Highlight
• Introduction of COVID-19 nonpharmaceutical interventions (NPIs) led to significant declines in respiratory pathogen circulation in young children.
• Children aged 3 to 4 years exhibited reduced serum antibodies to Streptococcus pyogenes and respiratory syncytial virus (RSV) post-NPIs, indicating delayed immunity acquisition.
• The antibody deficit was not observed in children under 3 years or for influenza viruses and SARS-CoV-2.
• This immunologic gap may provide a biological basis for the 2022-2023 rise in severe S pyogenes infections in European children.
