Maternal Vaccination and Risk of Hospitalization for Covid-19 Among Infants

COVID-19 during pregnancy is associated with a greater risk of adverse pregnancy outcomes and neonatal complications. Studies have shown that mRNA vaccines are highly effective in preventing severe COVID-19 during pregnancy, and the Centers for Disease Control and Prevention recommends vaccination, including boosters, for people who are pregnant or plan to be pregnant. The benefits of maternal vaccination are 2-fold: it provides pregnant people with protection again the virus and may transfer protection to their infants who are not eligible for vaccination. In the vaccinated pregnant person, maternal antibodies have been found in cord blood, breast milk, and infants' serum specimens, suggesting the transfer of maternal antibodies to infants. Antibody titers in infants are highest when maternal vaccination occurs late in the second or early third trimester of pregnancy. Other studies have found that at the peak of circulation for the omicron variant, rates of hospitalization for infants <6 months were 6 times as high as the rates during the peak of the delta variant. COVID-19 was the primary reason for hospitalization in 85% of infants. A related previous study, conducted during the circulation of the delta variant, found a 61% reduced risk of hospitalization for COVID-19 among infants <6 months when mothers were fully vaccinated with 2 doses of an mRNA vaccine during pregnancy. The aim of this study was to assess the effectiveness of maternal COVID-19 vaccination in preventing hospitalization in infants <6 months of age. This was a case-control, test-negative study conducted in 30 pediatric hospitals across 22 states. The case group included infants <6 months of age, who were hospitalized for COVID-19 or acute COVID-19-associated symptoms. The control group included those who were hospitalized without the virus. Excluded were infants born to women who had been partially vaccinated, vaccinated before or after pregnancy, vaccinated <2 weeks before delivery, had a booster or received a non-mRNA vaccine. To determine the effectiveness of vaccination, the odds of full maternal vaccination-defined as the completion of the 2-dose series of an mRNA vaccine-were compared between the case group and control group during the circulation of the delta variant (July 1, 2021, to December 18, 2021) and the omicron variant (December 19, 2021, to March 8, 2022). A total of 537 case infants and 512 control infants were included in the analysis. Among the case infants, 181 were hospitalized during the delta period and 356 during the omicron period. Also, 21% of case infants were admitted to the intensive care unit, and 12% received mechanical ventilation or vasoactive infusions. Two case infants whose mothers were not vaccinated during pregnancy died. Overall, maternal vaccination reduced the risk of hospitalization by 52% (95% confidence interval [CI], 33%-65%). During the delta period, the risk was reduced by 80% and during the omicron period by 38% (95% CIs, 60%-90% and 8%-58%, respectively). During the delta period, maternal vaccination reduced the risk of hospitalization by 47%(95% CI, 25%-62%) and the risk of ICU admission by 70%(95% CI, 42%-85%). When vaccination occurred during the first 20 weeks of pregnancy, the risk of hospitalization for COVID-19 among infants <6 months decreased by 38% (95% CI, 3%-60%), compared with a decreased risk of 69%(95% CI, 50%-80%) when vaccination occurred after 20 weeks. In conclusion, maternal vaccination during pregnancy-with the completion of the 2-dose series of an mRNA vaccine-was associated with a decreased risk of hospitalization and other neonatal complications related to COVID-19 among infants <6months of age. Vaccination was shown to be more effective during the delta-predominant period than the omicron-predominant period. These findings provide additional support for COVID-19 vaccination during pregnancy.