About two in three children ages one to four in the United States have been infected with SARS-CoV-2, according to a national analysis.
Infections in that age group increased more than any other during the Omicron wave, which researchers say demonstrates the variant’s high transmissibility.
Researchers looked for COVID-19 antibodies in blood samples from more than 86,000 children under the age of 18, including about 6,100 children between the ages of one and four. In the smallest ones, the number of infections more than doubled, from 33% to 68% between December 2021 and February 2022.
Although the analysis involved a small number of very young children, the results are consistent with the rapid increase in infections documented in that age group, says Pamela Davis, a physician and medical researcher at Case Western Reserve University in Cleveland, Ohio.
Overall, the researchers found that most children ages 1 to 17 had likely been infected by February of this year. Infections in children aged 5 to 11 years reached the highest level, 77%. Infection rates in children exceed those seen in adults.
These are important findings, especially for low- and middle-income countries where vaccination rates are low in adult populations and where children likely won’t be vaccinated for some time, says Fiona Russell, a pediatrician and infectious disease epidemiologist at the University from Melbourne in Australia.
The immunity generated by an infection could help prevent future infections and serious illnesses in children, but “the pandemic will not end until children around the world are also offered vaccination.”
Missed infections
Reported cases of COVID-19 in the United States suggest that about 17% of children under the age of 18 have been infected. “That’s just the tip of the iceberg,” says Kristie Clarke, a medical epidemiologist at the U.S. Centers for Disease Control and Prevention in Atlanta, Georgia, who led the study, which was published online this week without Peer Review. The reported cases, based on PCR and antigen testing, far underrepresent the true infection rate, especially in children because many asymptomatic and mild infections were likely not tested.
To assess the scale of unrecorded infections, Clarke and colleagues looked for the presence of SARS-CoV-2 antibodies in leftover blood samples taken during doctor visits between September 2021 and February 2022.
The antibodies they looked for target a specific protein on SARS-CoV-2 that is not present in the COVID-19 vaccines used in the United States, so they can know that children gained immunity from the vaccine. infection, rather than vaccination.
High infection rates in children are reflected in US hospitalization data. The number of children under five admitted to hospital with COVID-19 during the peak of the Omicron wave was 5 times higher than in the peak of the Delta wave, and intensive care admissions were 3.5 times higher.
Many factors could explain the rise in infections in toddlers and preschoolers, Clarke says, including that children under five are not eligible for vaccination and are less likely to wear masks or practice social distancing than older children and grown ups.
The numbers are both surprising and "may well have underestimated the strength of infection during the Omicron wave," says Shabir Mahdi, a vaccinologist at the University of the Witwatersrand, South Africa. Antibody tests can’t determine if someone has been infected multiple times and may have missed some infections because the number of antibodies declines over time, he says. The antibody test used in the study captured only about 80% of the infections that could be detected with a different antibody test, Madhi says.
Saving children
Studies suggest that the risk of serious illness is lower in children infected with Omicron than with Delta. A US study by Davis and Rong Xu, a data scientist also at Case Western Reserve University, looked at more than 650,000 children under the age of five and found that their risk of contracting a serious illness during Omicron wave was about a third of that during the Delta wave, measured by emergency hospital visits between November 2021 and January 2022. Xu says it will be important to watch the long-term effects of the surge in children.
Until now, hospitalization rates in children have been much lower than those seen in older age groups, Madhi says. “The children have been saved,” she says. “We should be grateful for that.”
