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Teen vaping has hit “epidemic” levels, according to federal health officials, with rates up by 78 percent over the last year alone. But strict laws on selling vape products to underage kids can reduce the number of teens who try e-cigarettes, according to research published Monday.

Teens living in cities or towns that more strictly policed retail sales of tobacco products were a third less likely to try cigarettes or e-cigarettes as those living in areas with more lax regulation, the team at the University of Southern California found. And they were half as likely to become regular users of tobacco, including e-cigarettes.

It supports what tobacco control advocates have been saying for years: strong laws, with equally strong enforcement, work to reduce tobacco use.

“It’s actually not at all surprising,” said Dr. Harold Farber, a pediatric lung expert at Texas Children’s Hospital, who was not involved in the study.

“If you have good laws and you enforce those laws to keep tobacco out of the hands of kids, fewer kids will get hold of tobacco,” Farber told NBC News.

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“If you have ineffective and loose laws, and no enforcement, then kids will get hold of tobacco.”

The study comes with a caveat. Specialty vape shops are often not regulated at the same level as other tobacco retailers, and that could provide a loophole. So could online sales. “The increase of poorly regulated e-cigarette Internet vendors, a relatively new way for minors to obtain tobacco products illegally at the time of data collection, may limit the future impact of tobacco retail licensing as a regulatory tool,” the team wrote in their report, published in the journal Pediatrics.

A team led by Dr. Rob McConnell, a specialist is preventative medicine and children’s health at USC, used an ongoing survey of students in southern California, asking more than 2,000 about whether they had tried vaping or smoking and then following up a year and a half later.

They also graded the communities where the teens lived on what kinds of laws and enforcement they had to limit tobacco sales. Four out of the 14 jurisdictions got an “A” rating based on American Lung Association criteria, meaning they required retailers to pay annual license fees to pay for regular compliance checks. These jurisdictions also used fines or revoked licenses to punish retailers who sold to underaged kids.

The rest of the communities all got “D” or “F” ratings, meaning they did not do much to stop retailers from selling tobacco products to teens.

Teens living in “A” grade communities were two-thirds as likely to have tried any tobacco product, including vaping, over the next year and a half and those living in the poorly regulated communities. They were 50 percent as likely to be current smokers or vapers, the team reported.

Other policies, such as smoke-free housing rules or bans on outdoor smoking had no effect on teen use, the researchers noted.

“Kids know who the retailers are who will sell to them,” Farber said.

The licensing fees are key, because they mean the regulation pays for itself, so it doesn’t cost the community anything extra to have good anti-tobacco regulations, he said.

“Raising the age for sale has been well-documented to work,” Farber added. “Raising the age for sale to 21, combined with enforcement, decreases youth smoking and is a very, very important initiative.”

Last November, the Centers for Disease Control and Prevention showed a 78 percent increase in vaping by high school students, with 3.6 million high school and middle school students now using e-cigarettes. The Food and Drug Administration has responded with a series of proposals aimed at reducing sales to teens.

Research shows that most smokers start before they are 21. Nicotine is extremely addictive, and children and young adults are more susceptible than older people to the effects. Teens who vape are also more likely to go on to smoke burned cigarettes.