Did vaccination reduce COVID-related preterm births?
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Before the vaccine rollout, unexpectedly high preterm birth rates (15.5%) were found among mothers with COVID-19 residing in privileged areas of California.

December 03, 2023 09:15 pm | Updated December 04, 2023 08:34 pm IST

The relative risk of preterm birth was just 1.2% between infected and non-infected mothers in California

The relative risk of preterm birth was just 1.2% between infected and non-infected mothers in California | Photo Credit: Getty Images/iStockphoto

In the early part of the pandemic, two sociologists found that mothers in California with COVID-19 had a greater risk of preterm delivery, as per a study published in the Proceedings of the National Academy of Sciences. The risk increased from 7.1% among non-infected mothers to 8.3% in those with COVID-19, a relatively small difference of 1.2% that the authors dramatically describe as “roughly equivalent to in utero exposure to high-intensity wildfire smoke for 20 days”. 

The study took an interesting twist as they tracked vaccine rollout in California, and divided the State into five zones based on vaccination coverage. The apparent adverse impact of COVID on preterm births diminished quickly in places where vaccines were widely accepted. The difference persisted for almost another year in areas that had low vaccine acceptance. 

The impact eventually vanished in January 2022, which coincided with the arrival of Omicron and widespread population immunity. Based on these observations, the authors state that vaccination prevented a large number of preterm births during the pandemic.

While this is no doubt a valid argument, the topic of preterm birth is rather complicated. Many factors influence the date of delivery, including patterns of obstetric practice during the pandemic. That a baby is born preterm need not necessarily be a direct effect of SARS-CoV-2 infection. In obstetrics, doctors consider multiple factors and make informed decisions about when to induce labour or perform a caesarean section. For instance, if a pregnant woman has severe COVID, for instance due to the delta variant, doctors might decide to deliver the baby as early as possible in an attempt to save the mother’s life. If that happens even one day before the 37th week, it gets counted as preterm, but this need not necessarily compromise the baby’s health. 

Before the vaccine rollout, unexpectedly high preterm birth rates (15.5%) were found among mothers with COVID-19 residing in privileged areas of California. The substantial 3.5% increase (compared to less vaccinated areas) suggests that doctors in well-served, highly vaccinated regions may have been extra cautious about maternal health.

A study by Northwestern University Chicago found no rise in preterm births during the pandemic, and oddly, a decrease in preterm deaths among low-income groups. The explanation is that despite economic challenges, government support allowed these women to stay home, and telemedicine improved healthcare access. This led to better maternal health.

Studies from India have reported mixed results. Tripathy et al. in Orissa found a higher frequency of preterm births among unvaccinated women compared to vaccinated. In a pre-vaccine era study from Mumbai by Malik et al., risk of preterm births among COVID-positive pregnant women was low, at only 4.3%. Kumari et al. from Jharkhand reported that being COVID-positive in 2020 made no difference in preterm birth rates, while some studies report otherwise. 

Experienced obstetricians and neonatologists agree that during the early immune naive phase of the pandemic, vaccination reduced severe COVID-19 outcomes among pregnant women, but the impact on preterm births is less clear.

In science, correlation need not always mean causation. There could well be additional unmeasured variables, perhaps socio-economic in nature that explain the observed difference in preterm births between highly vaccinated areas and less vaccinated areas.

(Rajeev Jayadevan is co-chairman, National IMA COVID Task Force)

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