Epidemic influenza in a hill tribe of northwest Thailand

An investigation of an epidemic of respiratory disease in a remote region of northwest Thailand revealed Influenza A/H3N2 as the etiologic agent. This epidemic would not have been recognized were it not for an unusual increase in respiratory disease following a meeting attended by residents of many villages. The influenza strains isolated most closely resembled A/Port Chalmers/1/73 when tested by hemagglutination inhibition but showed consistent antigenic differences when tested by quantitative neutralization. It is suggested that the differences noted between this strain and influenza viruses isolated elsewhere may have been due to the sequential transmission of influenza through partially immune people.