In 1977, a type of H1N1 virus, commonly known as the “Russian flu,” spread across the world, infecting people under 25 at much higher rates than their elders, who had been exposed to similar viruses in the ’40s and ’50s. In the first documented American outbreak, 70 percent of the students fell ill at a high school in Cheyenne, Wyoming, while their teachers proved immune. As the Air Force Academy’s chief medical officer said in 1978, “It’s one of the advantages of being middle-aged.”

Now, Leonard Mermel, an infectious disease specialist at Rhode Island Hospital, suggests the current flu virus could be similar enough to that ’70s strain that older people could again find themselves immune to a new virus. “It might be that the H1N1 circulating now (swine-origin influenza virus) has enough antigenic similarity to related H1N1 influenza strains of the past to protect older individuals exposed to them previously,” Mermel wrote in a letter to the journal The Lancet.

That would be good news for public health officials and explain one of the more puzzling aspects of the new swine flu outbreak: why young people seem to be more susceptible to the disease than their parents and grandparents. Regular seasonal flu tends to disproportionately strike the old, not the strapping youthful masses. That can lead to higher morbidity because the elderly population is not as healthy overall. If older people are already immune, public health organizations could allocate what are sure to be small amounts of vaccine to the right populations.

Larry Madoff, editor of the epidemiological news service, ProMED Mail, a program of the International Society of Infectious Diseases, said that Mermel’s hypothesis was plausible, but would require further investigation. “There may be other factors at play here as well,” Madoff said. The variance in infection rates among age groups “could also be due to the transmission patterns, the epidemiology. Kids are together more, confined in schools more. The immunology may not be the whole story.” Still, Madoff said that while the jury is still out, Mermel’s theory “looks pretty good,” particularly when combined with recent biological lab work.

In a Centers for Disease Control publication, the Mortality and Morbidity Weekly Report, scientists reported that older adults are, in fact, better immunologically prepared. “Results among adults suggest that some degree of preexisting immunity to the novel H1N1 strains exists, especially among adults aged >60 years,” they wrote. “One possible explanation is that some adults in this age group have had previous exposure, either through infection or vaccination, to an influenza A (H1N1) virus that is genetically and antigenically more closely related to the novel influenza A (H1N1) virus than are contemporary seasonal H1N1 strains.”

(Por Alexis Madrigal, Wired, 22/06/2009)