Influenza, 2008

The following summary includes seasonal influenza activity from the 2008-2009 season. It does not include pandemic or novel H1N1 influenza activity that began in late April 2009.

On December 8, 2008, the PHL isolated influenza virus from a Minnesota resident for the first time during the 2008-2009 influenza season. This represented an average start of influenza activity. Since 1990-1991, the first isolate typically has been between mid-November and mid-December. Influenza activity peaked in mid-February. Nationally, a similar activity pattern was seen.

Influenza surveillance in Minnesota relies on reporting of selective individual cases from clinics, hospitals, and laboratories, as well as outbreak reporting from schools and long-term care facilities. The current system for reporting outbreaks has been in place since the 1995-1996 influenza season, and a Sentinel Provider Influenza Network was initiated in 1998-1999 to conduct active surveillance. Twenty-six sentinel sites participated during the 2008-2009 season. While the program has surpassed its goal of 20 sentinel sites (ie, one site per 250,000 population), we plan to expand the network to ensure sites represent all areas of the state. Clinics are particularly needed in the southern and northeastern regions of the state, where coverage is sparse.

MDH requests reports of all suspected or confirmed cases of influenza-related encephalopathy or encephalitis in children <18 years of age, suspected or confirmed influenza-related deaths in children <18 years of age, suspected or confirmed cases of influenza and staphylococcal co-infection, suspected or confirmed influenza in hospitalized pregnant women, and suspected cases of novel influenza.

Surveillance for pediatric (<18 years of age) influenza-related hospitalizations was established during the 2003-2004 influenza season. During the 2006-2007 season, surveillance was expanded to include adult hospitalizations. During the 2008-2009 influenza season (October 2008 - April 2009), 160 persons residing in the metropolitan area were hospitalized with influenza infection, compared to 538 persons during the 2007-2008 influenza season. Among these 160 case-patients, 89 (56%) were children and 71 (44%) were adults. Incidence was highest among adults >69 years of age and among children <1 year of age. Fifty-five percent of case-patients were diagnosed with influenza by rapid antigen testing only, 22% by viral culture only, and 20% by rapid antigen and viral culture. Sixty-five percent of case-patients had type A influenza, 32% had type B influenza, and 3% had an unknown influenza type.

Thirty (19%) of 160 case-patients were diagnosed with pneumonia. Twenty-one (13%) case-patients required admission into an intensive care unit. Of these, nine (43%) were placed on mechanical ventilation. One (<1%) case-patient died. This case-patient was an older adult with multiple chronic medical conditions. Fifty-seven (80%) adult and 49 (55%) pediatric case-patients had at least one chronic medical condition that would have put them at increased risk for influenza infection. Four (3%) case-patients had an invasive bacterial co-infection (Acinetobacter iwoffii, Escherichia coli, Propionibacterium acnes, Streptococcus pneumoniae). Among those with a known influenza vaccine status, 34 (53%) adult case-patients and 37 (48%) pediatric case-patients received influenza vaccine (at least 2 weeks prior to their hospitalization) during the 2008-2009 season.

There were no influenza-related deaths identified during the regular 2008-2009 influenza season. Three pediatric influenza deaths were reported during the 2007-2008 season, and 6 pediatric influenza deaths were reported during the 2006-2007 season. Prior to 2006-2007, the last reported pediatric influenza death in Minnesota occurred during the 2004-2005 season.

A probable outbreak of influenza-like illness (ILI) in a school is defined as a doubled absenteeism rate with all of the following primary influenza symptoms reported among students: rapid onset, fever, illness lasting 3 or more days, and at least one secondary influenza symptom (eg, myalgia, headache, cough, coryza, sore throat, or chills). A possible ILI outbreak in a school is defined as a doubled absenteeism rate with reported symptoms among students, including two of the primary influenza symptoms and at least one secondary influenza symptom. During the 2008-2009 season, MDH received reports of probable ILI outbreaks from 70 schools in 22 counties throughout Minnesota and possible outbreaks in 65 schools in 21 counties. A total of 135 schools in 32 counties reported suspected outbreaks in 2008-2009. The 2008-2009 surveillance period for seasonal influenza in schools ended May 8, 2009 when enhanced surveillance for novel H1N1 influenza was initiated. Since 1988-1989, the number of schools reporting suspected influenza outbreaks has ranged from a low of 38 schools in 20 counties in 1996-1997 to 441 schools in 71 counties in 1991-1992.

An influenza outbreak is suspected in a long-term care facility when three or more residents in a single unit present with a cough and fever (>101º F), or chills during a 48- to 72-hour period. An influenza outbreak is confirmed when at least one resident has a positive culture or rapid antigen test for influenza. Three facilities in one county reported confirmed influenza outbreaks in 2008-2009. This represents the lowest number of long-term care facility outbreaks reported in a single season since surveillance began in the 1988-1989 season. In all facilities, influenza was laboratory-confirmed by rapid tests or culture. Previously, the number of long-term care facilities reporting ILI outbreaks has ranged from a low of six in 1990-1991 to a high of 140 in 2004-2005.

As of May 27, 2009, 405 (62%) of 650 influenza isolates in the PHL were well-matched to one of the three strains included in the vaccine for the 2008-2009 influenza season, compared to approximately 72% nationally. Of those, 388 were identified as influenza A/H1, 13 were identified as influenza A/H3, and 4 were identified as influenza B/Florida-like. Two hundred thirty-six isolates (36%) were identified as influenza B/Malaysia-like, a different lineage than the vaccine reference strain. For 8 influenza A isolates and 1 influenza B isolate, a vaccine match could not be determined.

• For up to date information see>> Influenza (Flu)
  
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2008