First outbreak of Hong Kong influenza in a general practice population in Great Britain. A field and laboratory study.

The first outbreak of influenza caused by Hong Kong (H.K.) variant of type A(2) influenza virus in a general practice (3,620 persons) in Cirencester, England, lasted 13 weeks, from 15 January to 15 April 1969, and is estimated to have attacked 4% of the practice population. The epidemic was too small to produce much effect on the total respiratory morbidity, but when symptom groups were examined separately it could be clearly followed in the class "febrile respiratory disease" in which 76 of the 78 isolations of the virus were made.Liability to attack was almost independent of age except that persons over 65 were less frequently attacked. In household outbreaks neither schoolchildren nor children under school age were more commonly the first case than their elders. The occurrence of influenza within a household increased the risk of an attack four-fold for the other members, and subsequent cases all occurred within 10 days of the first-consecutive cases within six days of one another.It is planned to continue surveillance throughout the period of dominance of Hong Kong variants of type A(2) influenza virus.