Could Older Population Have Enough Exposure To Past H1N1 Flu Strains To Avoid Infection?

Mermel notes that in the late 1970s, an influenza H1N1 reappeared in humans. It had a pandemic-like spread that began in younger aged individuals. This strain, known as the "Russian flu" H1N1, was similar to H1N1 strains that circulated internationally between 1946 and 1957. The Russian flu spread rapidly across the former Soviet Union, initially affecting individuals between the ages of 14 and 20 in schools, as well as young military personnel, and later spread to preschool children. Individuals older than age 30, however, had dramatically lower attack rates and the overall mortality was low. The epidemic peaked rapidly, with a relatively short duration.

In the United States, the first outbreak of the Russian flu occurred in a Wyoming high school. The attack rate there was over 70 percent, however, it affected students only; no faculty were reported to have the illness. High attack rates were seen in schools as well as military bases throughout the United States, similar to the outbreak in Russia. There were few reports of the H1N1 strain in individuals older than age 26, and again, the mortality rate was low.

In his commentary, Mermel hypothesizes that the H1N1 strain circulating now may have enough similarity to the previously circulating H1N1 strains or the H1N1 used in past vaccines so that it may lead to protection of older individuals. He concludes by noting that the weeks ahead should help us to determine if this will be the case.

His letter is published in the June 20 edition of the journal The Lancet 2009 (vol 373 p2108-09).