A study by the INTERCOVID Consortium, led by researchers from the University of Oxford and with the participation of Vall d’Hebron, provides new evidence on the potential benefits of COVID-19 vaccination during pregnancy. The results indicate that vaccination, especially when it includes a booster dose, is associated with a significant reduction in the risk of preeclampsia, one of the main serious complications of pregnancy.

The work, published in The Lancet, analyzed data from 6,527 pregnant women across 18 countries, recruited between 2020 and 2022. The study compared vaccinated and unvaccinated women, with and without SARS-CoV-2 infection, to evaluate the relationship between vaccination status and the occurrence of preeclampsia.

In Spain, in addition to Vall d’Hebron, the only participating hospital was the Hospital Clínico Universitario Lozano Blesa, which contributed to the analysis of results in the national clinical context.

The study data show that COVID-19 infection during pregnancy was associated with a 45% increase in the risk of preeclampsia, rising to 78% in unvaccinated women.

Conversely, among women who received a booster dose, vaccination was associated with a 33% reduction in the likelihood of developing preeclampsia, with statistical significance. This effect was even greater in women with preexisting conditions such as diabetes, hypertension, or thyroid disorders, where the risk reduction reached 58% after receiving the booster dose.

The protective effect remained after adjusting for relevant clinical factors, suggesting that the benefit could extend beyond preventing COVID-19 infection.

Additionally, vaccinated women had lower rates of preterm birth, maternal and perinatal morbidity, and mortality. Among those who received booster doses, the risk reduction was 33% for preterm birth, 32% for maternal morbidity and mortality, and 29% for severe perinatal morbidity and mortality.

“These results convey a highly relevant message for maternal health: getting vaccinated against COVID-19 during pregnancy, and especially receiving the booster dose, not only protects against infection and its complications, but is also associated with a significant reduction in the risk of preeclampsia and other adverse outcomes,” says Nerea Maiz, specialist in the Obstetrics Department at Vall d’Hebron Hospital and researcher in the Maternal and Fetal Medicine group at the Vall d’Hebron Research Institute (VHIR).

Preeclampsia: a complication of still unknown origin

Preeclampsia affects approximately 3–8% of pregnancies worldwide and is one of the leading causes of maternal and neonatal morbidity and mortality. Although its exact origin is not fully understood, systemic inflammation and vascular dysfunction play a key role—mechanisms also implicated in COVID-19 infection.

The study results support the hypothesis that vaccination could influence these biological pathways, offering potential additional protection even in the absence of active infection.

Implications for public health

These findings reinforce the importance of maintaining and strengthening vaccination programs for pregnant women, with special emphasis on booster dose administration and ensuring equitable access globally.

Funding and international collaboration

The INTERCOVID study was funded by the COVID-19 Research Response Fund of the University of Oxford. The consortium includes over 40 hospitals across 18 countries and represents one of the largest international initiatives aimed at understanding maternal and perinatal outcomes during the pandemic.