[Interepidemic influenza in Bulgaria based on laboratory research data].

During the interepidemic periods of 34 years (1955-1988) the National Influenza Centre of Bulgaria with the aid of 11 supporting stations investigated a total of 27240 nasopharyngeal washings and 42530 paired sera of patients with acute respiratory diseases. Eighty strains of subtype A (H1N1) influenza virus, 60 strains of subtype a (H2N2), 148 strains of subtype A (H3N2), 3 strains of subtype A (Heg2, Neg2), 2 strains of subtype A (Hsw1N1), 14 strains of type B, and 6 strains of type C were isolated. Serologically positive results for influenza were observed in 12.65% for influenza type A virus, 3.83% for type B, and 1.78% for type C (on the whole in 18.26%). The data obtained enable to recommend that practicing physicians should not hesitate to make the diagnosis of influenza during the interepidemic periods on the basis of sufficient clinical and especially epidemiological data and should actively try to get the laboratory verification of the preliminary diagnosis.