Association of COVID-19 Vaccination During Pregnancy With Incidence of SARS-CoV-2 Infection in Infants

In 2021, research conducted by Cui et al., indicated that the risk of critical illness resulting from COVID infection in infants younger than one year was significantly higher than in older children who contracted the disease.²

Research question & objectives

In this recent paper (JAMA Internal Medicine,1 June 22), Carlsen et al., set out to determine how effective maternal antibody transfer would be, in mothers who had been vaccinated in their second or third transfer, in reducing the incidence of SARS COV-2 infection during the first four months of their infant's life. As a secondary objective, the researchers wanted to determine if there was any difference between the efficacy of the maternal antibodies against Delta and Omicron variants

Methodology

The trial - a nationwide cohort study - was conducted on all live births in Norway between September 1, 2021, and February 28, 2022.  Infants born to mothers who “…had received a second or third dose of an mRNA vaccine after gestational day 83 and up to 14 days before delivery”¹, were considered as the sample group for the vaccinated mothers. 

Excluded from the trial were: mothers who had received a third or fourth vaccination 7-13 days before delivery; those who had received vaccinations before falling pregnant or had only received one vaccination during their pregnancy, and those who received vaccination outside of the country.

To assess associations between maternal vaccination, even if not fully vaccinated, and the incidence of a positive PCR test for SARS-CoV-2 among infants, a secondary analysis was conducted in which the researchers compared ..the risk of a positive SARS-CoV-2 test in infants born to unvaccinated women with those with mothers who had received a first dose of an mRNA vaccine during the second or third trimester.” ²

The incidence rates of SARS-COV-2 infection in all infants within 4 months of maternal vaccination were reviewed. The incidence of infants admitted to the hospital with a diagnosis of COVID before April 2022 was noted. 
The infants were further divided into two groups by follow-up time: The first group included those infants followed up until Dec 2021 in what was considered the Delta-dominated period, and the second group, occurring after this date, who were considered Omicron dominated.  

Findings 

A total of 21 643 live infants were born during the period under review. 9739 (45.0%) were born to women who received a second or third dose of a COVID-19 vaccine during pregnancy. 

The results as recorded by researchers were as follows:
 

“The first four months of life incidence rate of a positive test for SARS-CoV-2 was 5.8 per 10 000 follow-up days. Infants of mothers vaccinated during pregnancy had a lower risk of a positive test compared with infants of unvaccinated mothers and lower risk during the Delta variant–dominated period (incidence rate, 1.2 vs 3.0 per 10 000 follow-up days; adjusted hazard ratio, 0.29; 95% CI, 0.19-0.46) compared with the Omicron period (incidence rate, 7.0 vs 10.9 per 10 000 follow-up days; adjusted hazard ratio, 0.67; 95% CI, 0.57-0.79). The crude incidence rates for a positive SARS-CoV-2 test were 7.0 per 10 000 follow-up days among infants born to vaccinated mothers and 10.9 per 10 000 follow-up days among infants born to unvaccinated mothers (aHR, 0.67; 95% CI, 0.57-0.79). We observed no violation of the proportional hazard assumption.”¹

Conclusion

The researchers concluded that there was a positive association between COVID-19 vaccination and reduced risk of an infant receiving a positive PCR test for Covid within the first four months of life. The association was found to be stronger in the Delta period than in the Omicron period. 

Overall the findings indicate that there was evidence to suggest that the passive protection obtained following material mRNA vaccination during pregnancy was of benefit in preventing the risk of the disease in these vulnerable infants.

Access the original research here

Carlsen EØ, Magnus MC, Oakley L, et al. Association of COVID-19 Vaccination During Pregnancy With Incidence of SARS-CoV-2 Infection in Infants. JAMA Intern Med. Published online June 01, 2022. doi:10.1001/jamainternmed.2022.2442

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