Several surveillance methods are 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 2012-2013 season (September 30, 2012 – May 4, 2013), 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 2012-2013 influenza season, 3,086 laboratory-confirmed hospitalizations for influenza (57.7 hospitalizations per 100,000 persons compared to 10.4 per 100,000 during the 2011-2012 influenza season) were reported. Since September 30, 2012, hospitalized cases of influenza included 2,527 that were influenza A (1,413 H3, 36 A[H1N1]pdm09, and 1,078 unknown A type), 532 that were influenza B, 9 that were positive for both influenza A and B, and 18 were unknown influenza types. Among hospitalized cases, 16% were 0-18 years of age, 13% were 19-49 years of age, 15% were 50-64 years of age and 57% were 65 years of age and older. Median age was 71.7 years. Forty-nine percent of cases were residents of the metropolitan area.
Case report forms have been completed on 1,216 (80%) of 1,515 metropolitan area cases to date. Of these, 25% were diagnosed with pneumonia, 15% required admission into an intensive care unit, and 5% were placed on mechanical ventilation. Three percent of hospitalized influenza cases had an invasive bacterial co-infection. Eighty-one percent of cases received antiviral treatment. Overall, 90% of adult cases and 49% of pediatric cases had at least one chronic medical condition that would have 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, and 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.
For the 2012-2013 influenza season, there were 199 influenza-associated deaths (97 influenza A-type unspecified, 61 influenza A-H3, 22 influenza B, 2 influenza A & B, 1 influenza A/B non-distinguished, and 16 unknown type). The median age was 86 years; 5 (3%) 0-17 years, 5 (3%) 18-49 years, 15 (8%) 50-64 years, 40 (20%) 64- 79 years, and 134 (67%) age 80 and up. Forty percent of cases were from the metropolitan area. One hundred seventy-two (86%) had underlying medical conditions, and 129 (65%) were hospitalized for their illness. One hundred fourteen (57%) were residents of a long-term care facility. Six (3%) cases were identified through the UNEX/MED-X programs, 43 (22%) from hospital surveillance, 124 (62%) through death certificate review, 14 (7%) from long term care facility outbreaks, and 12 (6%) through other methods.
Novel Influenza Cases
In response to the identification of swine-origin H3N2 influenza in humans (H3N2v) associated with exposure to swine at county and state fairs in several states, MDH sent out a statewide health alert in August 2012. Healthcare providers and hospitals were asked to submit specimens to the PHL if they identified a patient with influenza-like illness (ILI) who had contact with swine within 7 days of onset or attended a county or agricultural fair in which swine were present, or had recent contact with someone with ILI who had recent exposure to swine or anyone hospitalized with ILI. MDH also initiated surveillance for ILI among 4H students exhibiting animals at the Minnesota State Fair. Participants were notified to see an onsite 4H nurse if they exhibited any illnesses. 4H nurses were asked to collect a specimen from anyone with ILI and submit those specimens to the PHL for testing.
Nine confirmed novel influenza cases were identified. Five were identified as H3N2v, and 4 were identified as swine-origin H1N2 variant (H1N2v). Both strains include the M gene from the influenza A(H1N1)pdm09 virus. Exposure occurred in August, September, and November. Six (4 H1N2v, 2 H3N2v) were associated with the Minnesota State Fair and 3 were associated with live animal markets in the metropolitan area. All cases had direct or indirect contact with swine. One case was hospitalized. All recovered from their illness.
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 (six labs) for influenza, RSV, and other respiratory viruses. Nine 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 September 30, 2012 - May 11, 2013, virology laboratories reported 277 viral cultures positive for influenza. Of these, 141 (51%) were positive for influenza A and 136 (49%) were positive for influenza B. The number of positive influenza cultures peaked during the week of December 30, 2012 - January 5, 2013 at 36. Between September 30, 2012 - May 11, 2013, laboratories reported data on 18,040 influenza PCR tests, 3,830 (21%) of which were positive for influenza. Of these, 58 (2%) were positive for influenza A(H1N1)pdm09, 2,115 (55%) were positive for influenza A/(H3), 732 (19%) were positive for influenza A-not subtyped, 54 (1%) were positive for influenza A non-typeable, 869 (23%) were positive for influenza B, and 2 (0.1%) were positive for both influenza A and B. Between September 30, 2012 and May 11, 2013, 632 influenza isolates were further characterized in the PHL; 21 (3%) were characterized as influenza A(H1N1)pdm09, 279 (44%) were characterized as influenza A/(H3), 1 (0.2%) was characterized as influenza A-type unspecified, 229 (36%) were characterized as influenza B/Victoria linage, 98 (16%) were characterized as influenza B/Yamagata lineage and 4 (1%) were characterized as influenza B-lineage unspecified.
Influenza Sentinel Surveillance
We conduct sentinel surveillance for 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 23-29, 2012 at 6.7%.
Influenza Incidence Surveillance Project
MDH was one of 12 nationwide sites to participate in an Influenza Incidence Surveillance Project for the 2012-2013 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. 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 29, 2012 – May 11, 2013, these clinics saw 1,562 ILI and 8,012 ARI patients. They submitted 1,076 specimens for influenza and respiratory pathogen testing, 254 (24%) of which were positive for influenza. Of those, 3 (1%) were positive for influenza A(H1N1)pdm09, 148 (58%) were positive for influenza A/(H3), 7 (3%) were positive for influenza A-type unspecified, and 96 (38%) were positive for influenza B. In addition to influenza, the following pathogens were detected by PCR: 25 (2%) adenovirus, 45 (4%) human metapneumovirus, 50 (5%) RSV, 149 (14%) rhinovirus, 1 (0.1%) parainfluenza virus 1, 17 (2%) parainfluenza virus 2, 37 (3%) parainfluenza virus 3, 1 (0.1%) parainfluenza virus 4, 6 (1%) coronavirus C229E, 52 (5%) coronavirus OC43, 3 (0.3%) coronavirus HKU1, and 28 (3%) coronavirus NL63 (note: these coronaviruses are not SARS-CoV or MERS-CoV).
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 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. Four hundred eighty-seven schools in 74 counties reported ILI outbreaks during the 2012-2013 school year. Since the 2009-2010 school year, the number of schools reporting ILI outbreaks has ranged from a low of 94 in 36 counties in 2011-2012 to 1,302 schools in 85 counties in 2009-2010.
An influenza outbreak is suspected in a long-term care facility (LTCF) when two or more residents in a facility develop symptoms consistent with influenza 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 and there are other cases of respiratory illness in the same unit. Two hundred nine facilities in 63 counties reported confirmed outbreaks during the 2012-2013 influenza season. This is the highest number of outbreaks reported since surveillance for outbreaks in LTCFs began in the 1988-1989 season. The number of LTCFs reporting outbreaks ranged from a low of three in 2008-2009 to a high of 209 in 2012- 2013.
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