‘Clear evidence’ of an immune response to COVID-19 mRNA vaccines in pregnant women

COVID-19 vaccines using messenger RNA technology are found ‘highly effective’ in pregnant and lactating women.

Published 30 March 2021

Largest prospective study to date finds that two mRNA vaccines induce an immune response likely to protect pregnant women and their infants from COVID-19 infection. ‘Excellent vaccine efficacy is very encouraging’ for pregnant and breastfeeding women.

The largest study of its kind so far has found that COVID-19 vaccines using messenger RNA technology are ‘highly effective’ in producing antibodies against the SARS-CoV-2 virus in pregnant and lactating women.(1) The study also showed that the vaccines confer protective immunity to newborns through breastmilk and the placenta.

This prospective study took place in a cohort of 131 women of reproductive age (84 pregnant, 31 lactating, and 16 non-pregnant) at two hospitals in Boston, USA, in whom antibody levels were measured at baseline, second vaccine dose, 2-6 weeks post second vaccine, and delivery. IgM, IgG and IgA titres were compared with those in pregnant women 4-12 weeks from natural infection.

Results showed that the vaccine-induced immune responses (from the Pfizer/BioNTech or Moderna vaccines only) were comparable in the pregnant and lactating women with those in the non-pregnant women - and notably all antibody measures in the vaccinated women were higher than those induced by native infection. Vaccine-generated antibodies were present in all umbilical cord blood and breastmilk samples, indicating the transfer of antibodies from mothers to newborns.

Commenting on the results in a press statement, one of study authors, Galit Alter from Massachusetts General Hospital, said: ‘We now have clear evidence the COVID vaccines can induce immunity that will protect infants. We hope this study will catalyze vaccine developers to recognize the importance of studying pregnant and lactating individuals, and include them in trials . . . developers must realize that pregnancy is a distinct immunological state.’

‘News of excellent vaccine efficacy is very encouraging for pregnant and breastfeeding women, who were left out of the initial COVID-19 vaccine trials,’ added fellow author Andrea Edlow.

So far guidance on COVID vaccination in pregnancy – as illustrated in both ESHRE and ASRM statements - has been somewhat frustrated by a lack of specific data.(2) Pregnant women were not included in any of the original vaccine trials, nor identified other than cursorily in the formal product information. In its guidance ESHRE took a more precautionary approach than ASRM, the latter recognising pregnancy itself as a risk factor for greater COVID-19 morbidity and urging vaccination in those pregnant or planning pregnancy, even though its effects on immunity were unclear. A statement reported by the American College of O&G earlier this year recommended that the COVID-19 vaccine ‘should only be offered to pregnant patients after discussing the lack of safety data and prioritized for women considered at highest risk’.(3)

This latest study also considered side effects from the vaccination and noted profiles that were no different in pregnant women from those found in non-pregnant subjects.

Overall, the authors conclude that, while side effects and immune response are only two of many considerations for pregnant women, ‘these data confirm that the COVID-19 mRNA vaccines result in comparable humoral immune responses in pregnant and lactating women to those observed in non-pregnant populations’. In their press statement they add that further ‘vaccine platforms’ will be studied as they become available.

1. Gray KJ, Bordt EA, Atyea C, et al. COVID-19 vaccine response in pregnant and lactating women: a cohort study. Am J Obstet Gynecol 2021;
2. See
3. Stafford IA, Parchem JG, Sibai BM. The coronavirus disease 2019 vaccine in pregnancy: risks, benefits, and recommendations. Al J Obstet Gynecol 2021; doi:10.1016/j.ajog.2021.01.022

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