Influenza pandemics-A Recurring problem

The most disastrous influenza outbreak was the 1918-1919 pandemic, which was worldwide in it's distribution and took more than 20 million lives. Although were not available at the time, the strain is belived to be similar to one isotated from swine in 1930 which possessed the following variant H and N antigen : HSW1N1 .This was deduced from the fact that the blood serum of person born between 1918 and 1929 contained antibodies to the swine virus H and N antigenes, whereas the serum of persons born after that period did not . In other words, those antibodies provided an immonological record of the influenza virus strains that had been prevalent during the 1918-1929 period.
Marked changes in the Hand N antigens occured in 1947,1957and1968 giving rise to strains having the antigenic variationH1N1 H2N2 and H3N2 respectively. Each change resulted in a pandemic. For example, after the emergence of the H2N2 varient, a pandemic begain in the central part of mainland China in Febtuary 1957. From there it spread widely in China, then to HongKong and then to other parts of the world, shortly after september 1957, epidemics swept the united states. Although not as server as the pandemic of 1918, it is estimated that nearly half the population of the united states become ill and more than 8,000 deaths were caused directly or indirectly by this new antigenic variant. Before this pandemic occurred, it had been found that the blood serum of people 70 to 90 years old already contained antibodies against the new strain. This was of interest because it implied that a similar strain may have caused an epidemic reported to have occurred in 1890. In 1968 the the H3N3 Variant appeared in HongKong and again a pandemic occurred. Still another variant was isolated in 1976 at Fort Dix, New Jersey; it's antigenic type was HSW1N1, resembling that of the highly virulent strain thought to be responsible for the 1918 pandemic. This subtype was predicated to give rise to a new and serious pandemic ; however, it failed to gain predominance over the H3N2 subtypes that were still prevalent. In 1977 still another subtype emerged, H1N1, similar to the subtype that had been prevalent in 1947 and spread through out the world. The sequence in which these variants have emerged suggests that antigenic variation in type A virus may occur in a repeating or cyclic fashion rather than as an endless progession of new subtypes. Future comparisons of new varienrs with past variants will help to confirm or negate this hypothesis.