With the spread of new strains of viruses (for example, “bird flu”), the urgency of studying their virulence is growing. According to statistics, H5N1 and H7N9 virus subtypes A annually lead to hundreds of deaths. At the same time, the risk of contracting one or another subtype of the disease differs depending on age. For example, some older people who have had H1N1 (“swine flu”) in childhood are less likely to contract H5N1. But the opposite cases contradict this, in addition, the hypothesis does not explain the differences in the severity of the disease.
To find the pattern, scientists at the Universities of California and Arizona conducted a meta-analysis of studies that looked at viral infections from 1918 to 2015. Then they divided all patients into two groups depending on the year of birth and immunity to a particular strain and subtype of viruses. In total, the authors identified 18 types of hemagglutins - proteins that are located on the envelope of the virion (viral particle) and allow the virus to attach to the host cell.
The bond of strains (the maximum bond is indicated in black), depending on the types of hemagglutin. / Katelyn M. Gostic et al., Science, 2016
The results showed that patients born between 1918 and 1968 were more likely to have the H7 strain and subtype H3N2 ("Hong Kong flu") during childhood, as a result of which they developed immunity to H7N9. Patients born later, in childhood, are more likely to encounter strains H1, H2 and subtype H5N1, and are defenseless against viruses of past years. According to the authors, the experience of infection in childhood guarantees 75% protection against severe disease in the future and 80% protection against death.
This effect is associated with the production of antibodies. In childhood, the immune system "remembers" the hemagglutin (they can be combined into two categories) of a particular strain and, upon subsequent infection, successfully destroys the virus. The data obtained allow predicting influenza epidemics and adjusting preventive measures, the authors are sure. At the same time, they emphasize that only two strains of "bird flu" were analyzed and the theory needs to be tested in further studies.