Half of People Who Get a Pfizer Booster Have These 2 Side Effects, CDC Says
Fifty percent experience one reaction and nearly 67 percent notice another, says a new CDC study.
It's only been a week since the Centers for Disease Control and Prevention (CDC) and Food and Drug Administration (FDA) officially endorsed a booster shot of the Pfizer COVID-19 vaccine for certain populations of the U.S. But with boosters being tested prior to that green light and since third doses were already approved for the immunocompromised, there is already some data on which side effects are most common with Pfizer boosters.
In the Oct. 1 issue of the CDC's Morbidity and Mortality Weekly Report, the agency looked at the responses of 212,191 voluntary registrants who used the agency's v-safe smartphone surveillance system to report their reactions to getting a booster shot of any vaccine between Aug. 12 and Sept. 19. Of those, 11,209 respondents got three doses of the Pfizer vaccine. The data showed that third doses of the shot yielded similar side effects to second doses, mainly "mild to moderate injection site and systemic reactions," according to the CDC.
The CDC says among v-safe respondents who received three doses of Pfizer, local reactions, meaning a side effect that occurs at the injection site, were reported slightly more frequently after the third dose than the second (74.1 percent versus 71.7 percent). Systemic reactions, meaning those that occur elsewhere in the body, were reported slightly less frequently after dose No. 3 than dose No. 2, however (69.2 percent versus 71.7 percent). Overall, however, a couple of side effects were much more common among Pfizer recipients than others, the data showed.
According to the CDC's study, only two reactions were reported in at least half of the people who got three doses of Pfizer: 51 percent said they felt fatigue and 66.6 percent experienced pain at injection site, which was the most common reaction among those who received the additional shot from Pfizer.
The CDC's findings are similar to those reported by the FDA in mid-September. That agency's data came from the third phase of Pfizer's clinical trials, tracking 309 participants between the ages of 18 to 55 who got a third Pfizer dose five to eight months after completing the initial two-dose regimen. Pain at the injection site and fatigue were also the most common reactions to the Pfizer booster in the FDA report but at slightly higher rates: 83 percent of the participants noted the former and 63.7 percent reported the latter.
In both the FDA and CDC reports, behind those two most frequently reported reactions were headaches and myalgia, AKA muscle aches.
In its report, the FDA also said reactions to the third and second Pfizer doses were comparable, with "one exemption": swollen lymph nodes, also known as lymphadenopathy. However, it's still relatively rare: More than 5 percent of adults who got Pfizer experienced the side effect after their third dose, compared to less than 1 percent who reported it after their initial two doses.
While the CDC's report did not measure lymphadenopathy, the FDA was not surprised to see the increase. "It is not surprising that stimulation of a lymph node reaction by vaccination would be present in the setting of a significant increase in neutralizing antibodies observed after Dose 3," the FDA stated. "While related to vaccination, this [adverse drug reaction] is generally mild and self-limited and is unlikely to impede a booster vaccination program."
Overall, the CDC says, their data "found no unexpected patterns of adverse reactions after an additional dose of COVID-19 vaccine," though they will continue to monitor booster recipients' reactions.