Abstract and Introduction
The B.1.1.529 (Omicron) variant of SARS-CoV-2, the virus that causes COVID-19, has been the predominant circulating variant in the United States since late December 2021.* Coinciding with increased Omicron circulation, COVID-19–associated hospitalization rates increased rapidly among infants and children aged 0–4 years, a group not yet eligible for vaccination. Coronavirus Disease 19–Associated Hospitalization Surveillance Network (COVID-NET)† data were analyzed to describe COVID-19–associated hospitalizations among U.S. infants and children aged 0–4 years since March 2020. During the period of Omicron predominance (December 19, 2021–February 19, 2022), weekly COVID-19–associated hospitalization rates per 100,000 infants and children aged 0–4 years peaked at 14.5 (week ending January 8, 2022); this Omicron-predominant period peak was approximately five times that during the period of SARS-CoV-2 B.1.617.2 (Delta) predominance (June 27–December 18, 2021, which peaked the week ending September 11, 2021).§ During Omicron predominance, 63% of hospitalized infants and children had no underlying medical conditions; infants aged <6 months accounted for 44% of hospitalizations, although no differences were observed in indicators of severity by age. Strategies to prevent COVID-19 among infants and young children are important and include vaccination among currently eligible populations such as pregnant women, family members, and caregivers of infants and young children.
COVID-NET conducts population-based surveillance for laboratory-confirmed COVID-19–associated hospitalizations in 99 counties across 14 U.S. states.¶ Among residents of a predefined surveillance catchment area, COVID-19–associated hospitalizations are defined as receipt of a positive SARS-CoV-2 real-time reverse transcription–polymerase chain reaction or rapid antigen detection test result during hospitalization or during the 14 days preceding admission. This analysis describes weekly hospitalization rates among infants and children aged 0–4 years during March 1, 2020–February 19, 2022, which includes the pre-Delta–, Delta- and Omicron-predominant periods; detailed clinical data were available through January 31, 2022. Unadjusted weekly COVID-19–associated hospitalization rates were calculated by dividing the total number of hospitalized patients by the population estimates within each age group for the counties included in the surveillance catchment area.** All rates are estimated per 100,000 infants and children aged 0–4 years. Rate ratios (RR) comparing Omicron- and Delta-predominant periods and 95% CIs were calculated. Three-week moving averages are presented for visualization purposes.
Trained surveillance staff conducted medical chart abstractions for all pediatric COVID-NET patients using a standardized case report form during March 2020 through November 2021. Because of the large surge in hospitalizations during December 2021 and January 2022, some sites examined clinical data on a representative sample of hospitalized infants and children.†† Data regarding primary reason for hospital admission,§§ symptoms at admission,¶¶ underlying medical conditions, and indicators of severe disease (i.e., hospital length of stay, intensive care unit [ICU] admission, need for respiratory support,*** and in-hospital death) were collected. Data on viral codetections (respiratory syncytial virus [RSV], influenza, rhinovirus/enterovirus, and other viruses)††† were collected for infants and children who received additional testing. Monthly ICU admission rates were calculated. Proportions were compared between periods of pre-Delta predominance (March 1, 2020–June 26, 2021), Delta predominance (June 27–December 18, 2021), and Omicron predominance (December 19, 2021–January 31, 2022); a variant that accounted for >50% of sequenced isolates was considered predominant. For the period of Omicron predominance, proportions were compared by age (<6 months, 6–23 months, and 2–4 years). Wilcoxon rank-sum tests and chi-square tests were used to compare medians and proportions, respectively; p-values <0.05 were considered statistically significant. Percentages were weighted to account for the probability of selection for sampled cases and adjusted to account for nonresponse. Data were analyzed using SAS (version 9.4; SAS Institute). This activity was reviewed by CDC and was conducted consistent with applicable federal law and CDC policy.§§§
During March 1, 2020–February 19, 2022, weekly hospitalization rates (hospitalized patients per 100,000 infants and children aged 0–4 years) peaked during Omicron predominance, in the week ending January 8, 2022, at 14.5. This peak hospitalization rate during Omicron predominance was approximately five times the peak during Delta predominance (2.9) (week ending September 11, 2021) (RR = 5.0; 95% CI = 3.8–6.8). Hospitalization rates among infants aged <6 months were approximately six times as high during the peak week of Omicron predominance (68.1) as during Delta predominance (11.1) (RR = 6.1; 95% CI = 3.9–10.0); Omicron-predominant versus Delta-predominant hospitalization RRs were also elevated among infants and children aged 6–23 months (16.9 versus 3.3; RR = 5.1; 95% CI = 3.1–8.5) and 2–4 years (4.7 versus 1.4; RR = 3.5; 95% CI = 2.0–6.3) (Figure). Monthly ICU admission rates were approximately 3.5 times as high during the Omicron predominance peak in January 2022 (10.6) as during the Delta predominance peak in September 2021 (3.0). Hospitalization rates among infants and children aged 0–4 years decreased by the week ending February 19, 2022 (3.9).
COVID-19–associated hospitalization rates* among infants and children aged 0–4 years, by age group (3-week moving average) — Coronavirus Disease 2019–Associated Hospitalization Surveillance Network, 14 states,† March 2020–February 2022§
Abbreviation: COVID-NET = Coronavirus Disease 2019–Associated Hospitalization Surveillance Network.
*Number of patients with laboratory-confirmed COVID-19–associated hospitalizations per 100,000 population; rates are subject to change as additional data are reported.
†COVID-NET sites are in the following 14 states: California, Colorado, Connecticut, Georgia, Iowa, Maryland, Michigan, Minnesota, New Mexico, New York, Ohio, Oregon, Tennessee, and Utah. Starting the week ending December 4, 2021, Maryland data are removed from weekly rate calculations.
§Periods of predominance are defined as follows: pre-Delta = March 1, 2020–June 26, 2021; Delta = June 27–December 18, 2021; Omicron = December 19, 2021–February 19, 2022.
Complete clinical data were available for 97% (2,562 of 2,637) of hospitalized infants and children aged 0–4 years, including 99% (1,200 of 1,209), 94% (790 of 841), and 97% (572 of 587) hospitalized during the pre-Delta–, Delta- and Omicron-predominant periods, respectively (Table 1). Although there was some variation across periods, most patients had COVID-19–related symptoms recorded at admission (87%) and COVID-19 as the primary reason for admission (85%). During Omicron predominance, 37% of hospitalized infants and children had one or more underlying medical condition.¶¶¶ Among 62% (1,582 of 2,562) of infants and children with testing for additional viral pathogens,**** the proportion hospitalized with RSV codetections was significantly higher during Delta predominance (20%) than during Omicron predominance (7%) (p<0.001). Compared with Delta predominance, hospital length of stay during Omicron predominance was shorter (2 versus 1.5 days, p = 0.002) and the proportion of hospitalized infants and children requiring ICU admission (27% versus 21%, p = 0.02) was lower.
During Omicron predominance, 44% of hospitalized infants and children aged 0–4 years were infants aged <6 months, similar to proportions during the Delta- (43%) and pre-Delta–predominant (46%) periods. Among 252 hospitalized infants aged <6 months, 146 (58%) were aged <2 months, 30 (21%) of whom received a diagnosis of COVID-19 during their birth hospitalization. A smaller proportion of infants and children aged <6 months was hospitalized with COVID-19–related symptoms at admission (82%) than the proportion aged 6–23 months (92%) or 2–4 years (89%) (Table 2), although no difference was observed when birth hospitalizations (91% of which were asymptomatic infections) were excluded. Approximately one half (51%) of hospitalized infants aged <6 months were febrile at the time of admission, including 44% of those aged <2 months and 61% of those aged 2–5 months. A higher proportion of infants aged <6 months (13%) were hospitalized with RSV codetections than were older infants and children (6–23 months = 4%; 2–4 years = 2%). Length of stay, ICU admission, and need for respiratory support did not significantly differ by age group.
Morbidity and Mortality Weekly Report. 2022;71(11):429-436. © 2022 Centers for Disease Control and Prevention (CDC)