In yet another twist to the debate over how best to protect children against the coronavirus, researchers reported on Wednesday that Covid vaccines conferred diminished protection against hospitalization among children 12 and older during the latest Omicron surge.

Vaccine effectiveness against hospitalization held steady in children aged 5 to 11 years, however, and among adolescents ages 12 to 18 years, two doses of the vaccine remained highly protective against critical illness requiring life support.

But effectiveness against hospitalization for less severe illness dropped to just 20 percent among these children. The findings were published in the The New England Journal of Medicine.

The data are broadly consistent with studies showing that, across all age groups, the vaccines lost much of their power against infection with the Omicron variant but still prevented severe illness and death.

While any hospitalization is unnerving, it is reassuring that the vaccines still protected children from the worst outcomes of infection, said Dr. Manish Patel, a researcher at the Centers for Disease Control and Prevention who led the study.

Among adolescents in the study who were critically ill, 93 percent were unvaccinated, and most had at least one underlying condition, Dr. Patel noted. “I think the big take-home message is that with the simple act of vaccination, you can prevent most critical illness in most children,” he said.

As of March 23, only about one in four children ages 5 to 11, and just over half of adolescents 12 to 17, were fully vaccinated in the United States. Those percentages have barely budged in the past few months.

For some parents still debating vaccination, the decision is complicated by the seeming retreat of the coronavirus. Cases and deaths have fallen to their lowest levels in a year, and no one yet knows whether the BA.2 subvariant of Omicron will bring another wave.

Some parents, believing their children’s risk of Covid to be trivial, have been reluctant to vaccinate them from the start. But while children remain much less likely than adults to become seriously ill, many more of them were hospitalized during the Omicron surge than at any other time in the pandemic.

In the new study, the researchers analyzed medical records and interviewed parents of children ages 5 and older who were hospitalized for Covid. They excluded children who tested positive for the coronavirus but had been admitted to the hospital for other reasons.

Because relatively few children are hospitalized for Covid, the researchers were able to identify only 1,185 children, comparing them with 1,627 others who did not have Covid. Among those hospitalized for Covid, 291 received life support and 14 died.

The study included data from 31 hospitals in 23 states, and spanned July 1 to Dec. 18, 2021, when the Delta variant was circulating, and Dec. 19 to Feb. 17, when the Omicron variant was dominant. During the Delta period, effectiveness against hospitalization was more than 90 percent among the adolescents up to 44 weeks after immunization.

During the Omicron surge, however, those numbers dropped sharply to about 40 percent for protection against hospitalization overall, regardless of the time since vaccination.

When the researchers parsed the data by severity of illness, they found that vaccine effectiveness against critical illness among hospitalized adolescents remained high, at 79 percent, but had fallen to 20 percent for less severe illness.

The new study is among the first to look at vaccine effectiveness in relation to severity of illness among hospitalized patients. It’s possible that this trend would appear among adult patients, too, if they were analyzed similarly, said Eli Rosenberg, deputy director for science at the New York State Department of Health.

“This split along critical, noncritical is interesting,” he said. “This definitely adds a new layer.”

In children ages 5 to 11 years, full vaccination had an effectiveness of 68 percent against hospitalization overall. Those data were gathered during the Omicron surge, because these children became eligible for vaccination only on Nov. 2. There were too few to analyze effectiveness by severity of illness.

About 78 percent of all hospitalized adolescents in the study, and 82 percent of younger children, had one or more underlying medical conditions, like obesity, autoimmune diseases or respiratory problems, including asthma.

The study suggests that the vaccine protected a majority of these children from the worst outcomes, said Dr. Luciana Borio, a former acting chief scientist at the Food and Drug Administration.

“It really validates the importance of vaccines for children 5 and older, and especially for those that are immunocompromised or have underlying medical conditions,” she said.

The Omicron variant can partly dodge immune defenses, so it is not surprising that the vaccines did not do as well as against the Delta variant, she and others said. Another recent study showed that in adolescents ages 12 to 17, two doses of the vaccine also offered virtually no defense against moderate illness caused by the Omicron variant. (Booster doses are now recommended for all Americans ages 12 and older.)

The large discrepancy in vaccine effectiveness between those who needed life support and those who did not may be due in part to the wide range of symptoms for which children were hospitalized. About one in four adolescents in the study required life-supporting interventions like mechanical ventilation or extracorporeal membrane oxygenation.

Dr. Marietta Vazquez, an infectious diseases specialist at Yale School of Medicine who was not involved in the study, said that, in her experience, most children who were hospitalized during the Omicron surge recovered quickly.

“The children who we saw admitted — they were either very, very sick, or they were mostly admitted because they were infected and they had high fevers or they had low oxygen saturation,” she said.

Parents also seemed more inclined to bring young children to the hospital during the Omicron surge, Dr. Vazquez added: “There’s such concern and fear about Covid.”

Some researchers have theorized that the decline in vaccine protection among adolescents resulted from waning effectiveness over time — that is, adolescents may not have been as well protected during the Omicron surge because too much time had elapsed since their immunizations.

But the new study found that vaccine effectiveness against the Omicron variant was 43 percent up to 22 weeks after immunization, and 38 percent between 23 and 44 weeks. Waning immunity appeared to be less a factor than the variant itself.

“It looked like it was more Omicron-related,” Dr. Patel said.

Most of the vaccinated adolescents in the new study had received just two doses. There were not enough of those who had received a third dose to evaluate its benefit, but a previous study suggested that a booster shot drastically improved protection against moderate illness in this age group, as it does in adults.

“I really think children should get three doses, and that I hope will raise these numbers,” said Akiko Iwasaki, an immunologist at Yale University. So far, only about 14 percent of children 12 and older have received a booster dose.