Vaccine effectiveness for preventing COVID-19 hospital admission during pregnancy: a population-based cohort study in England during the Alpha and Delta waves of the SARS-CoV-2 pandemic

Objective To estimate vaccine effectiveness (VE) for preventing COVID-19 hospital admission in women first infected with SARS-CoV-2 during pregnancy, and assess how this compares to VE among women of reproductive age who were not pregnant when first infected. Design Population-based cohort study using national, linked Census and administrative data. Setting England, United Kingdom, from 8 th December 2020 to 31 st August 2021. Participants 815,4777 women aged 18 to 45 years (mean age, 30.4 years) who had documented evidence of a first SARS-CoV-2 infection in NHS Test and Trace data or Hospital Episode Statistics. Main outcome measures A hospital inpatient episode where COVID-19 was recorded as the primary diagnosis. Cox proportional hazards models, adjusted for calendar time of infection and sociodemographic factors related to vaccine uptake and risk of severe COVID-19, were used to estimate VE as the complement of the hazard ratio for COVID-19 hospital admission. Results Compared with unvaccinated pregnant women, the adjusted rate of COVID-19 hospital admission was 76% (95% confidence interval 69% to 82%) lower for single-vaccinated pregnant women and 83% (75% to 88%) lower for double-vaccinated pregnant women. These estimates were similar to those found for non-pregnant women: 79% (76% to 81%) for single-vaccinated and 82% (80% to 83%) for double-vaccinated. Among those vaccinated more than 90 days before infection, being double-vaccinated was associated with a greater reduction in risk than being single-vaccinated. Conclusions COVID-19 vaccination is associated with reduced rates of severe illness in pregnant women infected with SARS-CoV-2, and the reduction in risk is similar to that for non-pregnant women. Waning of vaccine effectiveness occurs more quickly after one dose of a vaccine than two doses. What is already known on this topic Being pregnant is a risk factor for severe illness and mortality following infection with SARS-CoV-2. Existing evidence suggests that COVID-19 vaccines are effective for preventing severe outcomes in pregnant women. However, research directly comparing vaccine effectiveness between pregnant and non-pregnant women of reproductive age at the population level are lacking. What this study adds Our study provides real-world evidence that COVID-19 vaccination reduces the risk of hospital admission by a similar amount for both women infected with SARS-CoV-2 during pregnancy and women who were not pregnant when infected, during the Alpha and Delta dominant periods in England.