The viruses that cause influenza are genetically unstable. They change both gradually and abruptly, and each change may enable the virus to skirt around previously acquired immunity, invade the respiratory tract, and cause the flu. This genetic instability is why people can get influenza over and over again, why they get it some years and not others, why some attacks make them much sicker than others, and why it’s necessary to get a new flu shot every year for maximum protection.
The gradual changes in the virus, called antigenic drift, may or may not be enough to override most people’s immunity to it. But when the abrupt changes known as antigenic shift occur, large numbers of people—and sometimes the entire population—have no protection. Such shifts usually result in worldwide epidemics, or pandemics, and, when they involve type A flu viruses, they are often highly lethal.
Type A flu viruses have two major antigens on their surface called H and N (the “H” stands for hemagglutinin and the “N” for neuraminidase). When they encounter a person’s immune system, these antigens trigger the production of antibodies that protect the person against the viral infection. When major changes occur in H and/or N, most people have no preexisting antibody protection, and so they are susceptible to an invasion by the new form of the virus. The following table shows the major shifts that have occurred in the type A influenza virus during this century. The pandemics are usually named for the area where the virus was first isolated.
There is evidence that during the last decade of the nineteenth century, a type A virus very similar to the H2N2 Asian flu virus prevailed, and that during the first seventeen years of the twentieth century a virus similar to the H3N2 Hong Kong virus was the primary troublemaker.
The Spanish flu A-H1N1 virus that first appeared in 1918 lingered in the population until 1957, when the Asian virus, A-H2N2, emerged and supplanted it. However, in 1977 a virus reappeared that was virtually identical to the A-H1N1 that had circulated at least until 1950. Most people born before 1950 had immunity to the 1977 virus, but those who were younger or who had missed being exposed to the original cause of Spanish flu had no natural protection, and many became very ill.
In 1992, the type A virus genetically rearranged some of its attire, but did not do a total costume switch. A variant known as Beijing arose, an A-H3N2 virus that was a kissing cousin of the A-H3N2 virus that caused the Hong Kong flu in 1968. Thus, many people had at least a partial immunity to it because they had encountered its close relative in the past. In the 1994-95 flu season, yet another A-H3N2 virus called Shandong was a leading troublemaker.
What will come along next is anybody’s guess. But the nation’s flu watchers are always on guard, and as soon as any new variant arises that causes a significant outbreak of influenza anywhere in the world, the manufacturers of flu vaccine will be ready to add it to their mix.
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