Background: Safety monitoring of primary series mRNA COVID-19 vaccination in individuals 5 years and older identified an increased risk for myocarditis/pericarditis in younger males particularly following dose two. Limited data are available regarding safety in under 5 years.
Objectives: To assess the safety of mRNA COVID-19 primary vaccination in young children in the U.S. Vaccine Safety Datalink (VSD).
Methods: The VSD is a collaboration between the Centers for Disease Control and Prevention and 9 geographically diverse integrated healthcare systems with comprehensive medical record data on approximately 550,000 children under 5 years.
Using data and analyses updated weekly, we assessed 23 safety outcomes with sequential monitoring (n=19, including myocarditis/pericarditis and seizures) and descriptive analyses (n=4, including anaphylaxis). For sequential monitoring, we compared outcomes among vaccinees with those among vaccinated comparators who were concurrently, on the same calendar day, in the comparison interval. We estimated adjusted rate ratios (RRs) and corresponding 95% confidence intervals (CIs) using Poisson regression, comparing outcome incidence during postvaccination risk interval (1-21, except 0-7 and 0-21 days for seizures) with incidence during comparison interval (22-42 days postvaccination) adjusted for age, race, sex, site, and calendar day. Assuming one year of monitoring with uneven vaccine uptake, we pre-specified a 1-sided p-value < 0.011 as our signaling threshold. We reviewed medical records of all cases of myocarditis/pericarditis and anaphylaxis, and other selected outcomes as warranted.
Results: From June 18, 2022–January 28, 2023, 126,775 doses of Pfizer-BioNTech COVID-19 vaccine were given to children 6 months–4 years, and 106,159 doses of Moderna COVID-19 vaccine were given to children 6 months–5 years in the VSD population. There were no events in the risk interval for many outcomes, including myocarditis/pericarditis. RRs were not elevated for any pre-specified outcomes following any dose of Pfizer-BioNTech or Moderna vaccine and none of the outcomes met the signaling threshold of p-value < 0.011. For example, the RR for convulsions/seizures in the 0-7 days postvaccination was 0.58 (95% CI: 0.21–1.44, p-value: 0.92) after Pfizer-BioNTech and 0.92 (95% CI: 0.29–2.57 p-value: 0.65) after Moderna. In descriptive analyses, the one case of confirmed anaphylaxis was unrelated to vaccination (i.e., food allergy).
Conclusions: Interim analyses of >230,000 mRNA COVID-19 vaccine doses in young children did not identify any safety findings. These results provide early reassurance to clinicians, parents, and policymakers. Surveillance is ongoing.
