There are several methods of surveillance employed for influenza. Surveillance data are summarized by influenza season (generally October-April) rather than calendar year.
Surveillance for pediatric (<18 years of age), laboratory-confirmed hospitalized cases of influenza in the metropolitan area was established during the 2003-2004 influenza season. During the 2006-2007 season, surveillance was expanded to include adults. For the 2008-2009 season, surveillance was expanded statewide, although the collection of clinical information on hospitalized cases was limited to metropolitan area residents only. During the 2010-2011 season (October 3, 2010 – April 30, 2011), MDH requested that clinicians collect a throat or nasopharyngeal swab, or other specimen from all patients admitted to a hospital with suspect influenza, and submit a specimen to the PHL for influenza testing.
During the 2010-2011 influenza season, 965 laboratory-confirmed hospitalizations for influenza (18.3 hospitalizations per 100,000 persons, compared to 34.9 hospitalizations during the 2009-2010 influenza season [H1N1 pandemic period]) were reported. Since October 3, 2010, hospitalized cases of influenza have included 794 that were influenza A (353 H3, 129 2009 H1N1, and 312 unknown A type), 168 that were influenza B, and 3 that were influenza type unknown. Unknown types were tested locally with no material available at the PHL for testing for further subtyping.
Among hospitalized cases, 23% were 0-18 years of age, 20% were 19-49 years of age, and 57% were 50 years of age and older. Median age was 56.9 years. Forty-six percent of cases were residents of the metropolitan area. Of the 446 metropolitan area cases, 137 (31%) cases were also diagnosed with pneumonia. Seven (2%) had an invasive bacterial co-infection. Sixty-five (15%) required admission into an intensive care unit. Of these, 28 (43%) were placed on mechanical ventilation. Eighty-two percent of adult and 51% of pediatric cases had at least one chronic medical condition that would put them at increased risk for influenza disease.
Since the H1N1 pandemic, MDH has increased its surveillance efforts to identify deaths related to influenza.
Influenza-associated deaths are reported through several surveillance systems including hospital surveillance, Unexplained Critical Illnesses and Deaths of Possible Infectious Etiology (UNEX) reporting, Medical Examiner Infectious Deaths (MED-X) surveillance, death certificate review, nursing home outbreaks, as well as other sources. All reported cases are investigated to determine if there was a positive influenza laboratory result and symptoms of an infectious process consistent with influenza without recovery to baseline prior to death. In a small number of cases there may not be a positive influenza laboratory result due to the lack of specimens taken, in which case the person must have influenza noted as a cause of death on the death certificate, or the person must have had direct contact with a laboratory-confirmed influenza case to be included as an influenza-related death. Specimens are submitted to MDH and tested by PCR, culture, and serology at the PHL or immunohistochemistry at the CDC Infectious Diseases Pathology Branch.
From October 2010-April 2011, there were 70 influenza-associated deaths, 34 influenza A-type unspecified, 21 influenza A-H3, 9 influenza A-2009 H1N1, 5 influenza B, and 1 influenza A/B-type unspecified. The median age was 84 years; 3 (4%) age 0-17 years, 6 (9%) age 18-49 years, 7 (10%) age 50-64 years, 10 (14%) age 64-79 years, and 44 (63%) age 80 and up. Forty-four percent of cases were from the metropolitan area. Sixty-one (87%) had underlying medical conditions, and 44 (63%) were hospitalized for their illness. Forty (57%) were a resident of a long-term care facility.
Three (4%) cases were identified through the UNEX and MED-X programs, 25 (36%) through hospital surveillance, 24 (34%) through death certificate review, and 18 (26%) through other methods.
Between October 3, 2010 and May 21, 2011, virology laboratories reported data on 7,333 viral cultures, 344 (5%) of which were positive for influenza. Of these, 219 (64%) were positive for influenza A and 125 (36%) were positive for influenza B. Percent positive of influenza cultures peaked during the week of February 27-March 5, 2011 at 16%. Between October 3, 2010 and May 21, 2011, virology laboratories reported data on 9,424 PCR influenza tests, 1,842 (20%) of which were positive for influenza. Of these, 253 (14%) were positive for influenza A 2009 H1N1, 690 (37%) were positive for influenza A/(H3), 420 (23%) were positive for influenza A-type unspecified, and 479 (26%) were positive for influenza B. Between October 3, 2010 and May 21, 2011, 497 influenza isolates were further characterized in the PHL; 126 (25%) were characterized as influenza A 2009 H1N1, 158 (32%) were characterized as influenza A/(H3), 6 (1%) were characterized as influenza A-type unspecified, 206 (41%) were characterized as influenza B/Brisbane-like, and 1 (0.2%) was influenza B/Florida-like.
Influenza Sentinel Surveillance
MDH conducts sentinel surveillance for influenza-like illnesses (ILI; fever ≥100° F and cough and/or sore throat in the absence of known cause other than influenza) through outpatient medical providers including those in private practice, public health clinics, urgent care centers, emergency rooms, and university student health centers. For this report, there are 22 sites in 20 counties. Participating providers report the total number of patient visits each week and number of patient visits for ILI by age group (0-4 years, 5-24 years, 25-64 years, >65 years). Percentage of ILI peaked during the week of February 27-March 5, 2011 at 3.2%.
Influenza Incidence Surveillance Project
MDH was one of 12 sites nationally to participate in an Influenza Incidence Surveillance Project for the 2010-2011 influenza season. Four clinic sites reported the number of ILI patients and acute respiratory illness (ARI; recent onset of at least two of the following: rhinorrhea, sore throat, cough, or fever) patients divided by the total patients seen by the following age groups: <1 year, 1-4 years, 5-17 years, 18-24 years, 25-64 years, and ≥65 years, each week. These clinics also performed rapid influenza testing on all ILI patients and reported results to MDH. Clinical specimens were collected on the first 10 patients with ILI and the first 10 patients with ARI for PCR testing at the PHL for influenza and 12 other respiratory pathogens. Minimal demographic information and clinical data were provided with each specimen.
From August 1, 2010 – May 31, 2011, these clinics saw 1,321 ILI and 8,885 ARI patients. They submitted 1,277 specimens for influenza and respiratory pathogen testing, 242 (19%) of which were positive for influenza. Of those, 49 (20%) were positive for influenza A 2009 H1N1, 100 (41%) were positive for influenza A/(H3), 3 (1%) were positive for influenza A-type unspecified, and 90 (37%) were positive for influenza B. In addition to influenza, the following pathogens were detected by PCR: 39 (3%) adenovirus, 54 (4%) human metapneumovirus, 41 (3%) respiratory syncytial virus (RSV), 210 (16%) rhinovirus, 1 (0.1%) parainfluenza virus 1, 8 (0.6%) parainfluenza virus 2, 19 (1%) parainfluenza virus 3, 4 (0.3%) parainfluenza virus 4, 4 (0.3%) coronavirus C339E, 6 (0.5%) coronavirus HKU1, 24 (2%) coronavirus NL63, and 63 (5%) coronavirus OC43.
ILI Outbreaks (Schools and Long Term Care Facilities)
Between 1988 and 2009, a probable ILI outbreak in a school was defined by MDH as a doubled absence 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 (e.g., myalgia, headache, cough, coryza, sore throat, or chills). A possible ILI outbreak in a school was defined as a doubled absence rate with reported symptoms among students, including two of the primary influenza symptoms and at least one secondary influenza symptom. Prior to the 2009-2010 influenza season, the number of schools reporting probable 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.
The definition of ILI outbreaks changed beginning with the 2009-2010 school year. Schools reported when the number of students absent with ILI reaches 5% of total enrollment, or when three or more students with ILI are absent from the same elementary classroom. During the 2010-2011 school year 218 schools in 50 counties reported ILI outbreaks. During the previous school year, 1,302 schools in 85 counties reported ILI outbreaks.
An influenza outbreak is suspected in a long-term care facility (LTCF) when three or more residents in a single unit present with a cough and fever or chills during a 48- to 72-hour period. An influenza outbreak is confirmed when at least one resident has a positive culture, PCR, or rapid antigen test for influenza. Fifty-four facilities in 36 counties reported outbreaks from October 3, 2010 – May 21, 2011. Surveillance for outbreaks in LTCFs began in the 1988-1989 season. The number of LTCFs reporting ILI outbreaks has ranged from a low of three in 2008-09 to a high of 140 in 2004-2005.
- For up to date information see>> Influenza (Flu)
- Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2010