A recent study by the Centers for Disease Control and Prevention’s COVID-19 Response Team should help lay to rest any lingering concerns about the safety of the mRNA vaccines.

The Pfizer-BioNTech and Moderna mRNA vaccines for COVID-19 had strong safety records in their respective clinical trials, but in order to get a clearer picture of how safe both vaccines were once they had been approved for use in the pandemic, researchers from the Centers for Disease Control and Prevention reviewed the reports of negative reactions to the vaccines that came in during the first six-months of their use, starting in December 2020, when they became available.

Most of the adverse events reported were mild: headache, fever, chills and pain at the injection site. They were more common in women and those under 65. “The benefits of immunization in preventing serious illness and death strongly favor vaccination,” Hannah Rosenblum, lead author of the study, said.

Adverse reactions were reported most often the day after vaccination, and most of them were mild, such as fatigue, headache and injection site pain.

The Vaccine Adverse Event Reporting System (VAERS) has been run since 1990 by the Food and Drug Administration and the CDC. The v-safe program was developed specifically for COVID-19 vaccines by the CDC. The program sends daily surveys to participants via smartphone to monitor adverse events for the first month after vaccination.

“The two complementary surveillance systems used in this study should boost confidence that mRNA COVID-19 vaccines are safe,” Rosenblum added.

During the study period, almost three million mRNA vaccine doses were administered in the U.S. VAERS received more than 340,000 adverse event reports. Of these events, 92 percent were not serious in nature. The most common serious event was shortness of breath (15 percent).

Of the nearly eight million events reported to v-safe, almost five million were local reactions to the injection and almost four million were systemic reactions, such as fatigue or nausea. These reactions occurred more frequently after the second dose than after the first dose. Events were typically reported the day after vaccination, and most were mild, such as fatigue, headache and injection site pain. V-safe reports of inability to work or perform normal activities or seeking medical attention were more common after the second dose (32 percent) than the first dose (12 percent).

“[It is] 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 Tom Shimabukuro, a coauthor on the study and member of the CDC COVID-19 Response Team.

The researchers acknowledge some limitations to the study. The VAERS system, which relies on spontaneous reporting of adverse events, is not representative of the entire population. And the results do not prove adverse events are caused by mRNA vaccines. They can only show an association between adverse events and vaccines.

Because a smartphone is needed to participate in v-safe, those without a mobile device were excluded from the study. And although differences in adverse events between the Pfizer-BioNTech and Moderna vaccines have emerged, neither the VAERS reporting system nor the v-safe system can definitively measure the safety differences between the two mRNA vaccines.

The VAERS and v-safe reporting systems will continue to provide policymakers, healthcare providers and the public with updated information about the safety of COVID-19 vaccines.

The study and a related editorial are published in Lancet Infectious Diseases.