The 2000-2001 influenza season was mild nationally and in Minnesota; deaths attributable to influenza did not exceed the epidemic threshold at any time during the season. The season's first influenza isolate in Minnesota was confirmed by the MDH Public Health Laboratory on December 13, 2000. This represents a typical start of influenza activity; since the 1990-91 season, the first influenza isolate usually has been collected and identified between mid-November and mid-December.
Influenza surveillance in Minnesota relies on passive reporting from clinics/hospitals, laboratories, schools, and long-term care facilities. The current surveillance systems used in schools and long-term care facilities have been in place since the 1995-96 influenza season. A Sentinel Physician Influenza Surveillance Network consisting of three sentinel sites was initiated in Minnesota for the 1998-99 season. Eleven sentinel sites participated in influenza surveillance for the 2000-2001 season, and MDH plans to further expand the number of sites to 18 (representing one sentinel site per 250,000 population).
MDH received 338 influenza isolates from Minnesota residents for viral confirmation and strain identification. Of the influenza virus isolates received, 272 (80%) were identified as influenza type A(H1N1)/New Caledonia-like; 60 (18%) were influenza B/Yamanashi-like; one (<1%) was influenza type A (H3N2)/Panama-like, and two (<1%) were influenza type B but the strain could not be identified. Strains that circulated in Minnesota during 2000-01 were well matched to the strains included in the 2000-01 influenza vaccine. Influenza type A/New Caledonia and type B/Yamanashi also were the predominate strains circulating nationally.
A probable outbreak of influenza-like illness (ILI) in a school is defined as a doubled absence rate with all of the following primary influenza symptoms reported among students: rapid onset, fever of 101oF or greater, illness lasting at least 3 days, and at least one secondary influenza symptom (e.g., myalgia, headache, cough, coryza, sore throat, chills). A possible ILI outbreak in a school is defined as a doubled absence rate and reported symptoms among students, including two of the primary influenza symptoms and at least one secondary influenza symptom. Reports of probable ILI outbreaks were received from 144 schools in 49 counties throughout Minnesota; possible outbreaks were reported in 94 schools in 38 counties. Schools began reporting ILI outbreaks in mid-December; 89% of probable and possible ILI outbreaks were reported during January and February. Since 1988-89, the number of schools reporting suspected influenza outbreaks has ranged from 38 schools in 20 counties in 1996-97 to 441 schools in 71 counties during 1991-92.
Reported influenza activity in long-term care facilities was very mild compared to previous years. Eleven long-term care facilities reported confirmed or suspected ILI outbreaks. An ILI outbreak is suspected when three or more residents with a cough and fever (>101oF) or chills present on a single unit during a period of 48 to 72 hours. An ILI outbreak is confirmed when at least one resident has a positive culture or rapid-antigen test for influenza. In three (27%) long-term care facilities, influenza was laboratory-confirmed by direct antigen or culture. Five long-term care facilities reported outbreaks with onset in January, five in February, and one in March. Since 1988-89, the number of long-term care facilities reporting ILI outbreaks has ranged from six facilities in 1990-91 to 79 facilities in 1997-98.