What Are the Possible Side Effects of the Omicron-Specific COVID-19 Booster Shots?

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With the arrival of the long-awaited COVID-19 booster shots geared to target BA.4 and BA.5 omicron subvariants, many may be curious about its possible side effects.

Well, the new boosters actually may not be as different from what previous renditions of the vaccines entailed in terms of side effects.

"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.

Appointments to receive the updated shots have been ramping up in Chicago-area pharmacies, with Illinois health officials urging community members to get the new dose.

“These new bivalent vaccines are designed to offer extra protection against the omicron variants, which are now the dominant strain of the virus," Illinois Department of Public Health Director Dr. Sameer Vohra said in a statement. "Getting up to date now is especially important for those who are at risk of serious outcomes, as the updated vaccines offer protection from hospitalization and even death.”

The vaccine is now the primary shot that will be administered to those who are eligible and looking to get boosted throughout the country.

"With this new authorization, the monovalent — meaning the old version — are not authorized anymore as booster doses," Dr. Allison Arwady, Chicago's Department of Public Health commissioner, said earlier. "We really want folks to get these updated. So, anybody over the age of 12 who's looking for a booster, this is the one that they're going to get. Individuals 5 to 11 can still get the original boosters."

The Expected Side Effects

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.

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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