Children who have had COVID-19 aren't protected against the omicron variant, but vaccination does cut their chances of infection, a new study shows.

"I hear parents say, 'Oh, my kid had COVID last year,'" said senior study co-author Dr. Adrienne Randolph, from Boston Children's Hospital.

"But we found that antibodies produced by prior infections in children don't neutralize omicron, meaning that unvaccinated children remain susceptible to omicron," she added in a hospital news release.

For the study, Randolph and her colleagues analyzed blood samples from 62 children and teens hospitalized with severe COVID-19, 65 children and teens hospitalized with COVID-related multisystem inflammatory syndrome in children (MIS-C), and 50 youngsters who had mild COVID-19 and weren't hospitalized.

All the blood samples were taken during 2020 and early 2021, before the emergence of the omicron variant.

The researchers conducted lab tests to determine how well antibodies in the samples neutralized five COVID variants of concern: alpha, beta, gamma, delta, and omicron.

Overall, the samples showed some loss of antibody cross-neutralization against all five variants, but the loss was most pronounced for omicron, according to the study published online May 27 in the journal Nature Communications.

"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."

But the researchers found that children and teens who had received two doses of COVID-19 vaccine had higher levels of neutralizing antibodies against the five variants, including omicron.

Randolph said she hopes the findings will encourage parents to have their children and teens vaccinated.

Only 28% of U.S. children aged 5 to 11 and only 58% of youth aged 12 to 17 had received two doses of COVID-19 vaccine as of May 18, according to the U.S. Centers for Disease Control and Prevention.

A U.S. Food and Drug Administration advisory panel is set to meet on June 14-15 to consider authorization of COVID-19 vaccines for children under age 5.