting an omicron-specific Covid booster shot, you might be wondering what its side effects are — and how severe they might be.
Rest assured: They're not expected to be much different from what you may have experienced with previous vaccine and booster doses.
"We just don't have any data on this [yet], essentially giving two vaccines in one shot — but biologically, I just wouldn't expect the side effects, severity or the safety profile of the shots to be different from the current mRNA vaccines and boosters," Dr. Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia and member of an independent advisory group to the U.S. Food and Drug Administration, tells CNBC Make It.
The reformulated shots from Pfizer and Moderna are bivalent, which means they target both the original Covid strain and omicron's BA.5 and BA.4 subvariants. Side effect data isn't available yet because the new boosters were approved by the FDA and Centers for Disease Control and Prevention before fully completing their clinical trials.
The federal agencies based their approvals off several other pieces of safety data, including evidence from the original Covid vaccines — the updated formulations are merely a tweak to those originals — and lab data on the shots' BA.5 element in mice.
They also examined clinical trial data on earlier versions of bivalent boosters targeting omicron's BA.1 subvariant. Those shots were never released to the public, because BA.1 was quickly surpassed by other omicron subvariants — but their design is extremely similar to the shots now available at pharmacies and clinics nationwide.
Together, the evidence shows a potential roadmap for the side effects you can expect after getting one of the new boosters, and how severe those side effects might be.
The expected side effects
In Pfizer and Moderna's clinical trials for the BA.1 shots, participants who were already fully vaccinated with a booster shot received an updated booster dose. In both clinical trials, the most commonly reported side effects within seven days of receiving the shot were:
- Pain
- Fatigue
- Headache
- Muscle pain
- Chills
- Joint pain
- Redness and swelling at the injection site
- Fever
That's a familiar list: It's the same group of side effects that came with the original formulations. But notably, in those clinical trials, the severity of the side effects was very mild.
Pfizer's trial found that about 52% of participants that received the BA.1 shot experienced mild pain at the injection site, 8% experienced moderate pain and only 0.3% experienced severe pain. Roughly 26% of participants experienced a mild or moderate headache, while only 0.3% experienced a severe one.
Moderna's trial found that nearly 59% of participants experienced fatigue, but only about 4% experienced that at a Grade 3 level, which is defined as significant fatigue that prevents daily activity.
Severe side effects are "generally" most common after receiving a second dose of a vaccine, not after receiving a third or fourth dose, says Offit. You're only eligible for the new boosters if you've completed a primary vaccination series, meaning most people will have already received at least two doses ahead of time.
The same concept held true during the last round of booster shots. The new shots have the same dosage amounts as the original vaccines, which further suggests that their safety profiles could be similar, Offit says.
A single dose of Pfizer's monovalent vaccine contains 30 micrograms of mRNA targeting the original Covid strain. The updated booster shots contain the same number of micrograms, with 15 targeting the original strain and the other 15 targeting BA.4 and BA.5.
Moderna's monovalent shot contains 50 micrograms of mRNA per dose targeting the original strain. Its updated booster has 25 micrograms targeting the original strain, and 25 targeting the omicron subvariants.
The BA.1 trials only tested a few hundred people, which is a relatively small sample size compared to the thousands of Americans set to receive the new BA.5 doses, Offit notes. You can still be confident going in, he says — just don't be 100% sure what to expect.
"We should keep our eyes wide open to what side effects and adverse events might occur, and still keep in mind that this is a new product," Offit says.
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