What is the risk of Omicron infection in pregnant women?

In a recent study published in BMJ Medicine, researchers evaluated the risk of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in pregnant women during the Omicron predominance period in the United Kingdom (UK).

Background

Previous studies characterized SARS-CoV-2 infection in pregnant women when the infecting variants of concern (VOCs) were Alpha (B.1.1.7) and Delta (B.1.617.2) compared to the wild-type (wt) ancestral strain. These studies pointed out that these VOCs caused more severe maternal infections at high frequency than the wt SARS-CoV-2 strain and with worse perinatal outcomes.

The UK began vaccinating pregnant women in April 2021 and recognized them as a risk group that needed vaccination as a priority. However, studies barely assessed the SARS-CoV-2 infection severity in pregnant women during the Omicron era when vaccination coverage was high.

It is crucial to determine the risk of Omicron infection in unvaccinated pregnant women. This data could help inform mitigation strategies to prevent severe coronavirus disease 2019 (COVID-19) during pregnancy by Omicron breakthrough infections and from new SARS-CoV-2 VOCs that continue to emerge. Robust national data would also inform women planning a pregnancy or already pregnant regarding the care they need and healthcare professionals to provide care.

About the study

In the present study, researchers used the UK Obstetric Surveillance System data to assess the severity of maternal infection during the Omicron predominance period, i.e., between 15 December 2021 and 14 March 2022. The study data covered 194 UK hospitals with a consultant-led maternity department.

The study population comprised all pregnant females admitted to these hospitals with a SARS-CoV-2 positive reverse transcription-polymerase chain reaction (RT-PCR) test during the study duration. The study primarily assessed the infection type (asymptomatic or symptomatic) in pregnant women, their vaccination status before hospital admission, and severity of infection per recent World Health Organization (WHO) criteria. Additionally, the researchers evaluated perinatal outcomes.

Study findings

A total of 3699 women were admitted to 194 hospitals of the UK Obstetric Surveillance System with RT-PCR confirmed SARS-CoV-2 infection during the study period. Of these, 986 (26.7%) had symptoms, and 2713 (73.3%) were asymptomatic.

The researchers had vaccination status of 3064 women, of which 845 pregnant women were in the symptomatic group, and 2219 were in the asymptomatic group. Only three of the 129 women for whom vaccination status was known and had moderate to severe infection had completed a three-dose vaccine regimen per the UK government advisory. The remaining symptomatic women were either unvaccinated (58.9%) or had received one (12.7%) or two doses (23%).

Over 14% of symptomatic women exhibited at least one indicator of moderate to severe infection, and 99 required respiratory support. However, only a few of these women received any anti-SARS-CoV-2 drug treatment. While 4.4% received corticosteroids for maternal indication, 0.6%, 0.3% and 0.9% received monoclonal antibodies, antivirals, and tocilizumab, respectively.

The researchers could observe perinatal outcomes among 2841 symptomatic and 2285 asymptomatic pregnant women. Nearly one-third of the former group were discharged while still pregnant, although childbirth had to be expedited for 4.2% of them. In 2821 infants, the researchers documented ten and nine stillbirths in the symptomatic and asymptomatic groups, respectively.

Conclusions

The current national-level prospective cohort study showed the role of vaccination in preventing pregnant women from progression to severe COVID-19 in the Omicron era. The proportion of women in the current study cohort with symptoms and moderate to severe infection was 14.6%. The percentage of such women was lower during the wild type, Alpha, and Delta VOCs predominance periods in the UK at 24.5%, 36.2%, and 42.8%, respectively.

Since most symptomatic pregnant women had received one or more vaccine doses than in previous variant periods, vaccination prevalence was accounted for when comparing outcomes across variant periods. Vaccination likely conferred some degree of protection from severe illness and symptomatic infection. Thus, although one in five symptomatic pregnant women had moderate to severe infection, this was reduced to one in ten with two and one in twenty with three vaccine doses.

Mostly unvaccinated women were at the risk of severe respiratory failure with Omicron, similar to observations during the first wave of the COVID-19 pandemic in the UK. Although severe outcomes were less frequent in the Omicron era, the risk of hospital admission due to Omicron infections was higher in the UK than in other European countries. Since the study data showed that the incidences of respiratory failure among pregnant women are preventable, thus policymakers need to make concerted efforts to improve vaccine uptake during pregnancy.