3 SUMMERTIME OUTBREAKS OF INFLUENZA TYPE-A

In the Northern Hemisphere, sporadic cases of influenza occur during t he summer, yet summertime outbreaks are rare. From 12 August through 2 September 1993, three influenza outbreaks in Louisiana were investiga ted using medical-record review, interviews, viral cultures, serology, and active surveillance for influenza-like illness in Louisiana. Atta ck rates in the outbreaks were 61% (69/114), 42% (24/57), and 45% (23/ 51). Viruses isolated were most closely related to influenza A/ Beijin g/32/92 (H3N2). The identification of influenza A as the cause of the first two outbreaks led to the recommendation for amantadine use in th e third outbreak. Active surveillance did not detect any other outbrea ks of influenza-like illness during August or September 1993. Out-of-s eason influenza A outbreaks can therefore occur when little influenza- like illness is present in a community. Evaluation of outbreaks of acu te, febrile respiratory illness outside the influenza season should in clude this possibility, since rapid detection can lead to the timely u se of amantadine or rimantadine.