In January 2023 results in The Lancet from a network led by the Oxford Maternal and Perinatal Health Institute (OMPHI) emphasised the importance of vaccination in pregnancy. The team evaluated the effect of the COVID-19 omicron variant on maternal and neonatal outcomes between November 2021 and June 2022, at which point omicron was the variant of concern.  

The variant was associated with “increased risks of severe maternal morbidity, especially in symptomatic and unvaccinated” patients. Risk of preeclampsia was increased among patients with “severe symptoms”. Furthermore, overweight patients were at the highest risk of complications.  

COVID-19 complications 

The study refers to previous research, which established in 2021 that there was an “increased risk with COVID-19 in pregnancy”. However, since that time, COVID-19 has undergone genetic mutations.  

The team acknowledges “early reports” that suggest that omicron, “although more transmissible, produces milder disease in non-pregnant individuals than previous variants”. Despite this, other evidence indicates that hospitalisation and mortality rates might be comparable to other variants “after adjusting for confounders and vaccination status”. 

Thus, the INTERCOVID-2022 Study aimed to “examine the effects on maternal and perinatal outcomes of COVID-19 during pregnancy, as compared with pregnant women without a COVID-19 diagnosis, enrolled during the first 6 months of omicron as the variant of concern”. Furthermore, it evaluated “vaccine effectiveness against COVID-19 diagnosis, severe maternal complications, and hospitalisation or death, as compared with non-vaccinated pregnant women, according to vaccine type and dose; and clarify disease risk and prevention effectiveness to challenge misinformation in the medical community and general public”.  

The study 

The study was a prospective, observational, cohort study. It involved 41 hospitals, part of the OMPHI network, across 18 countries. Patients were enrolled at “any time during pregnancy or delivery” 27th November 2021 and 30th June 2022. Enrolment opened the day after WHO declared omicron a variant of concern.  

“After each woman with a COVID-19 diagnosis was enrolled, to minimise risk of bias, two unmatched women without a COVID-19 diagnosis, as representative of the pregnant population at each study site, were enrolled concomitantly and consecutively, at delivery or at the same level of care”. 

The authors indicate that the “primary outcomes were the same as in [their] previous report”. The findings show a “marked increased risk for the same outcomes among symptomatic, unvaccinated women, close to the effect seen in pregnant women before vaccines became available”.  

“Reassuringly, the vaccine effectiveness of all vaccines combined to prevent severe maternal morbidity and referral, ICU admission, or death was 76% amongst all women given a booster dose.” 

Furthermore, “respective vaccine effectiveness values for mRNA vaccines with a booster dose was 81%”. Although “vaccination during pregnancy did not prevent infection with the omicron variant”, as in the general population, “doses of the two widely used vaccine types are highly effective in preventing severe maternal complications”. 


“In our view, pregnant women must be included in the assessment of modified vaccines and prioritised for vaccination to avoid past shortcomings.”  

Professor José Villar of Oxford’s Nuffield Department of Women’s and Reproductive Health described the study as “robust, evidence-based information”. A “concern” is that “severe symptoms of the disease occurred in 4% to 7% of unvaccinated women diagnosed” with omicron during pregnancy. This “clearly indicates the need for a complete vaccination course during pregnancy”.  

“Antenatal services worldwide should strive to include vaccination against COVID-19 in the routine care of pregnant women.” 

Professor Aris Papageorghiou, also from the Nuffield Department of Women’s and Reproductive Health, acknowledges that the omicron variant “may be less harmful than previous variants in the general population”. However, the “large proportion of unvaccinated pregnant women worldwide are still at major risk”.  

“Universal complete vaccination is required.”  

Professor Stephen Kennedy praised the “well-coordinated, multi-national, collaborative research” that the study employed to “improve the health of mothers and babies worldwide”.  

“We hope our work will help to negate the considerable misinformation circulating regarding the pandemic and effectiveness of vaccines.”  

For more on COVID-19 vaccines and recommendations for pregnant patients at the World Vaccine Congress in Washington 2023 get your tickets now.