COVID-19 hospitalization rates among US infants younger than 6 months rose during Omicron variant predominance compared to the Delta period, but indicators of severe infection didn't, according to a study published today in Morbidity and Mortality Weekly Report.
A team led by researchers from the Centers for Disease Control and Prevention (CDC) analyzed data from the Coronavirus Disease 2019-Associated Hospitalization Surveillance Network on infant COVID-19 hospitalizations in 13 states from Jun 20, 2021, to Aug 31, 2022. The study spanned the Delta-predominant (Jun 20 to Dec 18, 2021) and the Omicron (Dec 19, 2021, to Aug 31, 2022) waves.
The researchers noted that hospitalizations of children younger than 5 years increased faster than those in other age-groups. On Jun 22, 2022, the (CDC began recommending COVID-19 vaccination of children 6 months and older. Infants remain ineligible.
An 11-fold climb from April to July 2022
Weekly COVID-19 hospitalizations of infants younger than 6 months rose 11-fold from a low point of 2.2 the week of Apr 9, 2022, to a peak of 26.0 the week of Jul 23 and then declined.
The average weekly hospitalization rate in infants was higher during the Omicron BA.2/BA.5 subvariant period (13.7) than during Delta (8.3) (rate ratio [RR], 1.6). Relative to the Delta period, rates were also higher during BA.2/BA.5 among children 6 months to 4 years old (RR, 1.9) and adults 75 and older (RR, 1.4) but lower among children 5 to 17 (RR, 0.9), adults 18 to 64 (RR, 0.5), and adults 65 to 74 (RR, 0.8).
The average weekly hospitalization rate among infants during the Omicron BA.2/BA.5 wave (13.7) was less than that of adults 75 and older (39.4), similar to that of those 65 to 74 (13.8) and higher than that in other preschoolers (2.3 and 0.8 for children aged 6 to 23 months and 2 to 4 years, respectively) and in adults younger than 65.
Indicators of severe disease among 1,116 hospitalized infants younger than 6 months with clinical data were generally lower during Omicron than Delta, and in-hospital deaths were rare, at less than 1%. Such indicators included length of hospital stay, the proportion of intensive care unit admissions, the use of supplemental oxygenation via high-flow nasal cannula or bilevel positive airway pressure/continuous positive airway pressure, and mechanical ventilation.
Among 473 infants hospitalized during Omicron, 84% had COVID-19 symptoms, and 38% were younger than 1 month; 39% were birth hospitalizations. Fully 87% of infants who tested positive during their birth hospitalization showed no symptoms. Similar proportions of infants with non-birth hospitalizations had symptoms, including 94% of those younger than 1 month, 97% of those aged 1 to 2 months, and 96% of those aged 3 to 5 months.
Twenty-six percent of hospitalized infants 1 to 2 months old and 36% of those aged 3 to 5 months had at least one underlying medical condition. The most common underlying condition was prematurity, at 20% of those aged 1 to 2 months and 25% of those 3 to 5 months. Most infants had a fever at hospitalization (74% of those 1 to 2 months and 68% of those 3 to 5 months old.
Importance of maternal vaccination
Multiple factors may be behind the high COVID-19 hospitalization rate among infants younger than 6 months during Omicron, including the high infectivity and community transmission of that variant and the relatively low threshold for hospitalizing infants for COVID-19 signs and symptoms in infants, the researchers said.
"High relative hospitalization rates in infants compared with older children, adolescents, and adults aged <65 years during the Omicron BA.2/BA.5 variant–predominant period also reflect lower rates of hospitalization in these other age groups compared with those during the Delta variant–predominant period, as immunity in older age groups has increased through vaccination, previous infection, or both," they wrote.
The authors noted that maternal COVID-19 vaccination has been proven to protect infants younger than 6 months, and both the CDC and the American College of Obstetricians and Gynecologists recommend the vaccine for women who are pregnant, breastfeeding, or planning a pregnancy.
"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," the authors wrote.