Fig. 2: Effectiveness of maternal immunization status against infant COVID-19-related hospitalization during the first 180 days of life, stratified according to COVID-19 variants.

Cox proportional hazards models were used to estimate the hazard ratios (HR) and 95% confidence intervals (CI) for COVID-19 hospitalizations in the various groups of maternal immunity status, compared to the maternal naïve group. Models were adjusted for maternal age, gestational age at delivery, parity, and neonatal sex; multifetal gestation, birthweight, and number of maternal documented SARS-CoV-2 tests during pregnancy. Maternal Immunization effectiveness was calculated as (1−adjusted hazard ratio) × 100. Similar analysis was performed when stratified by COVID-19 variants. The delta-predominant period was defined as 24 August 2021 to 1 December 2021. The omicron-predominant period was defined as 15 December 2021 to 15 March 2022. Case infants, n = 661; Control infants, n = 59,460. Boxes and error bars represent the median and 95% CI. Dotted line shows no effect point. Subgroups are indicated by color: Hybrid, red; Natural immunity, green; Full vaccination (3–4 doses), orange; Partial vaccination: 1–2 doses, navy blue.