- Booster studies are ongoing but limited data suggests similar side effects after third dose as after the initial doses.
- Side effects after the third dose are also likely to vary from person to person, with some people experiencing minimal discomfort.
- The most common side effects for a third dose of the Pfizer/BioNTech vaccine were pain at the injection site, fatigue, headache, muscle and joint pain, and chills.
People in the United States could begin receiving third doses of mRNA COVID-19 vaccines as early as the week of Sept. 20, President Joe Biden’s administration announced in mid-August.
This is pending approval by the Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC).
But some people may already be wondering about the side effects of a third dose — especially those who were hard hit with fatigue or muscle pain after their second dose.
Several clinical studies on the safety and efficacy of vaccine boosters are ongoing, but the limited available data suggests that the side effects will be similar to those of the first two doses.
Booster side effects similar to initial doses
Dr. Michael Cackovic, an obstetrician from The Ohio State University Wexner Medical Center, says there’s a lot of variation in side effects after the COVID-19 vaccines.
One study in the United Kingdom found that only 22 percent of people had systemic side effects such as fatigue, headache, or chills after the second dose of the Pfizer-BioNTech vaccine.
Sixty-eight percent had only local reactions such as pain or tenderness at the site of injection.
Side effects after the third dose are also likely to vary from person to person, with some people experiencing minimal discomfort.
“The few patients I have talked to that received the booster report fewer side effects [than after the first two doses],” said Cackovic.
Last week, Pfizer and BioNTech released results from a phase 3 clinical trial in which people were given a booster between 4 and 8 months after their second dose.
As with studies on the initial vaccine series, the most common side effects were pain at the injection site, fatigue, headache, muscle and joint pain, and chills.
The frequency of these side effects was “similar to or better than after dose two,” the company said in a press release.
The results have not yet been published in a peer-reviewed journal.
Booster dose clinical trials ongoing
A Pfizer spokesperson told the CDC’s vaccine advisory committee on August 30 that it may have additional booster study data by late September or early October, reported STAT’s Helen Branswell on Twitter.
The company also announced last week that it has started submitting data to the FDA for its application seeking approval of a booster dose of its vaccine for Americans 16 years and older.
This is the age range for which the two-dose regimen of the vaccine is fully approved.
The FDA has not indicated when it will finish reviewing Pfizer’s booster application.
In August, the agency approved third doses of the mRNA vaccines for people with weakened immune systems. These are not considered booster doses.
People who are immunocompromised may need an additional dose to help them generate a robust immune response more in line with that of people with healthy immune systems.
In another recent booster study, a group of Israeli researchers looked at the effectiveness of a third dose of the Pfizer-BioNTech vaccine in people 60 years or older.
Israel began rolling out boosters to this age group at the end of July, with people eligible if they had been fully vaccinated with their second dose at least 5 months earlier.
Researchers found that the third dose reduced the risk of severe COVID-19. However, they did not release any data on the side effects after the booster.
This study has not yet been peer-reviewed.
For the Moderna-NIAID and Johnson & Johnson vaccines, no side effects data from the booster studies have been released.
The Biden administration has indicated that it expects people who received the J&J vaccine will be advised to get a second dose, but it is waiting for data from the clinical trials before setting a date for rollout.
Data needed on rare but serious side effects
Most of the side effects from two doses of the mRNA vaccines or one dose of the J&J vaccine are mild to moderate and short-lived.
But there have also been more concerning, but rare, side effects.
The Pfizer-BioNTech and Moderna-NIAID vaccines both carry a small risk of heart inflammation — myocarditis and pericarditis. This is more common in younger people, especially males.
The majority of cases reported to the CDC have recovered with treatment, according to data presented at the agency’s vaccine advisory committee on August 30.
However, in terms of heart inflammation, “the risk of the coronavirus itself affecting you is much, much higher than the risk of the vaccine affecting you,” said Dr. Ira Taub, a pediatric cardiologist at Akron Children’s Hospital.
Two recent studies confirmed a higher risk of myocarditis after acquiring the coronavirus — a preprint study by Taub and his colleagues and a large Israeli study in the New England Journal of Medicine.
While Taub says we still need to see safety data from booster studies of the mRNA vaccines, he expects this side effect will remain rare even after the booster dose.
“I would be quite surprised if [myocarditis] became anything more than a rare event, even with the third vaccination,” he said.
In addition to mild and moderate side effects, the J&J vaccine carries a small risk of blood clots with low levels of platelets. This side effect is more common in females ages 18 to 49 years.
No data was available on the chances of this condition happening after a second dose of the J&J vaccine.
Before the FDA and CDC approve boosters for people in the United States, the agencies will review data on the safety and efficacy of these additional doses.
They will also continue to monitor the vaccines after booster doses are rolled out.
Although booster doses are not widely available to people in the United States, doctors can prescribe a third dose of the Pfizer-BioNTech vaccine “off-label” because the vaccine is fully approved.
Dr. David Juurlink, a drug safety researcher at ICES in Toronto, cautions against doing this before the safety data is available.
“Before giving (already-protected, relatively young) people third doses, we need solid evidence that the expected benefits of doing so exceed the potential harms,” he wrote on Twitter.
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