COVID-19 hospitalization rates among infants younger than 6 months jumped during the Omicron wave, outpacing hospitalizations during the earlier Delta wave, CDC researchers reported.
During the months when the Omicron BA.2/BA.5 subvariants were dominant, weekly hospitalizations per 100,000 infants younger than 6 months increased 11-fold (95% CI 4.3-33.3), rising from a low of 2.2 during the week ending April 9 to a peak of 26.0 during the week ending July 23 before then declining again, noted Sarah Hamid, PhD, of the CDC’s National Center for Immunization and Respiratory Diseases, and colleagues in the Morbidity and Mortality Weekly Report.
The average weekly hospitalization rate per 100,000 for this youngest demographic during the Omicron BA.2/BA.5-predominant periods — 13.7 — was higher than the rate of 8.3 during the Delta-dominant period (rate ratio RR 1.6, 95% CI 1.4-1.8), and was similar to rates among adults ages 65 to 74 (13.8), and higher than rates among all other age groups, except for adults ages 75 and older, which were far higher, at 39.4 per 100,000.
COVID-19 “can and does cause severe and fatal outcomes in children, including infants,” Hamid and team wrote. “To help protect infants too young to be vaccinated, prevention should focus on nonpharmaceutical interventions and vaccination of pregnant women, which might provide protection through transplacental transfer of antibodies.”
Curiously, the share of hospitalized infants with indicators of severe disease — such as longer hospital length of stay, intensive care unit admission, and need for respiratory support — were “consistently lower” during the Omicron BA.2/BA.5 periods compared with the Delta period, the authors noted.
Still, there is “continued risk for COVID-19-associated hospitalization among infants aged <6 months, who are ineligible for vaccination," they stressed.
Possible reasons for the higher hospitalization rates among young infants during the Omicron-dominant periods include high community transmission with these subvariants, as well as the “relatively low threshold” for hospitalizing infants with signs and symptoms linked to COVID-19 compared with older children, the authors said.
Moreover, increased immunity in older age groups due to vaccination, previous infection, or both could have played a role, they added.
Hamid and colleagues used data from the Coronavirus Disease 2019-Associated Hospitalization Surveillance Network (COVID-NET) to look at weekly COVID-19-associated hospitalization rates and clinical characteristics among infants younger than 6 months across the following four periods:
- the Delta-dominant period (June 20, 2021 to Dec. 18, 2021)
- the Omicron BA.1-dominant period (Dec. 19, 2021 to March 19, 2022)
- the Omicron BA.2-dominant period (March 20 to June 18, 2022)
- the Omicron BA.5-dominant period (June 19 to Aug. 31, 2022)
They combined the BA.2 and BA.5 periods due to small sample sizes.
Demographic data related to COVID-19-associated hospitalizations were collected from 13 states. Clinical data — such as symptoms during admission, underlying medical problems, and indicators of severe diseases — were only available for infants younger than 6 months in 12 states.
In all, Hamid and team were able to access complete clinical data for 1,116 hospitalized infants younger than 6 months with laboratory-confirmed COVID-19, including 321, 322, and 473 infants during the Delta, Omicron BA.1, and Omicron BA.2/BA.5 periods, respectively.
Given that population estimates for infants younger than 6 months weren’t available, the authors used data on half of infants younger than 1 year to calculate an estimate, ignoring “seasonality in births,” which was a limitation to the study.
“Births typically peak in the summer, leading to potential small overestimates of rates during Omicron BA.2/BA.5 variant-predominant periods,” they noted.
Another limitation was that changes occurred in public health policies, testing, and treatment over the study period, which the authors said they could not account for. They also did not assess rates of vaccination or previous infection among mothers, which may have given some infants higher immunity. Lastly, the COVID-NET catchment areas represent only about 10% of the U.S. population, so results based on their data may not be generalizable.
Hamid reported no conflicts of interest. One co-author reported multiple relationships with industry, and another reported support from the Council of State and Territorial Epidemiologists for the population-based Influenza Hospitalization Surveillance Project and COVID-NET activities.