Isolation and identification of influenza virus strains circulating in Thailand in 2001

Local influenza surveillance plays an important role in preparing for, and responding to, epidemics and pandemics. Between January and December 2001, the National Institute of Health of Thailand collected a total of 711 throat swab specimens from outpatients affected with acute respiratory symptoms from several centers throughout Thailand, of which 374 were virus-positive. Of these, 338 (90.4%) were positive for influenza virus by immunofluorescence testing. By hemagglutination-inhibition (HI) testing, 155 of the type A viruses were found to be subtype H1N1 strains closely related to A/New Caledonia/20/99, and 70 were subtype H3N2 A/Moscow/ 10/99-like viruses. For type B, the isolates were antigenically B/Sichuan/379/9-like by HI, although a number of the strains could be shown to be more closely related to earlier influenza B strains by genetic analysis. The strains circulating in Thailand were antigenically similar to strains isolated worldwide during the same period and to strains recommended by the WHO for inclusion in the vaccines for use in 2001-2002.