Risk of SARS-CoV-2 transmission from mother to baby is extremely low, say researchers

Less than 2% of babies born to mothers with SARS-CoV-2 (the virus responsible for covid-19 infection) also test positive around the time of birth, finds an analysis of the latest evidence published by The BMJ today.

While the risk of transmission is extremely low, the results do suggest that babies of mothers with more severe infection may be more likely to test positive. However, current evidence does not support measures such as cesarean sections, mother-baby separation at birth, or formula feeding to avoid covid-19 transmission to babies.

Pregnant women can carry the SARS-CoV-2 virus in their blood, placenta, amniotic fluid, and breast milk, so have the potential to pass it onto their babies. But major knowledge gaps remain about the risks to babies when their mothers are infected with SARS-CoV-2.

To address this, an international team of researchers reviewed nearly 500 studies involving 28,952 mothers with covid-19 who sought hospital care for any reason.

Their aim was to assess rates of positivity among babies born to mothers with the virus, the likely timing of mother-to-child transmission, outcomes among babies with the virus, and whether factors associated with labour, delivery, or breastfeeding increased the risk of babies becoming infected.

Most of the studies were from Europe, Central Asia and North America, and although they were designed differently and were of varying quality, the researchers were able to allow for that in their analysis.

Overall, 1.8% of the 14,271 babies born to mothers with SARS-CoV-2 infection also tested positive with a PCR test. This fell to just 1% when analyses were limited to babies exposed to the virus before or during birth.

Of the 592 positive babies with data on the timing of exposure and type and timing of tests, 14 had confirmed mother-to-child transmission: seven before birth (in utero), two during labour or delivery (intrapartum), and five during the early postnatal period (up to 10 days after birth).

Of the 800 positive babies with outcome data, 20 were stillbirths, 23 were neonatal deaths (during the first 28 days of life), and eight were early pregnancy losses; 749 babies were alive at the end of follow-up.

Babies seemed more likely to test positive for covid-19 if their mothers had severe covid-19 infection, were admitted to an intensive care unit, developed an infection, or died shortly after giving birth.

But no associations were found between a positive test in babies and the stage of pregnancy (trimester) when the mother was infected, preterm birth, mode of delivery, breastfeeding, or mother-baby separation at birth.

What's more, positivity rates varied between regions, ranging from 0.1% in studies from North America to 5.7% in studies from Latin America and the Caribbean.

The researchers acknowledge some limitations that may have affected their results, including differences in definitions of severe and mild covid-19 as well as the types, timing and accuracy of tests used.

Nevertheless, their extensive analyses enabled them to assess the robustness of their findings according to the timing of maternal infection and testing in babies, and across regions. And as this is a living systematic review, the findings can be updated regularly as new evidence emerges.

As such, they conclude: "The overall rates of SARS-CoV-2 positivity in babies born to mothers with SARS-CoV-2 infection is low" and that "severe maternal covid-19 may be associated with SARS-CoV-2 positivity in babies, but not vaginal delivery, breastfeeding, or mother-baby contact after birth."

It is important to consider the changing landscape of the covid-19 pandemic, they add, "including the prevalence of covid-19 in various regions, the impact of vaccination, and the effects of known and emerging SARS-CoV-2 variants on mother-to-child transmission."

Overall, the findings from this review seem reassuring, but they also highlight the paucity of high quality data on risks to infants from covid-19, says Catherine McLean Pirkle in a linked editorial.

While there is currently still no evidence that changes need to be made in postnatal care best practices, she points out that hospital deliveries often put new mothers and babies in contact with dozens of people, so pinpointing the source of infection in newborn babies is challenging.

The variation found between regions also suggests that when proper preventive measures are taken during intrapartum and early postpartum periods, such as consistent and appropriate use of personal protective equipment, infection of newborn babies is unlikely, she adds.

"Given that vaccines are not available for babies and young children, it is critical that better data become available to inform appropriate shared decision making on perinatal care between parents and healthcare providers," she concludes.