Children who previously had COVID-19 or the inflammatory condition MIS-C, associated with the virus, are not protected against the newer Omicron variant, finds a study.

The study, published in Nature Communications, showed that vaccination, however, does afford protection. Though COVID was rare and mild in children, some of those infected faced severe multisystem inflammatory syndrome (MIS).

The US Centers for Disease Control and Prevention (CDC) defines MIS-C as a condition where different body parts can become inflamed, including the heart, lungs, kidneys, brain, skin, eyes, or gastrointestinal organs.

"I hear parents say, 'Oh, my kid had COVID last year'. But we found that antibodies produced by prior infections in children don't neutralise Omicron, meaning that unvaccinated children remain susceptible to Omicron," said Adrienne Randolph, from the Boston Children's Hospital.

The researchers obtained blood samples from 62 children and adolescents hospitalised with severe COVID, 65 children and adolescents hospitalised with MIS-C, and 50 outpatients who had recovered from mild COVID-19. All the samples were taken during 2020 and early 2021, before the emergence of the Omicron variant.

In the laboratory, they exposed the samples to a pseudovirus (derived from SARS-CoV-2, but stripped of its virulence), and measured how well antibodies in the samples were able to neutralise five different SARS-CoV-2 variants of concern: Alpha, Beta, Gamma, Delta, and Omicron.

Overall, children and adolescents showed some loss of antibody cross-neutralisation against all five variants, but the loss was most pronounced for Omicron. "Omicron is very different from previous variants, with many mutations on the spike protein, and this work confirms that it is able to evade the antibody response," Randolph said. "Unvaccinated children remain susceptible."

In contrast, children who had received two doses of the COVID-19 vaccine showed higher neutralising antibody titers against the five variants, including Omicron. Randolph hopes these data will encourage parents to have their children and teenagers vaccinated. The Food and Drug Administration panel will meet on June 15 to consider the authorisation of COVID vaccines for children under age 5.