The Covid-19 vaccines by Pfizer and Moderna are “highly effective” in producing antibodies against the coronavirus in pregnant and breastfeeding individuals, according to new research.

In the largest study of its kind to date, researchers at Massachusetts General Hospital, Brigham and Women’s Hospital and the Ragon Institute demonstrated the vaccines are not just effective in protecting pregnant women, but they pass on protective immunity to newborn babies through breastmilk and the placenta.

In the UK, pregnant women aren’t able to have the Covid-19 vaccine unless they have an underlying health condition that puts them more at risk from the virus, or they work in a profession that increases their exposure.

This is because there’s a lack of safety data from trials of the jabs. The Joint Committee on Vaccination and Immunisation (JCVI), which is responsible for prioritising who gets the vaccine, says although available data doesn’t indicate safety concerns or harm to pregnancy, there’s insufficient evidence to recommend routine use of Covid-19 vaccines during pregnancy.

Women are, however, able to have the jab if they’re breastfeeding.

In the US, women can choose to have the vaccine. The study, published in the American Journal of Obstetrics and Gynaecology (AJOG), looked at 131 women of reproductive age – 84 were pregnant, 31 were lactating and 16 were not pregnant. All of the women received one of two new mRNA vaccines: Pfizer/BioNTech or Moderna.

Scientists discovered that in all three groups, antibody levels were present and similar – and, reassuringly, side effects after vaccination were rare and comparable across the study participants.

The news of “excellent vaccine efficacy” is “very encouraging” for pregnant and breastfeeding women, who were left out of the initial Covid-19 vaccine trials, said Dr Andrea Edlow, a maternal-foetal medicine specialist at Massachusetts General Hospital and co-senior author of the new study.

“Filling in the information gaps with real data is key,” she said, “especially for our pregnant patients who are at greater risk for complications from Covid-19.”

The study is important because we know individuals who are pregnant are more vulnerable to Covid-19. Research led by the University of Birmingham and the World Health Organisation (WHO) suggests the risk of pregnant women being admitted to intensive care or needing ventilation is higher than non-pregnant reproductive-aged women with the virus.

Pregnant women are also at increased risk of severe Covid-19 if they’re from ethnic minority backgrounds, or if they have pre-existing conditions like obesity, high blood pressure and diabetes.

For the latest study on antibodies in pregnant people, the team also compared vaccination-induced antibody levels to those induced by natural infection with Covid-19 in pregnancy, and found significantly higher levels of antibodies from vaccination.

Vaccine-generated antibodies were also present in all umbilical cord blood and breastmilk samples taken from the study, showing the transfer of antibodies from mothers to newborns. 

In Florida recently, a new mother made headlines after her baby girl was born healthy and with Covid-19 antibodies – the frontline health care worker had received the Moderna jab three weeks before giving birth. 

“We now have clear evidence the Covid vaccines can induce immunity that will protect infants,” said Galit Alter, of the Ragon Institute and co-senior author of the study.

The research was also able to provide an insight into potential differences between the immune response elicited by the Pfizer vaccine compared to the Moderna vaccine. Levels of mucosal (IgA) antibodies were higher after the second dose of Moderna compared to the second dose of Pfizer.

The finding is important for all individuals, since SARS-CoV-2 is acquired through mucosal surfaces like the nose, mouth and eyes, said Kathryn Gray, an obstetrician at Brigham and Women’s Hospital and another author of the study.

“It also holds special importance for pregnant and lactating women because IgA is a key antibody present in breastmilk,” she added.