Influenza surveillance during winter 1997-1998 in Israel.

Background: Each winter influenza activity is a major cause of morbidity and mortality both in Israel and worldwide. Objectives: To identify the influenza viruses active in Israel during the winter season and to assess the extent of influenza morbidity. Methods: Information was collected on a population of 18,684 individuals enrolled in two community clinics in central Israel. It included the total number of visits for acute respiratory infection - including influenza and inflpenza-like illness (ARI/ flu-like)- during a 20 week surveillance period (23 November 1997 to 27 March 1998) and the percent of influenza virus isolates in nasopharyngeal specimens from a sample of patients with ARI/flu-like collected on a weekly basis during the same period. Results: A total of 5,947 visits for ARI/flu-like were recorded among 18,684 enrolled patients in two community clinics (18,1%). The progressive increase in the number of visits for ARI/flu-like reached a peak on week 2/98 with 597 visits and a rate of 31.95 visits per 1,000 population. After this, a decrease to the initial values was evident by week 12/98. Most affected patients were in the age groups 5-14 and 65 years and over, with a rate of 733.5 and 605.3 visits per 1,000 population, respectively. Influenza virus was isolated from 92 of the 426 nasopharyngeal specimens (21.6%). The most commonly detected strain was A/Sydney/5/97(H 3 N 2 ) like (77.2%). The peak rate of isolates was recorded at the beginning of January (01/98). Conclusions: A/Sydney/5/97(H 3 N 2 ) like-strain was the dominant influenza virus. Its presence did not prevent the simultaneous activity of influenza A/H 1 N 1 virus. The dynamic of the clinical disease as expressed by the weekly visit rate for ARI/flu-like was similar to the temporal pattern of the virological findings. The extent of morbidity suggests moderate epidemic activity.