Vaccination provides effective protection against increased risk of pregnancy complications due to COVID-19 Omicron variant

17 January 2023

The global network led by the Oxford Maternal and Perinatal Health Institute (OMPHI) at the University of Oxford has today published, in the Lancet, the results of the ‘2022 INTERCOVID Study’ conducted in 41 hospitals across 18 countries.

To evaluate the impact of the COVID-19 Omicron variant on maternal and neonatal outcomes the researchers studied 1,545 pregnant women diagnosed with the variant and 3,073 non-diagnosed, concomitant pregnant women as controls. The study was conducted between 27 November, 2021, and 30 June, 2022, during which time Omicron was the variant of concern. Vaccine effectiveness against the variant was also assessed.

COVID-19 Omicron variant during pregnancy was associated with increased risks of maternal morbidity, severe pregnancy complications, and hospital admission, especially among symptomatic and unvaccinated women. In particular, the risk of preeclampsia was increased among women with severe symptoms. Obese/overweight women with severe symptoms were at the highest risk for maternal morbidity and severe complications.

Vaccinated women were well protected against severe COVID-19 symptoms and complications and had a very low risk of admission to an intensive care unit. Prevention of severe COVID-19 symptoms and complications requires women to be completely vaccinated, preferably with a booster dose as well.

In the study, mRNA vaccines were most effective in preventing severe COVID-19 symptoms and complications, although viral vector vaccines with a booster also provided adequate protection – for at least 10 months after the last dose for both mRNA vaccines and viral vector vaccines with a booster.

José Villar, Professor of Perinatal Medicine at the University of Oxford, who co-led INTERCOVID 2022, says: “We have provided robust, evidence-based information on the increased risk of the COVID-19 Omicron variant during pregnancy for severe maternal complications among symptomatic and unvaccinated women. Of concern is that severe symptoms of the disease occurred in 4% to 7% of unvaccinated women diagnosed with the COVID-19 Omicron variant during pregnancy. The study clearly indicates the need for a complete vaccination course during pregnancy, preferably with a booster, to provide protection for at least 10 months after the last dose. Antenatal services worldwide should strive to include vaccination against COVID-19 in the routine care of pregnant women.”

Aris Papageorghiou, Professor of Fetal Medicine, University of Oxford, who co-led INTERCOVID 2022, says: “Although the Omicron variant may be less harmful than previous variants in the general population, the large proportion of unvaccinated pregnant women worldwide are still at major risk. As it is impossible to predict who will develop severe symptoms or complications, universal complete vaccination is required. Unfortunately, full vaccination coverage among pregnant women is still inadequate even in developed countries.”

Stephen Kennedy, Professor of Reproductive Medicine, University of Oxford, who co-led INTERCOVID 2022, says: “The present study is a shining example of how well-coordinated, multi-national, collaborative research can, in a very short time, provide robust evidence to improve the health of mothers and babies worldwide. The findings from this study and our previous INTERCOVID studies have contributed to changes in clinical practice and public health policy recommending vaccination for all pregnant women. We hope our work will help to negate the considerable misinformation circulating regarding the pandemic and effectiveness of vaccines.”

Notes to Editors

Previous publications of the INTERCOVID project

Our original findings relating to the first wave of the pandemic were published in April 2021: 'Maternal and Neonatal Morbidity and Mortality Among Pregnant Women With and Without COVID-19 Infection: The INTERCOVID Multinational Cohort Study', can be read in JAMA Pediatrics.

Subsequent research based on the 2020 cohort were published in:

June 2021, 'Preeclampsia and COVID-19: results from the INTERCOVID prospective longitudinal study', can be read in the American Journal of Obstetrics and Gynecology.

April 2022, 'Effect of prenatal exposure to maternal COVID-19 and perinatal care on neonatal outcome: results from the INTERCOVID Multinational Cohort Study', can be read in the American Journal of Obstetrics and Gynecology.

July 2022, 'Diabetes mellitus, maternal adiposity, and insulin dependent gestational diabetes are associated with COVID-19 in pregnancy: the INTERCOVID study', can be read in the American Journal of Obstetrics and Gynecology.

For further information and interview requests please contact: University of Oxford press office, [email protected] or +44 (0) 1865 280528 or Dr. Aris Papageorghiou, Aris [email protected] or +44 (0)1865 221019.

Nuffield Department of Women's & Reproductive Health
The Nuffield Department of Women's & Reproductive Health at the University of Oxford encompasses multidisciplinary research across four overarching themes; Cancer, Global Health, Maternal & Fetal Health and Reproductive Medicine & Genetics. It focuses on genetic studies, the dissection of molecular, biochemical and cellular mechanisms underlying normal and aberrant reproductive tissue function, clinical studies in women’s health and pregnancy, and growth and development across the first 1000 days of life (INTERGROWTH-21st Project).

More information: https://www.wrh.ox.ac.uk/

Oxford Maternal & Perinatal Health Institute (OMPHI)
The Oxford Maternal & Perinatal Health Institute, based at Green Templeton College, Oxford, works alongside the Nuffield Department of Women's & Reproductive Health, bringing together world experts to resolve priority maternal and perinatal health problems on a global scale.

More information: https://www.wrh.ox.ac.uk/research/omphi

Oxford University’s Medical Sciences Division is one of the largest biomedical research centres in Europe, with over 2,500 people involved in research and more than 2,800 students. The University is rated the best in the world for medicine and life sciences, and it is home to the UK’s top-ranked medical school. It has one of the largest clinical trial portfolios in the UK and great expertise in taking discoveries from the lab into the clinic. Partnerships with the local NHS Trusts enable patients to benefit from close links between medical research and health care delivery.
The set-up of the original INTERCOVID Study network was supported by a grant from Oxford University’s COVID-19 Research Response Fund; this study of the Omicron variant was undertaken without additional financial support.

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