People in their 60s and over are relatively immune to swine flu because it is similar to H1N1 viruses that circulated before 1943, a group of researchers said Friday.
The group, comprising Japanese, U.S. and Dutch researchers, including Hiroshi Nishiura from the Japan Science and Technology Agency, came up with the finding by comparing the gene sequence of the novel virus' hemagglutinin — a protein related to immune reaction in humans — with those of H1N1 viruses from 1918 to the present.
Using data from 11 countries available as of last July, the study, released by British publisher Biomed Central Ltd., also showed that over 75 percent of confirmed cases of the new H1N1 infection occurred in people up to 30 years old.
Peak incidence was found in the age range of 10 to 19, while less than 3 percent occurred in people over 65.
In Japan, people aged 60 years and older were found to be much less, or 0.17 times, susceptible to the virus than those aged 20 to 39, Nishiura and other scientists said in a separate study also released by the publisher Friday. The findings were based on the country's epidemic data from May 29 to July 14.
The study of sequence data, published on the Web site of the BMC Infectious Diseases journal, also showed that the new H1N1 strain lacks glycosylation sites — where sugars are added to proteins — on the globular head of hemagglutinin near regions that stimulate production of antibodies, a pattern shared with the 1918 pandemic strain and H1N1 viruses that circulated until the early 1940s.
Later H1N1 viruses progressively added new glycosylation sites, and people exposed to these strains, particularly after the 1977 re-emergence of H1N1 strains, are likely to be unable to generate antibodies that can recognize the new H1N1 influenza, it said.