During recent years, although influenza B has given rise to epidemics every 3-5 years, influenza A has caused greater concern to those responsible for the surveillance of the disease. The usual cycle of influenza A waves every 2 or 3 years was modified in many countries by the yearly appearance of new variants of the virus A/Hong Kong/68 (A/England/42/72, A/Port Chalmers/1/73). However, a number of countries, mostly located in Eastern Europe, were not attacked by virus A infections in 1973-74, when the modified variant A/Port Chalmers/1/73 caused a small wave in many areas of the world (this wave occurred in addition to, and often followed, a wave with virus B). Then, in 1974-75, the same countries of Eastern Europe were affected by an epidemic associated with viruses related to A/Port Chalmers, which was generally more widespread in these countries than in those already attacked by this variant the previous year. The epidemiological circumstances which may have led to such differences are not clear, all the more so as these differences were found even in neighbouring countries.
Moreover, the antigenic drift which led from A/England/72 to A/Port Chalmers/73 was of the same magnitude as the previous one which led from A/Hong Kong/68 to A/England/72, but the wave of A/England was much sharper than that of A/Port Chalmers. Also, during the epidemics with A/Hong Kong/68 of influenza year 1971-72, the variant A/Hong Kong/107/71 became fairly widespread. In haemagglutination inhibition tests, A/Hong Kong/107/71 was much more remote from A/Hong Kong/68 than was the subsequent variant A/England/42/72. In spite of this, A/England/42/72 caused vast epidemics the following year whilst A/Hong Kong/71 disappeared.
Although antigenic drifts constitute an element of prediction for patterns of spread of influenza, they are only one of the determining factors which govern the propagation of the viruses. There is at present no clear explanation for the differences in epidemic potential between influenza viruses (as well as for differences in clinical virulence). There is a need for a continuing study of the parameters governing the differences in spread between countries. For this purpose, health administrations should develop further the use of epidemiological indices, the significance of which should be carefully assessed.
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