Covid vaccination during pregnancy protects infant from virus

Covid-19 vaccination during pregnancy may protect the unborn infant from the virus as well as the mother, according to a study published in The BMJ.

Although vaccines are not licensed for infants under six months of age, the study found that two doses of an mRNA Covid-19 vaccine during pregnancy are linked to a lower risk of infection and hospital admission in infants under six months of age.

This is because antibodies are transferred across the placenta and through breastmilk (a process known as passive immunity). To study this further, researchers in Canada set out to estimate the effectiveness of Covid vaccination during pregnancy against delta and omicron infection and hospital admission in infants.

mRNA vaccines were better at protecting against Delta than Omicron

The study included 8,809 infants in the main analysis. All infants were younger than six months of age and were born in Ontario between 7 May 2021 and 31 March 2022.

All infants were PCR tests for Covid-19 infection and this information was linked to data on the mother’s mRNA Covid-19 vaccination during pregnancy.

A range of potentially influential factors were considered, including mother’s age at delivery, number of previous pregnancies, pre-pregnancy conditions such as diabetes, heart disease and asthma, and infant sex.

In total, there were 99 positive delta cases compared with 4,365 negative controls, and 1,501 omicron cases compared with 4,847 controls.

Two vaccine doses during pregnancy were found to be highly effective (95%) against delta infection in infants, as well as against infant hospital admission due to delta (97%).

Two vaccine doses were less protective against Omicron, at 45% for infected infants and 53% for hospitalisation. However, this improved with a third dose during pregnancy (73% and 80%, respectively).

Effectiveness of two doses against omicron infection was highest when the second dose was given later in the pregnancy: 53% in the third trimester of pregnancy compared with 47% in the first and 37% in the second.

Effectiveness of two doses against omicron infection waned over time, from 57% between birth and eight weeks to 40% after 16 weeks of age.

More studies needed to better inform vaccine recommendations

The researchers highlight some limitations, such as the fact that unmeasured factors such as breastfeeding could have differed between infants of vaccinated and unvaccinated mothers, potentially affecting their results.

However, they used detailed information on hospital deliveries, mother’s vaccination status, and PCR test results. Furthermore, previous studies from four different countries have had similar results, suggesting they are likely to be robust.

They are now calling for more studies to look into the effects of mRNA vaccinations on babies, saying they are needed to “better inform vaccination recommendations in an evolving landscape of new SARS-CoV-2 strains and novel vaccines.”