Vaccine protection against moderate disease declined in adolescents, new CDC data suggest.

Five months after immunization, two doses of the Pfizer-BioNTech vaccine appeared to provide virtually no protection against moderate illness caused by the Omicron variant — as measured by emergency room visits and emergency care clinics — in adolescents ages 12 to 17, according to data published Tuesday by the Centers for Disease Control and Prevention.

But booster shots dramatically increased protection, supporting the agency’s recommendation of booster shots for everyone 12 years and older.

The findings should be interpreted with caution. The agency’s study excluded unvaccinated adolescents who had some immunity to a previous infection, making vaccination seem less effective than it was.

And the researchers provided only limited data on hospital admissions, a more reliable proxy for serious illness than emergency and emergency care.

“One limitation of this data is that parents can take their children to an emergency room or emergency department for a variety of reasons, and the effectiveness of vaccines by immunocompromised status, underlying health status, or vaccine product has not yet been studied,” the CDC said. in a statement.

Several studies have shown that while the vaccine’s effectiveness against infection declines over time, the immune response protects very well against hospitalization and death, even against the highly contagious Omicron variant.

A separate analysis of data from 29 jurisdictions posted to the CDC’s website reported nine Covid-related deaths among vaccinated children and adolescents ages 5 to 17 between early April 2021 and January 2022, compared with 121 deaths among unvaccinated people. children of that age.

Still, the findings suggest that scientists should carefully monitor vaccine 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 think it’s likely, and we have to be prepared as parents, that it’s going to happen again.”

The results take on special significance for parents as school districts across the country consider ending mask mandates. The CDC released new guidelines last week that suggest that about 70 percent of Americans can safely drop their masks in indoor public areas.

Vaccination in young children is slow; less than one in four children aged 5 to 11 are now fully vaccinated. More than half of adolescents ages 12 to 17 have been fully vaccinated, with two injections, and about 12 percent have received a third booster dose.

The findings follow data published on Monday showing that two doses provided little protection against infection with the Omicron variant in children ages 5 to 11 after just one month. The vaccine has been shown to provide diminishing protection against infection even in adults, especially against the Omicron variant. New data published by the CDC on its website reflects this trend.

In the new study, the researchers analyzed data on 39,217 emergency room and emergency care clinic visits and 1,699 hospitalizations among children ages 5 to 17 in 10 states, from April 9, 2021 to January 29, 2022.

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In children ages 5 to 11, the vaccine’s ability to prevent moderate illness fell to 46 percent about two months after full vaccination (two weeks after the second injection). Most emergency and emergency care visits occurred during the Omicron peak, when older children and adults were also more vulnerable than before in the pandemic.

Vaccine efficacy against moderate disease in adolescents remained stable during the Delta era. But 150 days after full vaccination, effectiveness dropped sharply to 38 percent in adolescents ages 12 to 15 and to 46 percent in youths ages 16 and 17.

When the researchers analyzed data specifically intended for protection in the Omicron era, protection against moderate disease virtually disappeared in adolescents vaccinated more than 150 days earlier. But a third vaccine dose restored effectiveness to 81 percent.

The findings are consistent with studies in adults showing that the vaccine’s effectiveness against infection and mild disease declined sharply over time, especially after the advent of the Omicron variant.

Effectiveness is a comparison between protection in vaccinated and unvaccinated groups of people. But as a larger portion of the population becomes immune from infection, it becomes more difficult to get an accurate picture of vaccine effectiveness, said Paul Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia and advisor to the Food and Drug Administration.

“Are we comparing apples to apples when we say vaccine efficacy is declining?” he said.

Protection from serious illness was even more difficult to parse. There were too few hospital admissions among the younger children to draw firm conclusions. Among adolescents vaccinated more than 150 days previously, effectiveness against major illnesses remained strong, at 70 percent or higher.

But most of those hospitalizations occurred during the Delta era, so the data doesn’t provide insight into its effectiveness against hospitalization when the Omicron variant arrived and spread.

The CDC recommends booster shots for Americans ages 12 and older. Pfizer and BioNTech evaluate the benefit of a third dose in younger children.

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