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.