The MDH Public Health Laboratory confirmed the 2001-2002 season's first influenza isolate in Minnesota on December 11, 2001, which represents a typical start of influenza activity. Since 1990-1991, the first influenza isolate usually has been collected and identified between mid-November and mid-December. Despite a typical start, influenza activity in Minnesota began slowly, and activity peaked late, during the last week in February. Similarly, national influenza activity peaked in late February, and deaths attributable to influenza and pneumonia exceeded the epidemic threshold at that time.
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 1995-1996 influenza season. A Sentinel Physician Influenza
Surveillance Network, consisting of three sentinel sites,
was initiated in Minnesota for the 1998-1999 season. Eleven
sentinel sites participated in influenza surveillance in 2000-2001,
and 21 sites participated during the 2001-2002 season. While
the program has surpassed its enrollment goal of 18 sentinel
sites (one site per 250,000 population), MDH plans to expand
the number of participating sites in order to provide coverage
in all areas of the state. The summer Influenza Sentinel Surveillance
program began in May 2002, with 15 participating sites; the
program's purpose is to establish baseline rates of influenza-like
illness activity and to monitor influenza year-round.
During 2001-2002, the MDH Public Health Laboratory received 692 influenza virus isolates collected from Minnesota residents for viral confirmation and strain identification. Of these isolates, 555 (80%) were identified as influenza type A(H3N2)/Panama-like; 119 (17%) were influenza B/Victoria-like; 10 (1%) were B/Sichuan-like; three (<1%) were influenza A with unidentifiable strains, and five (1%) were influenza type B with unidentifiable strains. The predominant influenza A strain circulating in Minnesota during 2001-2002 was well-matched to the influenza A strains included in the 2001-2002 influenza vaccine. Of the two influenza B strains, only B/Sichuan was included in the vaccine, which includes one influenza B strain each year. Influenza type A/Panama, type B/Victoria, and B/Sichuan also were the predominant strains circulating nationally.
The 2001-2002 influenza season was highlighted by the identification of a new strain of influenza, designated A/Wisconsin/12/2001(H1N2). This strain first was identified in a resident of Outagamie County in Wisconsin and has since been reported in England, Israel, and Egypt. The new H1N2 strain appears to be the result of the reassortment of the genes in the circulating A/New Caledonia (H1N1) and the A/Moscow (H3N2) [A/Panama (H3N2)] strains. The identification of this new strain further emphasizes the importance of collecting viral cultures as a part of influenza surveillance.
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 101°F or greater, illness lasting at least 3 days, and at least one secondary influenza symptom (e.g., myalgia, headache, cough, coryza, sore throat, or chills). A possible outbreak of ILI in a school is defined as a doubled absence rate with reported symptoms among students, including two primary influenza symptoms and at least one secondary influenza symptom. Since 1988-1989, the number of schools reporting ILI outbreaks has ranged from 38 schools in 20 counties in 1996-1997 to 441 schools in 71 counties during 1991-1992. Reports of probable ILI outbreaks were received from 165 schools in 46 counties throughout Minnesota during 2001-2002; possible outbreaks were reported in 118 schools in 45 counties. Schools began reporting ILI outbreaks in early October; 74% of probable and possible ILI outbreaks were reported during February, March, and April.
An outbreak of ILI is suspected in a long-term care facility when three or more residents with a cough and fever (>101°F) or chills present from 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 positive rapid-antigen test for influenza. Since 1988-1989, the number of long-term care facilities reporting ILI outbreaks has ranged from six facilities in 1990-1991 to 79 facilities in 1997-1998. Sixty long-term care facilities reported confirmed or suspected ILI outbreaks during 2001-2002. In 47 (78%) facilities, influenza was laboratory-confirmed by direct antigen or culture. Twenty-three (38%) facilities reported outbreaks in February, 30 (50%) in March, and two (3%) in April.