COVID-19 hospitalization rates in infants increased during omicron

COVID-19-associated hospitalization rates increased among infants during the omicron-predominant period compared with the delta-predominant period, according to a new MMWR.

In light of the findings, researchers said prevention efforts should focus on vaccination during pregnancy, which might confer some protection in infants.

A growing body of research shows that COVID-19 vaccines are safe and provide benefits at any time during pregnancy for both mothers and newborns. A meta-analysis revealed that receiving a COVID-19 vaccine while pregnant did not increase the risk for peripartum outcomes and may in fact lower the infant’s risk for NICU admission and intrauterine fetal death.

The new report indicates that the rates of COVID-19-related hospitalizations are higher for infants aged younger than 6 months — who are not eligible for vaccination — compared with those of any other age group, save for those aged 65 years or older.

To protect themselves and their infants, “both CDC and the American College of Obstetricians and Gynecologists recommend COVID-19 vaccination for women who are pregnant, breastfeeding, trying to get pregnant now, or might become pregnant in the future,” Sarah Hamid, PhD, of the CDC’s National Center for Immunization and Respiratory Diseases and Epidemic Intelligence Service, and colleagues wrote.

For the study, Hamid and colleagues analyzed data from the COVID-19–Associated Hospitalization Surveillance Network, which “conducts population-based surveillance for laboratory-confirmed COVID-19–associated hospitalizations,” the researchers wrote. They specifically sought to understand changes in hospitalizations during the omicron-predominant period, from March 20 to Aug. 31.

Hamid and colleagues found that, from April 9 to July 23, weekly hospitalization rates per 100,000 infants aged younger than 6 months increased elevenfold (95% CI, 4.3-33.3), from a nadir of 2.2 to a peak of 26. The average weekly hospitalization rate was 13.7 per 100,000 infants, which was similar to that of people aged 65 and 74 years, who had the highest hospitalization rate at 13.8 per 100,000 population.

Among the 473 infants who were hospitalized during this period, 84% had COVID-19-related symptoms, 39% were birth hospitalizations and 38% were aged younger than 1 month. Notably, “at least one underlying medical condition was present in 26% of hospitalized infants aged 1 to 2 months and 36% of those aged 3 to 5 months,” with prematurity being the most frequently reported underlying condition, according to Hamid and colleagues.

The researchers wrote that “multiple factors likely contributed to high COVID-19–associated hospitalization rates among young infants during the omicron variant–predominant period,” like community transmission, the high infectivity of the variant “and the relatively low threshold for hospitalizing infants for signs and symptoms consistent with COVID-19 (eg, fever) relative to that in older children.”

Getting vaccinated during pregnancy is estimated to be about 52% effective against COVID-19 hospitalization for infants aged younger than 6 months, the researchers noted, suggesting “that young infants might receive protection through passive transplacental transfer of maternal antibodies acquired through maternal vaccination.”

However, they also wrote that the “effectiveness of maternal vaccination in preventing disease in young infants was lower during the early Omicron variant–predominant period (38%) than during the delta variant–predominant period (80%).”

“Because of immune evasion when novel variants have emerged and waning immunity as time since the last dose increases, infants born during the omicron BA.5 variant–predominant period might have had less protection,” they wrote.

Though the results are troubling, Hamid and colleagues wrote that the findings “do not suggest increased severity of COVID-19 among hospitalized infants.”

“To help protect both pregnant women and infants too young to be vaccinated, prevention should focus on ensuring that pregnant women stay up to date on COVID-19 vaccines (including receiving a bivalent booster dose),” the researchers concluded.