Huge study finds most COVID-19 vaccine side effects were mild for Pfizer-BioNTech and Moderna

A new study involving millions of participants has found most side effects from mRNA COVID-19 vaccines were mild and faded substantially after one day.

The findings, published Monday in The Lancet Infectious Diseases, should reassure Pfizer-BioNTech and Moderna mRNA vaccine recipients that the shots, which were granted U.S. Food and Drug Administration emergency authorization in late 2020, are safe, experts said.

"These data are reassuring that reactions to both mRNA vaccines are generally mild and subside after one or two days – confirming reports from clinical trials and post-authorization monitoring," said the Center for Disease Control and Prevention's Tom Shimabukuro, one of the authors of the large-scale study.

The study looked at data from over 7 million self-reported instances of negative side effects from mRNA vaccine doses administered between December 2020 and June 2021. Researchers found that pain at the site of injection, fatigue and headache were the most commonly reported negative side effects.

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During the study period, more than 298 million doses of mRNA vaccines were administered nationwide: 132 million Moderna and 167 million Pfizer, according to the study. Researchers said their data suggests 92% of negative side effects were not serious, and less than 1% of people who self-reported side effects reported seeking any medical care following vaccination.

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The data examined in the study came from the Vaccine Adverse Event Reporting System, which was established in 1990 and is run by the CDC and the FDA. Data also came from the v-safe system, a similar reporting system managed by the CDC that was developed specifically for the COVID-19 vaccination rollout.

“COVID-19 vaccine safety monitoring is the most robust in U.S. history, and the two complementary surveillance systems used in this study should bolster confidence that mRNA COVID-19 vaccines are safe," said the CDC's Hannah Rosenblum, who helped author the study.

Out of all the negative side effects reported via VAERS, 20% were headache, 17% were fatigue, 16% were fever, 16% were chills and 15% were pain, according to the study.

The study says about 4,500 deaths were noted, mostly among people 60 and older. Those deaths were reported regardless of any potential association with vaccination, and study authors say that no unusual patterns were detected.

The CDC's David Shay, who also helped author the study, explained that data reflects deaths researchers would expect to find when studying an older population.

Broadly, the study was not able to suggest cause and effect between vaccination and the adverse events being studied. Authors say that is a constraint of the self-reported surveillance system used to collect data.

A release quotes Elizabeth Phillips – of Vanderbilt University Medical Center, who was not involved in the study – who commented that the study showed "no unusual patterns emerged in the cause of death or serious adverse effects."

Data from the v-safe monitoring program showed more people were likely to experience negative side effects after the second vaccine dose compared with to the first.

This article originally appeared on USA TODAY: COVID vaccine side effects: Most were mild for Pfizer, Modena: Study