Several influenza surveillance methods are employed. 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 and expanded to include adults for the 2005-2006 influenza season. For the 2008-2009 season surveillance was expanded statewide. Since the 2013-2014 season, clinicians have been encouraged to 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 2017-2018 season (October 1, 2017 – April 30, 2018), 6,434 laboratory-confirmed hospitalized cases (116.6 cases per 100,000 persons) were reported, compared to 70.9 cases per 100,000 in 2016-2017, and 27.5 cases per 100,000 in 2015-
2016. Cases included 5,374 influenza A (152 A [H1N1] pdm09, 2,440 H3, and 2,782 unknown A type), 1,018 influenza B (403 of Yamagata lineage and 19 of Victoria lineage), 7 positive for both influenza A and B, and 35 of unknown influenza type. Among the cases, 8% were 0-18 years, 11% were 19-49 years, 16% were 50-64 years, and 65% were 65 years of age and older. Residents of the metropolitan area made up 58% of cases.
Case report forms have been completed on 33% of the 2,603 metropolitan area cases selected for chart abstraction. Of these, 18% were diagnosed with pneumonia, 13% required admission into an intensive care unit, and 6% were placed on mechanical ventilation. An invasive bacterial co-infection was present in 9% of hospitalized cases. Antiviral treatment was prescribed for 93% of cases. Overall, 94% of adult cases and 6% of pediatric cases had at least one chronic medical condition that would have put them at increased risk for influenza disease.
There were 6 pediatric influenza- associated deaths; 4 were positive for influenza A (H3) and 2 were positive for influenza A-not subtyped.
The Minnesota Laboratory System (MLS) Laboratory Influenza Surveillance Program is made up of more than 110 clinic- and hospital-based laboratories which voluntarily submit testing data on a weekly basis. These laboratories perform rapid testing for influenza and respiratory syncytial virus (RSV). Significantly fewer laboratories perform viral culture testing. Nine laboratories perform PCR testing for influenza, and three also perform PCR testing for other respiratory viruses. The PHL provides further characterization of submitted influenza isolates to determine the hemagglutinin serotype. Tracking laboratory results assists healthcare providers with patient diagnosis of influenza-like illness (ILI), and provides an indicator of the progression of the influenza season as well as prevalence of disease in the community. Between October 1, 2017 – May 19, 2018, laboratories reported data on 42,188 influenza PCR tests, 8,260 (20%) of which were positive for influenza. Of these, 326 (4%) were positive for influenza A (H3), 75 (1%) were positive for influenza A (H1N1) pdm09, 6,110 (74%) were positive for influenza A-not subtyped, and 1,749 (21%) were positive for influenza B.
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. There were 29 sites in 22 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 4-10, 2017 at 6.7%.
Influenza Incidence Surveillance
MDH was one of eight nationwide sites to participate in an Influenza Incidence Surveillance Project. Five clinic sites reported the number of ILI 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 for PCR testing at the PHL for influenza and 13 other respiratory pathogens.
Minimal demographic information and clinical data were provided with each specimen. From November 1, 2017 – May
11, 2018, these clinics saw 2,136 ILI patients. They submitted 234 specimens for influenza testing; 77 (33%) were positive for influenza. Of those, 49 (64%) were positive for influenza A (H3), 6 (8%) was positive for influenza A (H1N1) pdm09, 4 (5%) were positive for influenza A-type unspecified, 13 (17%) were positive for influenza B/Yamagata lineage, and 5 (6%) were positive for influenza B/ Victoria lineage.
ILI Outbreaks (Schools and Long Term Care Facilities)
Since 2009, schools reported outbreaks when the number of students absent with ILI reached 5% of total enrollment, or when three or more students with ILI were absent from the same elementary classroom. Six hundred ninety-eight schools in 72 counties reported ILI outbreaks during the 2017-2018 school year. The number of schools reporting ILI outbreaks since the 2009-2010 school year ranged from a low of 92 in 2013-2014 to a high of 1,302 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 twelve facilities in 68 counties reported confirmed outbreaks during the 2017-2018 influenza season. The number of LTCFs reporting outbreaks ranged from a low of three in 2008- 2009 to a high of 212 this season.
- For up to date information see>> Influenza (Flu)
- Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2017