Figure 3. Effectiveness of Maternal Two-Dose mRNA Vaccination against Hospitalization for Covid-19 among Infants, Stratified According to Vaccination Timing, Variant, and Type of Admission.

The delta-predominant period was defined as July 1, 2021, to December 18, 2021. The omicron-predominant period was defined as December 19, 2021, to March 8, 2022. The timing of maternal vaccination was based on the date of receipt of the second dose of a two-dose series of an mRNA Covid-19 vaccine during pregnancy. Gestational age was missing for 3 infants who had been born to vaccinated mothers with known timing of the second dose; for these infants, classification of vaccination timing was based on a gestational age of 40 weeks. For 16 infants who had been born to mothers who had been fully vaccinated during pregnancy, dates of vaccination were not available to determine the vaccination timing during pregnancy. Vaccine effectiveness was calculated as (1 − adjusted odds ratio) x 100, where the odds ratio is the odds of maternal vaccination among mothers of case infants as compared with control infants. The widths of the confidence intervals have not been adjusted for multiplicity and should not be used in place of a hypothesis test.