That e-cigarettes are less harmful is a message American smokers rarely hear, partly because American regulation prevents it. Companies are banned from making such claims unless they go through a long process to prove it, and so far, no e-cigarette maker has done so. More states are also passing laws that lump e-cigarettes in with traditional cigarettes, levying taxes on them and banning their use as part of local smoke-free rules.

“When they are regulated just like tobacco, people draw the conclusion that they are just as dangerous,” said Daniel I. Wikler, an ethicist at the Harvard School of Public Health. “You didn’t say it, but you didn’t have to. People make that assumption and you don’t try to disabuse them of it.”

Last week, Georgia State University published a report finding that the percentage of Americans who thought e-cigarettes were as bad as cigarettes or worse than them had tripled, to 40 percent in 2015 from 13 percent in 2012.

Image A vape and a safety notice at Sababa Vapes in Philadelphia. The United States’ top public health authorities have often discouraged vaping. Credit... Mark Makela for The New York Times

If smokers have tried everything else, and use an e-cigarette to quit completely, “that’s a good thing,” said Dr. Thomas R. Frieden, the director of the C.D.C. He has heard anecdotes about that happening, he added.

“But the plural of anecdote is not data,” he said. “And counterbalancing that good trend, there are at least three negative things that might be happening,” like people who have never smoked using them, children picking them up as a path to smoking, or smokers using them to perpetuate their habit.

But smoking has been declining. The 2015 adult smoking rate dropped by more than 10 percent from the previous year, the biggest decline since the government began tracking the measure in 1965, said Kenneth E. Warner, a professor of public health at the University of Michigan, citing the government’s National Health Interview Survey. Past dips have always been linked to some event, he said, like a tax. But there was no single event to explain this one, and some suspect e-cigarettes may have played a role.