Vaccine protection against moderate illness waned among adolescents, new C.D.C. data suggest.

Five months after immunization, two doses of the Pfizer-BioNTech vaccine appeared to offer virtually no defense against moderate illness caused by the Omicron variant — as measured by visits to emergency departments and urgent care clinics — among adolescents aged 12 to 17 years, according to data published on Tuesday by the Centers for Disease Control and Prevention.

But booster shots drastically increased the protection, lending support to the agency’s recommendation of booster shots for everyone 12 and older.

The findings must be interpreted with caution. The agency’s study did not exclude unvaccinated adolescents who had some immunity from a prior infection, which may have made vaccination seem less effective than it was.

And the researchers offered only limited data on hospitalizations, a more reliable proxy for severe disease than emergency room and urgent care visits.

“One limitation of this data is that parents may bring their children to an urgent care or emergency department for a variety of reasons, and vaccine effectiveness by immunocompromised status, underlying health status, or vaccine product have not yet been examined,” the C.D.C. said in a statement.

Several studies have shown that even though vaccine efficacy against infection wanes over time, the immune response remains highly protective against hospitalization and death, even against the highly contagious Omicron variant.

A separate analysis of data from 29 jurisdictions posted on the C.D.C.’s website reported nine Covid-associated deaths among vaccinated children and adolescents aged 5 to 17 between early April 2021 and January 2022, compared with 121 deaths in unvaccinated children of those ages.

Still, the findings suggest that scientists must carefully monitor the vaccine’s performance over time in children and adolescents, bearing in mind that boosters may be needed.

“We need to see more of these studies to see if this is consistent,” said Deepta Bhattacharya, an immunologist at the University of Arizona. “But I do think it’s likely, and we should be prepared as parents, that it’s going to take another shot.”

The results take on particular import for parents as school districts nationwide consider ending mask mandates. The C.D.C. last week published new guidance suggesting that about 70 percent of Americans can safely drop their masks in public indoor spaces.

Vaccine uptake among young children has been slow; fewer than one in four children aged 5 to 11 are now fully vaccinated. More than half of adolescents 12 to 17 have been fully vaccinated, with two shots, and about 12 percent have received a third booster dose.

The findings follow data published on Monday showing that two doses offered little protection against infection with the Omicron variant in children aged 5 to 11 after just one month. The vaccine has been shown to offer diminishing protection against infection even in adults, particularly against the Omicron variant. New data published by the C.D.C. on its website reflect this trend.

In the new study, the researchers analyzed data on 39,217 visits to emergency departments and urgent care clinics and 1,699 hospitalizations among children aged 5 to 17 years in 10 states, from April 9, 2021, to Jan. 29, 2022.

In children aged 5 to 11, the vaccine’s ability to prevent moderate illness dropped to 46 percent about two months after full vaccination (two weeks after the second shot). Most of the visits to emergency rooms and urgent care clinics occurred during the Omicron surge, when older children and adults also were more vulnerable than they had been earlier in the pandemic.

The vaccine’s effectiveness against moderate illness in adolescents held steady during the Delta era. But 150 days after full vaccination, effectiveness dropped sharply to 38 percent in adolescents aged 12 to 15 years, and to 46 percent in those aged 16 and 17 years.

When the researchers analyzed data specifically for protection in the Omicron era, protection against moderate illness all but disappeared in adolescents who had been vaccinated more than 150 days earlier. But a third vaccine dose restored effectiveness to 81 percent.

The findings are consistent with those from studies of adults showing that the vaccine’s effectiveness against infection and mild illness declined sharply over time, particularly after the arrival of the Omicron variant.

Effectiveness is a comparison between protection in vaccinated and unvaccinated groups of people. But as more of the population gains immunity through infection, it becomes harder to gain a true picture of vaccine effectiveness, said Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia and an adviser to the Food and Drug Administration.

“Are we comparing apples to apples when we say that the vaccine efficacy is going down?” he said.

Protection against severe illness was still harder to parse. There were too few hospitalizations in the younger children to draw firm conclusions. Among adolescents who had been vaccinated more than 150 days earlier, effectiveness against severe illness remained strong, at 70 percent or higher.

But most of those hospitalizations occurred during the Delta era, so the data do not provide a window into the effectiveness against hospitalization as the Omicron variant arrived and spread.

The C.D.C. recommends booster shots for Americans aged 12 and older. Pfizer and BioNTech are evaluating the benefit of a third dose in younger children.