Pfizer's COVID-19 booster shot yields mild to moderate side effects, much like its second dose.
Injection-site pain is most common after a booster, according to data released Wednesday.
People who received a third dose of Pfizer's vaccine also frequently reported headaches and fatigue.
A third dose of Pfizer's coronavirus vaccine yields mild to moderate side effects, much like the second shot, according to data released Wednesday by the Food and Drug Administration.
The data is included in a document from Pfizer that outline the case for a third dose, or "booster," of its coronavirus vaccine. In the document, Pfizer says that protection conferred by the two-dose vaccine declines over time, meaning that people should get a booster shot about six month after their second dose.
In results from 289 participants ages 18 to 55, Pfizer found that a third dose of its approved vaccine was safe. The most frequent side effects among the group were injection-site pain, fatigue, headache, muscle and joint pain, and chills.
Here's how common each of these side effects was among the trial participants:
Injection site pain: 83%
Muscle and joint pain: 39.1%
Of the 306 trial participants monitored for adverse events, 24 reported adverse events that were likely related to the vaccine. Only one had an adverse event considered to be severe. That participant experienced lymphadenopathy, or swelling of the lymph nodes, which came on two days after receiving the third dose and resolved after the fifth day. Overall, 16 participants reported some level of swelling in their lymph nodes.
Booster shots increased antibody levels for all age groups in the study, giving them more protection against the virus. Participants saw 3.3 times higher antibody levels one month after their booster than one month after their second dose.
Data from Israel suggests side effects resolve after 1-3 days
Other reports have also indicated that booster side effects are both mild and fleeting. A September report from Maccabi Health Services, one of Israel's four major health providers, found that booster side effects usually go away within one to three days.
Maccabi surveyed more than 9,200 adults in Israel who'd recently received a third dose of Pfizer's vaccine. In general, the following portions of adults reported these side effects:
Injection site pain: 87%
Weakness and fatigue: 57%
Muscle pain: 26%
Swollen lymph nodes: 19%
Joint pain: 14%
Fever over 38 degrees Celsius (100 degrees Fahrenheit): 9%
Fever up to 38 degrees Celsius: 8%
Half of the surveyed adults said the booster shot yielded worse side effects than the second dose. But the other half said the booster side effects were the same or milder compared to dose two.
In another survey from Clalit Health Services, Israel's largest healthcare provider, 88% of participants said they felt "similar or better" after their Pfizer booster than after dose two. That survey included around 4,500 people. Just 31% of them reported localized side effects like pain or swelling at the injection site.
How the first 2 doses compare to a booster shot
The mRNA vaccines from Pfizer and Moderna instruct the body to produce a harmless viral protein, then develop antibodies against it. The first dose of Pfizer's vaccine generally produces the mildest side effects, since our bodies are being introduced to the instructions and the resulting protein for the first time.
By the time we receive a second dose, our bodies have learned to recognize that protein quickly, so are ready to attack it. That's why side effects are generally more pronounced. (If you've had COVID-19 before, though, your first dose may yield more intense side effects than your second.)
After their first Pfizer shot, 47% of adults ages 18 to 55 reported fatigue. That number rose to 59% after the second dose. Reports of a headache also rose from 42% to 52% from the first to second dose, while reports of muscle pain rose from 21% to 37%.
Scientists don't expect boosters to yield worse side effects than the second shot, since our bodies are responding in the same way - by fighting off a familiar threat. Boosters shouldn't carry a higher risk of severe allergic reactions, either.
"If you've tolerated two doses of a vaccine, you're far less likely to have an anaphylactic reaction to a third dose," Kawsar Talaat, an infectious-disease physician at Johns Hopkins Medicine, recently told STAT.
Boosters haven't been authorized for all US adults yet
So far, the US has only authorized third doses for immunocompromised people, including cancer and HIV patients, as well as those who've had organ transplants. Disease experts have suggested that most elderly people may also require booster shots, since their immunity from vaccines tends to wane more quickly than the average.
But experts are still torn as to whether boosters are necessary for everyone.
In a review published Monday in The Lancet, 18 scientists said there was no need for the widespread use of boosters yet. They noted that vaccines still offer strong protection against severe disease and death. The scientists also worried that boosters might unnecessarily lead to higher instances of rare side effects like myocarditis, a type of heart inflammation. Pfizer didn't record any cases of myocarditis in its booster-shot trial.
But the US is nonetheless expected to start rolling out booster shots for the general public starting on September 20 in order to shore up immunity against the Delta variant. First, though, the FDA must authorize third doses for the general public. An advisory committee meets Friday to decide whether the FDA should green-light Pfizer's booster shot.
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