Amid early signs that insurance premiums under the Affordable Care Act might rise significantly next year, administration officials Tuesday previewed their plans to increase enrollment in the marketplaces, particularly among young adults who have been slow to sign up.

Open enrollment starts Nov. 1 and ends Jan. 31.

For the first time, the administration said it would send letters about marketplace coverage to uninsured people and to families who paid the individual mandate penalty for not having coverage or claimed an exemption from the health law requirement that they have coverage.

About 7.9 million Americans paid a penalty for lack of coverage in 2014. The Internal Revenue Service has not disclosed how many paid the fine for lacking coverage last year.

About 45 percent of 2014 taxpayers who paid a penalty or claimed an exemption from the penalty were under age 35, according to the Health and Human Services Department.

“This new strategy … will let us directly reach millions who were recently uninsured and may appreciate the value of marketplace coverage,” HHS said.

The administration will also:

• Email people if they open an account on www.healthcare.gov but do not select a plan and pay a premium.
• Encourage insurers to contact young adults before they turn 26 and move off their parents’ health plans to tell them about marketplace coverage options. New guidance from the Department of Labor makes clear that the sponsors of employer plans are allowed to provide specific information on health insurance plans that 26-year-olds can buy on the marketplace, HHS said Tuesday. The health law allows people to stay on their parents’ health insurance until 26.
• Help people pay for transportation to open enrollment events this fall where they can find help signing up. HHS has contracted with the ride-sharing service Lyft to provide discounts for those customers.

Nearly 13 million people signed up for health coverage under the Obamacare marketplaces this year. About 28 percent of them were between ages of 18 and 34. That group has the highest uninsured rate and generally enjoys the best health, which helps insurers control costs and balance the risks of covering less-healthy people.

Since the health law was fully implemented in 2014, the uninsured rate in the United States has fallen below 10 percent for the first time. In May, the rate fell to 9.1 percent, according to the Centers for Disease Control and Prevention.

Recent reports suggest health insurance premiums could increase by double-digit rates in several states in 2017. About 80 percent of marketplace enrollees are insulated from such increases by the government subsidies they receive based on their income levels.