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Credit…Tamir Kalifa for The New York Times

Five months after vaccination, two doses of the Pfizer-BioNTech vaccine appeared to offer virtually no defense against moderate disease caused by the Omicron variant – as measured by visits to emergency departments and urgent care clinics – in adolescents ages 12 to 17, according to data released Tuesday by the Centers for Disease Control and Prevention.

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

The results should be interpreted with caution. The agency’s study did not exclude unvaccinated teens who had some immunity to a previous infection, which may have made the vaccination less effective than it was.

And the researchers offered only limited data on hospitalizations, a more reliable indicator of serious illness than emergency room and urgent care visits.

“One of the limitations of these data is that parents may bring their children to urgent or urgent care for a variety of reasons, and vaccine effectiveness based on immunocompromised status, underlying medical condition or vaccine product has not yet been investigated,” the CDC said. in a report.

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

A separate analysis of data from 29 jurisdictions published on the CDC’s website reported nine Covid-associated deaths among vaccinated children and adolescents ages 5 to 17 between early April 2021 and January 2022, compared to 121 deaths among children. unvaccinated of these ages.

Still, the results suggest scientists need to carefully monitor vaccine performance over time in children and adolescents, bearing in mind that booster shots may be needed.

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

The results are of particular importance to parents as school districts across the country consider ending mask mandates. The CDC released new guidelines last week suggesting that about 70% of Americans can safely lay down their masks in public indoor spaces.

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

The findings follow data released 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 provide decreasing protection against infection, even in adults, particularly against the Omicron variant. New data released by the CDC on its website reflects this trend.

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

In children aged 5 to 11 years, the ability of the vaccine to prevent moderate disease dropped to 46% approximately two months after full vaccination (two weeks after the second injection). Most visits to emergency rooms and urgent care clinics took place during the Omicron surge, when older children and adults were also more vulnerable than they had been at the start of the pandemic.

Vaccine efficacy against moderate disease in adolescents remained stable during the Delta era. But 150 days after full vaccination, efficacy dropped sharply to 38% in adolescents aged 12-15 and 46% in those aged 16-17.

When the researchers analyzed the data specifically for protection in the Omicron era, protection against moderate disease virtually disappeared in adolescents who had been vaccinated more than 150 days earlier. But a third dose of vaccine restored efficacy to 81%.

The results are consistent with studies in adults showing that the vaccine’s effectiveness against mild infections and illnesses declined sharply over time, particularly after the arrival of the Omicron variant.

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

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

Critical illness protection was even more difficult to analyze. There were too few hospitalizations among young children to draw firm conclusions. Among adolescents who had been vaccinated more than 150 days earlier, efficacy against severe disease remained strong, at 70% or more.

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

The CDC recommends booster shots for Americans ages 12 and older. Pfizer and BioNTech are evaluating the benefits of a third dose in young children.

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