In an international research collaboration including several centers, we showed that even in the youngest children, boys and girls can respond differently to vaccines. In a study of 7801 children under 5 in Germany, we found that after receiving the COVID-19 vaccine BNT162b2 (Pfizer/BioNTech), girls were more likely than boys to experience local reactions at the injection site and general symptoms like fever or tiredness. By contrast, after standard childhood vaccines, muscle-related symptoms were more common in boys. These findings highlight the importance of considering sex differences when evaluating vaccine safety and effectiveness.

Publication: Sex differences in symptoms following the administration of BNT162b2 mRNA COVID-19 vaccine in children below 5 years of age in Germany (CoVacU5): a retrospective cohort study

In a related analysis of 4615 children during the Omicron wave, we also found that two doses of BNT162b2 provided substantial protection against infection and symptoms, but without any sex differences. Together, these projects give early, independent evidence on how the COVID-19 vaccine works in children under 5, a group for whom data have been scarce. 

These studies were made possible by an international collaboration of researchers and caregivers, providing a unique dataset during a critical phase of the COVID-19 pandemic. By working across borders and pooling expertise, we were able to generate insights that are directly relevant to parents, clinicians, and policymakers worldwide.

Publication: Effectiveness of the BNT162b2 mRNA COVID-19 vaccine in children under 5 years