COVID-19 Vaccination in Pregnant Patients Reduces Hospitalization, Preterm Birth Across Variants

If administered before infection, COVID-19 vaccination during pregnancy was associated with a lower risk of adverse events and preterm birth, regardless of the circulating variant, according to a study published today in JAMA.1

COVID-19 vaccination during pregnancy lowers hospitalization and preterm birth risks, providing key guidance for maternal care strategies. | Image credit: Leigh Prather - stock.adobe.com

Gaps in Understanding COVID-19 Variant and Vaccination Effects on Pregnant Patients

Although certain clinical and demographic factors are known to increase the risk of severe COVID-19 in pregnancy, the specific risks associated with each variant and the protective role of vaccination remain unclear. This uncertainty is partly due to the concurrent emergence of new COVID-19 variants and the rollout of vaccines.

The researchers noted that previous vaccination-focused studies have shown a protective effect against adverse maternal and infant outcomes following COVID-19 infection. However, many of these studies were limited by small sample sizes, leaving gaps in understanding how variant type and vaccination status independently influence outcomes in large prospective cohorts.

To address this, the researchers analyzed data from the Canadian Surveillance of COVID-19 in Pregnancy program, which tracks maternal and infant outcomes of pregnancies affected by COVID-19 and assesses the impact of vaccination on adverse pregnancy outcomes.2

The analysis included pregnant patients diagnosed with COVID-19 between April 5, 2021, and December 31, 2022, across 9 Canadian provinces and territories: British Columbia, Alberta, Manitoba, Ontario, Quebec, New Brunswick, Prince Edward Island, Nova Scotia, and Yukon.1 Cases were followed into 2023 to capture pregnancy conclusions and infant outcomes.

Reducing Adverse Pregnancy Outcomes Through COVID-19 Vaccination

Among 26,584 identified cases, vaccination status was available for 19,899. Of these, most resided in Ontario (n = 13,679), followed by Alberta (n = 2514), British Columbia (n = 1813), Quebec (n = 977), Manitoba (n = 386), Prince Edward Island (n = 308), Nova Scotia (n = 159), New Brunswick (n = 60), and Yukon (n < 6).

Overall, 69.2% (n = 13,799) of infections occurred during the Omicron period, while 30.8% (n = 6120) occurred during the Delta period. Among the study population, 46.3% (n = 8763) of cases were in individuals aged between 30 and 35, aligning with the mean age of pregnancy in Canada, and 55.9% (n = 4765) were in White patients.

Additionally, 72.2% (n = 14,367) of COVID-19 cases occurred in patients who received at least 1 vaccine dose before diagnosis, while 27.8% (n = 5532) remained unvaccinated. Of those vaccinated, 20% (n = 2942) received a vaccine during pregnancy, at a median of 18 (IQR, 11-25) weeks before their COVID-19 diagnosis. Generally, vaccinated patients were older, more often White, had a lower body mass index, and had lower rates of gestational hypertension. However, they had higher rates of preexisting and gestational diabetes.

During both Delta and Omicron periods, vaccination was associated with a lower risk of hospitalization (Delta: relative risk [RR], 0.38 [95% CI, 0.30-0.48]; absolute risk difference [ARD], 8.7% [95% CI, 7.3%-10.2%]; Omicron: RR, 0.38 [95% CI, 0.27-0.53]; ARD, 3.8% [95% CI, 2.4%-5.2%]), critical care unit admission (Delta: RR, 0.10 [95% CI, 0.04-0.29]; ARD, 2.4% [95% CI, 1.8%-2.9%]; Omicron: RR, 0.10 [95% CI, 0.03-0.29]; ARD, 0.85% [95% CI, 0.27%-1.44%]), and preterm birth (Delta: RR, 0.80 [95% CI, 0.66-0.98; ARD, 1.8% [95% CI, 0.3%-3.4%]; Omicron: RR, 0.64 [95% CI, 0.52-0.77]; ARD, 4.1% [95% CI, 2.0%-6.2%]).

Multivariable analyses confirmed that vaccination remained associated with lower hospitalization risk in both periods after controlling for comorbid conditions. Compared with vaccinated patients, the unvaccinated group had an adjusted RR of hospitalization of 2.43 (95% CI, 1.72-3.43) during Omicron and 3.82 (95% CI, 2.38-6.14) during Delta.

Informing Future COVID-19 Vaccination, Maternal Care Strategies

The researchers acknowledged several limitations of the study, including a decrease in formal tracking of COVID-19 cases during the Omicron period by public health authorities, which likely led to some cases being missed. Additionally, the observational nature of the study means that causation cannot be assumed. Still, they expressed confidence in the clinical relevance of their findings.

“This robust analysis of the association of vaccination status and reductions in adverse outcomes in both the Delta and Omicron variant periods provides new insights into the impact of vaccination during the COVID-19 pandemic,” the authors wrote.

References

1. McClymont E, Blitz S, Forward L, et al. The role of vaccination in maternal and perinatal outcomes associated with COVID-19 in pregnancy. JAMA. Published online December 15, 2025. doi:10.1001/jama.2025.21001
2. Canadian surveillance of COVID-19 in pregnancy: epidemiology, maternal and infant outcomes. University of British Columbia. Updated September 13, 2023. Accessed December 15, 2025. https://ridprogram.med.ubc.ca/cancovid-preg/