Three summertime outbreaks of influenza type A.

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