What’s Behind the Numbers: Flavor bans may have had an effect.

One thing is clear about underage e-cigarette use: Adolescents like flavors. About 90 percent of the students who reported vaping said they used flavored products, citing favorites that tasted like fruit and candy.

Teenagers identified Elf Bar and Esco Bar as their favorite brands, well-known for flavors like strawberry kiwi and watermelon ice.

Public health advocates in California recognized the allure, leading to a yearslong fight to pass a ban on flavored tobacco products, which took effect in December. It quickly led to falling sales, according to data from the C.D.C. Foundation. From December 2022 to June of this year, flavored e-cigarette sales fell by nearly 70 percent, to 179,000 from about 575,000 vapes or refills.

The ban no doubt made it harder for young people to buy vapes in California, where you must be 21 to buy tobacco products.

Public health experts also linked other state and local flavor bans and education campaigns to the falling high school vaping rate, which is the lowest in nearly a decade. And a few years ago, under public pressure, Juul, which had once been the most popular brand, withdrew most of its flavors from the market.

The survey was given in about 180 schools nationwide, and was released by the C.D.C. and the Food and Drug Administration. It reported on e-cigarette use in the last 30 days but did not include any state-specific information.

In all, about 2.1 million middle and high school students reported using e-cigarettes, down from 2.5 million last year. But surveys conducted during a few previous years since the peak of the vaping crisis in 2019 have carried notes of caution about drawing strict comparisons year-to-year because of pandemic conditions when students were in and out of school.

Why It Matters: Studies identify health risks of vaping for teenagers.

Federal officials who regulate e-cigarettes see their use as an aid in helping adult smokers quit traditional cigarettes, given the well-known cancer risks.

But e-cigarette use has become wildly popular among nonsmokers. About 40 percent of people who use e-cigarettes are under 25, including many who started when Juul was first introduced. A majority of those young people never smoked before vaping, according to the C.D.C.

The health effects are well known by now. One recent University of Southern California study noted the toxicity of the chemicals in e-cigarettes and sent questionnaires to adolescents who vaped. It found significant increases in wheezing, shortness of breath and bronchitis symptoms. And many experts have expressed concerns about the effect of nicotine addiction on the adolescent’s developing brain.

What’s Next: A proposed menthol ban and heightened enforcement of illegal imports.

The F.D.A. is moving toward a ban on menthol cigarettes and is advancing a proposal to drastically cut nicotine levels in cigarettes. That has led legacy tobacco companies to embrace e-cigarette sales as the way forward in the marketplace to offset overall declining cigarette sales.

Yet those companies — along with many lawmakers in Congress and antismoking groups — say they have been dismayed with what they consider lax enforcement by the F.D.A. While the agency has authorized about two dozen vaping products for sale, thousands of illicit candy-colored flavored vapes have flooded the country and are top sellers.

The F.D.A. said it would press ahead with its enforcement efforts, including its import ban on Elf Bar and Esco Bar products and fines on retailers who continue to sell them. The agency has issued warning letters to makers of those vapes and many others.

Brian King, the F.D.A.’s tobacco division chief, welcomed the findings, but said: “We can’t rest on our laurels. There’s more work to be done to build on this progress.”

Dr. Neff said her agency needed to better understand why there was a small but significant increase in middle school use of any tobacco product, to 6.6 percent this year from 4.5 percent last year.

“Our work is far from done,” Dr. Neff said.

Other researchers noted that the combined general use of tobacco products by middle and high school students barely fell, to 10 percent this year from 11 percent last year. “On balance, it’s no change in the overall youth tobacco use,” said Karen Knudsen, chief executive of the American Cancer Society. “And that’s concerning.”