More than 2 million existing customers with insurance under the Affordable Care Act have had coverage renewed automatically for 2016 by HealthCare.gov, after they ignored government warnings to shop around to avoid surprise spikes in prices of health plans.

According to data released Tuesday, 8.2 million people already have chosen — or have been automatically assigned to — health coverage next year through the federal insurance exchange. In good news for ACA proponents, the number of new customers and people under age 35 is running ahead of last year’s enrollment season.

Federal officials said the “vast majority” of automatic renewals have been fed into the exchange’s computer system in recent days. They declined to specify the exact number of such renewals but said in a briefing for reporters that “it is safe to say” they account for more than 2 million of the overall tally. That is slightly more than last year.

This renewal niche is significant because 2016 insurance premiums in the federal exchange are climbing by 7.5 percent on average, nearly four times as much as a year ago. Health and Human Services Secretary Sylvia Mathews Burwell and her aides have been urging current customers to return to HealthCare.gov and consider switching plans to avoid substantial rate increases.

People who are automatically enrolled have until Jan. 31 to choose a different health plan or to withdraw from coverage. So it remains unclear how many might switch or drop out of the marketplace by the end of the sign-up season.

Separate HHS figures released Tuesday show that the number of Americans with insurance under the ACA continued to decline during the summer and early fall. By the end of September, 9.3 million people had health coverage in effect through the federal exchange or separate ones run by a dozen states and the District. That was 600,000 less than in late June — a bigger drop than during the previous three months.

Federal health officials say enrollment will always ebb and flow as some people’s lives change in ways that mean that they no longer need or qualify for insurance on the exchanges, which are intended for consumers without access to affordable health benefits from an employer. Burwell has forecast that 9.1 million people will have ACA coverage at the end of 2015 — a prediction HHS did not change Tuesday. The actual level will be reported in about three months.

Burwell sought to draw attention to the 2.1 million people younger than 35 who had enrolled through HealthCare.gov as of late last week — about 1 million more than at the same stage last year. The new figure means that children and young adults account for 35 percent of those enrolled by the deadline to have coverage in place as of New Year’s Day.

Health policy experts have long said that the stability of the ACA marketplaces depends on young, healthy adults to balance out the expense of older people who havemore medical problems. When the exchanges opened in October 2013, Obama administration officials said they hoped that nearly two in five of the people enrolled would be between 18 and 34.

HHS officials said Tuesday that they did not yet have a new breakdown for that age group.