Influenza, 2011

Several surveillance methods are employed for influenza. Surveillance data are summarized by influenza season (generally October-April) rather than calendar year.

Hospitalized Cases

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 2011-2012 season (October 2, 2011 – April 30, 2012), we requested clinicians collect a throat or nasopharyngeal swab, or other specimen from all patients admitted to a hospital with suspect influenza, and submit the specimen to the PHL for influenza testing.

During the 2011-2012 influenza season, 552 laboratory-confirmed hospitalizations for influenza (10.4 hospitalizations per 100,000 persons compared to 18.3 per 100,000 during the 2010-2011 influenza season) were reported. Since October 2, 2011, hospitalized cases of influenza have included 522 that were influenza A (295 H3, 18 2009 H1N1, and 209 unknown A type), 29 that were influenza B, and 1 was influenza type unknown. The unknown type was tested locally with no material available to the PHL for testing for further subtyping.

Among hospitalized cases, 22% were 0-18 years of age, 22% were 19-49 years of age, and 56% were 50 years of age and older. Median age was 54.4 years. Forty-five percent of cases were residents of the metropolitan area. Of the 248 metropolitan area cases, 98 (39%) cases were also diagnosed with pneumonia. One (<1%) had an invasive bacterial co-infection. Twentythree (9%) required admission into an intensive care unit. Of these, 6 (26%) were placed on mechanical ventilation. Ninety-four percent of adult and 47% of pediatric cases had at least one chronic medical condition that would put them at increased risk for influenza disease.


Since the H1N1 pandemic, we have increased our efforts to identify deaths related to influenza. Influenza-associated deaths are reported through several 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 outbreak investigations, 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 laboratoryconfirmed influenza case to be included as an influenza-related death.

For the 2011-2012 influenza season, there were 33 influenza-associated deaths (16 influenza A-type unspecified, 13 influenza A-H3, and 4 influenza B). The median age was 86 years; 1 (3%) 25-49 years, 2 (6%) 50-64 years, 5 (15%) 64-79 years, and 25 (76%) age 80 and up. Thirty percent of cases were from the metropolitan area. Thirty-one (94%) had underlying medical conditions, and 23 (70%) were hospitalized for their illness. Twenty-one (64%) were residents of a long-term care facility. Two (6%) cases were identified through the UNEX/MED-X programs, 15 (45%) from hospital surveillance, 8 (24%) through death certificate review, 7 (21%) from nursing home outbreaks, and 1 (3%) through other methods.

Laboratory Data

The Minnesota Laboratory System (MLS) Laboratory Influenza Surveillance Program is made up of more than 310 clinic- and hospital-based laboratories, voluntarily submitting testing data on a weekly basis. These laboratories perform rapid testing for influenza and respiratory syncytial virus (RSV). Significantly fewer labs perform viral culture testing (6 labs) for influenza, RSV, and other respiratory viruses. Five laboratories perform PCR testing for influenza and three also perform PCR testing for other respiratory viruses. The PHL also provides further characterization of submitted influenza isolates to determine the hemagglutinin serotype to indicate vaccine coverage. Tracking laboratory results assists healthcare providers with patient diagnosis of influenza-like illness and provides an indicator of the progression of the influenza season as well as prevalence of disease in the community.

Between October 2, 2011 and May 19, 2012, virology laboratories reported 86 viral cultures positive for influenza. Of these, 71 (83%) were positive for influenza A and 15 (17%) were positive for influenza B. The number of positive influenza cultures peaked during the week of March 11 - March 17, 2012 at 14. Between October 2, 2011 and May 19, 2012, laboratories reported data on 11,459 influenza PCR tests, 1,437 (13%) of which were positive for influenza. Of these, 60 (4%) were positive for influenza A 2009 H1N1, 949 (66%) were positive for influenza A/(H3), 295 (21%) were positive for influenza A-not subtyped, 60 (4%) were positive for influenza A non-typeable, 71 (5%) were positive for influenza B, and 2 (0.1%) were positive for both influenza A and B. Between October 2, 2012 and May 19, 2012, 348 influenza isolates were further characterized in the PHL; 20 (6%) were characterized as influenza A 2009 H1N1, 267 (77%) were characterized as influenza A/(H3), 9 (3%) were characterized as influenza A-type unspecified, 8 (2%) were characterized as influenza B/Brisbane-like, and 44 (13%) were influenza B/Wisconsin-like.

Influenza Sentinel Surveillance

We conduct sentinel surveillance for influenza-like illness (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 these data there are 22 sites in 18 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 December 25-31, 2011 at 3.3%.

Influenza Incidence Surveillance Project

MDH was one of 12 nationwide sites to participate in an Influenza Incidence Surveillance Project for the 2011-2012 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 us. 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 July 31, 2011 – May 19, 2012, these clinics saw 1,865 ILI and 8,390 ARI patients. They submitted 913 specimens for influenza and respiratory pathogen testing, 61 (7%) of which were positive for influenza. Of those, 10 (16%) were positive for influenza A 2009 H1N1, 43 (70%) were positive for influenza A/(H3), 3 (5%) were positive for influenza A-type unspecified, and 5 (8%) were positive for influenza B. In addition to influenza, the following pathogens were detected by PCR: 43 (5%) adenovirus, 18 (2%) human metapneumovirus, 75 (8%) respiratory syncytial virus (RSV), 170 (19%) rhinovirus, 36 (4%) parainfluenza virus 1, 14 (2%) parainfluenza virus 2, 2 (0.2%) parainfluenza virus 3, 7 (0.8%) parainfluenza virus 4, 22 (2%) coronavirus C229E, 11 (1%) coronavirus OC43, 9 (1%) coronavirus HKU1, and 4 (0.4%) coronavirus NL63.

ILI Outbreaks (Schools and Long Term Care Facilities)

Between 1988 and 2009, a probable ILI outbreak in a school was defined 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 reached 5% of total enrollment, or when three or more students with ILI are absent from the same elementary classroom. Ninety-one schools in 36 counties reported ILI outbreaks during the 2011- 2012 school year. During the previous school year 218 schools in 50 counties reported ILI outbreaks. During the 2009- 2010 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. Forty-one facilities in 26 counties reported outbreaks during the 2011-2012 influenza season. Surveillance for outbreaks in LTCFs began in the 1988-1989 season. The number of LTCFs reporting ILI outbreaks has ranged from a low of 3 in 2008-2009 to a high of 140 in 2004-2005.

Updated Monday, 27-Jan-2014 09:20:35 CST