Vaccination protects against complications in pregnancy due to Omicron variant of COVID-19

The vaccines offer high protection against severe COVID-19 symptoms, with an effectiveness of up to 91% in infected women who had received the complete vaccination regimen and a booster dose.


The Vall d'Hebron University Hospital and the Lozano Blesa University Hospital of Zaragoza have participated in the INTERCOVID 2022 study, which has shown that infection with the Omicron variant of SARS-CoV-2 increases the risk of complications during pregnancy and that vaccination is effective in offering protection to pregnant women. The work, coordinated by the University of Oxford, has been published in The Lancet.

The project was carried out in 41 hospitals across 18 countries to assess the impact of the Omicron variant of COVID-19 during pregnancy on mothers fetuses and newborns. In April 2021, it was already confirmed that the risk of complications was increased in pregnant women infected with SARS-CoV-2 and, on this occasion, evidence is provided on the effect of Omicron and the efficacy of vaccination against this variant.

The researchers studied 1545 pregnant women diagnosed with the variant and 3073 pregnant women without a diagnosis of COVID-19. The study was conducted between November 2021 and June 2022, during which time Omicron was the variant of concern.

The results showed that the Omicron variant during pregnancy was associated with an increase in complications, such as preterm delivery, and hospital admissions. "The study provides us with evidence on the increased risk during pregnancy due to this variant, especially among symptomatic and unvaccinated women", says Dr. Nerea Maiz, a specialist in the Obstetrics Department at Vall d'Hebron Hospital and a researcher in the Maternal and Fetal Medicine group at Vall d'Hebron Research Institute (VHIR). Severe symptoms appeared in 4-7% of unvaccinated women diagnosed with the Omicron variant, and it was particularly high in women with obesity or overweight. Specifically, among women with severe symptoms, the risk of preeclampsia (high blood pressure during pregnancy) was increased.

On the other hand, it was observed that vaccination offered high protection against severe symptoms of COVID-19 and against pregnancy complications, and vaccinated women had a low risk of needing admission to intensive care units. The effectiveness of the vaccine against the development of severe symptoms in women with COVID-19 was 74% in women with the complete vaccination regimen and was as high as 91% in those who had received a booster dose. "The study clearly indicates the need for full vaccination, preferably with booster doses, to offer protection to pregnant women", Dr. Maiz emphasizes. "Unvaccinated pregnant women infected with the Omicron variant are still at significant risk of complications. Fortunately, in our environment a wide vaccination coverage has been achieved in recent years, but we must continue to insist on the benefits of receiving booster doses, in accordance with the calendar established by the health authorities", adds Dr. Marta Fabre, researcher in the Placental pathophysiology and fetal programming group of the Instituto de Investigación Sanitaria Aragón and physician in the Clinical Biochemistry Department of HCU Lozano Blesa in Zaragoza.

Regarding the type of immunization received, mRNA vaccines (such as those from Pfizer-BioNTech and Moderna) were more effective in preventing severe symptoms of COVID-19 and complications for at least 10 months from the last dose. Viral vector vaccines (such as those from AstraZeneca and Janssen) also offered adequate protection if a booster dose had been administered.

As for the side effects of vaccination, none were observed either in the mother or in the fetus or newborn, even if the vaccine was given during gestation.

INTERCOVID 2022 is an example of an international collaborative study that provides robust evidence to improve the health of mothers and babies worldwide. Findings from this study and previous results have contributed to changes in clinical practice and public health policy recommending vaccination for all pregnant women.

The study has confirmed that the Omicron variant of COVID-19 is associated with increased complications during pregnancy, especially among women with severe symptoms and who have not been vaccinated.

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