An ongoing study of more than 6 million Americans found no serious side effects linked to mRNA COVID-19 vaccines.
"These results from our safety surveillance are reassuring," said Dr. Nicola Klein, director of the Kaiser Permanente Vaccine Study Center in Oakland, Calif. She spoke in a Kaiser Permanente news release.
Klein is also a leader of Vaccine Safety Datalink (VSD), which studies patient records for 12 million Americans. The project is supported by the U.S. Centers for Disease Control and Prevention.
The Journal of the American Medical Association published Datalink's first comprehensive findings on Sept. 2.
The study found no serious health effects that could be tied to the Pfizer-BioNTech or Moderna COVID-29 vaccines. Monitoring will continue for two years.
The findings span mid-December 2020 through June 26, 2021. Researchers compared specific side effects in the first three weeks after mRNA shots with those three to six weeks later. In all, 6.2 million people were studied after the first dose and 5.7 million after the second dose.
The researchers looked for 23 potential health problems that have followed other vaccinations or had been reported by patients.
These included neurological disorders such as encephalitis and myelitis, seizures and Guillain-Barré syndrome, as well as cardiovascular problems such as acute heart attack, stroke and pulmonary embolism. Others examined included Bell's palsy, appendicitis, anaphylaxis and multisystem inflammatory syndrome.
None rose to a level that was statistically significant after mRNA COVID shots, the analysis found.
Researchers highlighted their findings about cases of confirmed myocarditis and pericarditis among young people, which has become an outcome of concern. Myocarditis is an inflammation of the heart, and pericarditis is inflammation of the sac surrounding it.
The study found 34 such cases in patients between 12 and 39 years of age -- 85% were male and 82% were hospitalized for a median time of one day, meaning half were hospitalized longer, half for a shorter time. Nearly all recovered by the time the review took place, the researchers said.
They calculated that in the 12- to 39-year-old age group, there is a risk of 6.3 additional cases of myocarditis per million in the week after vaccination. That's significantly less than the risk linked to COVID-19 itself, the study authors noted.
"The results of this study are a great example of how seriously CDC takes vaccine safety, and how thorough and transparent we are in our safety monitoring efforts," said Dr. Tom Shimabukuro, deputy director of CDC's Immunization Safety Office.
"It is our top priority to do the science and communicate quickly and clearly with health care providers and the public, as COVID-19 vaccines continue to undergo the most intensive safety monitoring in U.S. history," he added. "Getting vaccinated remains the best way to protect yourself and your loved ones against a virus that has taken millions of lives."
Some early findings had been summarized and reported at public meetings of the CDC's Advisory Committee on Immunization Practices, though the JAMA article is the VSD's first major report of its safety surveillance of the Pfizer and Moderna vaccines.
More information
To learn more about COVID-19 vaccines, visit the U.S. Centers for Disease Control and Prevention.
SOURCE: Kaiser Permanente, news release, Sept. 3, 2021
