New York Time | Age has its privileges, and a new study suggests that one of them may be immunity to some flu pandemics.

In ordinary seasonal flu, people over 65 are generally at higher risk for death. But in each of the 20th-century flu pandemics — the global outbreaks that struck in 1918-19, 1957 and 1968-69 — those under 65 were most likely to die.

That was also true for the swine flu pandemic caused by the novel A/H1N1 strain in 2009-10, a new study published online in PLoS One found. About 20 percent of the United States population was infected, with male death rates higher than female. Death rates considered over the entire age range were not especially high, but there were 1.325 excess deaths per 100,000 in people 25 to 64, higher than in any year since 1959, when such detailed data collection began. In people over 65, there were 0.228 fewer deaths per 100,000 than usual.

The reason is what the scientists call antigenic cycling. “As we get older, the intrinsic strength of the immune system declines, but the memory aspect is maintained,” said the study’s senior author, Andrew Noymer, an associate professor of public health at the University of California, Irvine. “So you have quite elderly people who are nevertheless protected by exposure when they were younger.”