Hong Kong influenza in Czechoslovakia, 1969: a preliminary surveillance report.

Cases of Hong Kong influenza did not appear in Czechoslovakia until the middle of January 1969, although cases had occurred in neighbouring Poland in December 1968.Altogether 426 strains of A2 influenza virus were identified between the 5th and 22nd weeks of 1969 and their biological and antigenic properties were investigated. The strains isolated were highly sensitive to non-specific inhibitors present in horse, guinea-pig and human sera: in human sera, the inhibitors could be removed by means of receptor-destroying enzyme but not by treatment with KIO(4). The haemagglutinin of the new variant was much less sensitive to a temperature of 56 degrees C-65 degrees C than the former A2 strains. The results of all these investigations, which also included complement-fixation and haemagglutination-inhibition tests, showed that the epidemic was caused by A2 influenza strains antigenically and biologically identical with the A2/Hong Kong/68 strain.Surveillance studies showed that the outbreak started in the north-western part of Bohemia and Moravia and spread to the south-east. The rural areas were much less affected than the industrial and densely populated areas. The number of deaths due to influenza and pneumonia was only slightly higher than the seasonal average, probably because of the mild clinical course of the disease.