Vaccine effectiveness against mild and severe disease in pregnant mothers and their infants in England
Authors: Kirsebom Freja C M , Andrews Nick , Mensah Anna A , Stowe Julia , Ladhani Shamez N , Ramsay Mary , Lopez Bernal Jamie , Campbell Helen 
Published: June 2023

Background

Pregnant women are at increased risk of adverse outcomes following SARS-CoV-2 infection, including venous thromboembolism, admission to critical care and death. Their infants may also suffer from pre-term birth, stillbirth and severe disease. Vaccination may protect both mothers and their infants against severe COVID-19 disease.

Methods

We used a test-negative, case-control study design to estimate vaccine effectiveness against symptomatic disease and hospitalisation with the Delta and Omicron variants in pregnant women who gave birth in 2021 in England. We also estimated the protection conferred by prior infection and maternal vaccination against symptomatic disease and hospitalisation with the Delta and Omicron variants in their infants.

Findings

Vaccine effectiveness against symptomatic disease (Delta and Omicron) and against hospitalisation (Delta only) was high and similar to that observed in the general population. Maternal vaccination during and post-pregnancy as well as previous infection also provided sustained protection from symptomatic disease and hospitalisation following Delta and Omicron infection in infants up to 8 months of age, with the highest protection being observed when maternal vaccination occurred during later pregnancy. Unlike non-pregnant women, a booster dose provided sustained protection with no evidence of waning up to 15 weeks after vaccination.

Interpretation

Maternal vaccination prevents mild and severe disease in pregnant women and their infants up to 6-8 months after birth. Our findings support the promotion of both primary and booster vaccination for pregnant women, irrespective of prior infection status, to protect themselves and their infants.

Funding

None.

Research in Context

Evidence before this study

Pregnant women were included in the UK’s priority risk groups for COVID-19 vaccination from 2 December 2021 when they were encouraged to complete vaccination with an mRNA booster vaccine of either Pfizer BioNTech or Moderna. We searched PubMed using the terms ‘pregnancy’, ‘COVID-19’, ‘vaccine’ and ‘vaccine effectiveness’, with no date restrictions on 1 March 2023, and used the snowball process to identify additional relevant publications. We also scoped preprint databases for relevant COVID-19 vaccine effectiveness studies undertaken after the emergence of the more immune-evasive Omicron variant from December 2021. Studies have shown moderate COVID-19 vaccine effectiveness after a second dose in pregnant women against symptomatic Omicron disease with evidence that booster doses of mRNA vaccines confer higher protection against serious Omicron disease, comparable with population-based immunity. In addition to evidence of transplacental transfer of maternal antibody, real-life evidence from test-negative case-control studies have demonstrated protection in infants following maternal vaccination which is highest after vaccination in the third trimester and wanes with increasing infant age.

Added value of this study

Ours is the largest study of the effectiveness of maternal COVID-19 vaccines against both maternal and infant disease, in addition to the protection conferred by past infection in the mother to the infant. In pregnant women, vaccine effectiveness against symptomatic Delta and Omicron infection, and against hospitalisation with Delta, remained high after vaccination with limited waning observed at the longest time points investigated post vaccination. Both prior infection and maternal vaccination protected infants after birth against symptomatic disease and hospitalisation with Delta and Omicron. Vaccine effectiveness was highest when maternal vaccination occurred in the later stages of pregnancy.

Implications of all the available evidence

These findings support the benefits of maternal vaccination in preventing disease in the mother and in her infant in the first months of life, regardless of prior infection status in the mother. Policy decisions need to balance the suggestion of higher protection after vaccination later in pregnancy with the need to ensure adequate opportunities for vaccination before women reach the pregnancy stage when they are at greatest risk from COVID-19 disease and to optimise the infant benefit even in pre-term births.