WASHINGTON — For the second year in a row, the proportion of Americans without health insurance declined in 2012, even though real household income and the poverty rate were not significantly different from their 2011 levels, the Census Bureau reported on Tuesday.

In 2012, the bureau said, 15.4 percent of people were uninsured, down from 15.7 percent in 2011. The number of uninsured people, 48 million, was not statistically different from the estimate of 48.6 million in 2011.

David S. Johnson, the chief of social and economic statistics at the bureau, said that much of the increase in coverage last year was attributable to government programs. Medicare covered 15.7 percent of the population, compared with 15.2 percent the previous year.

Census Bureau data showed significant changes in coverage over the last 13 years.

From 1999 to 2012, the bureau said, the proportion of people with private health insurance declined to 63.9 percent, from 73 percent, while the proportion with government coverage rose to 32.6 percent, from 24.2 percent.