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. For the 2014-2015 season, influenza B subtyping was added.
During the 2018-2019 influenza season (October 1, 2018 – April 30, 2019), there were 2,490 laboratoryconfirmed hospitalized cases (45.5 cases per 100,000 persons compared to 116.6 cases per 100,000 in 2017- 2018 and 70.9 cases per 100,000 in 2016-2017) reported. Cases included 2,377 influenza A (670 A[H1N1] pdm09, 287 H3, and 1,420 unknown A type), 101 influenza B (12 of Yamagata lineage and 4 of Victoria lineage), 4 positive for both influenza A and B, and 8 of unknown influenza types. Among the cases, 13% were 0-18, 18% were 19-49, 25% were 50-64, and 45% were 65 years of age and older. Median age was 62 years. Residents of the metropolitan area made up 53% of cases.
Case report forms have been completed on 66% of the 1,326 metropolitan area cases. Of these, 29% were diagnosed with pneumonia, 21% required admission into an intensive care unit, and 8% were placed on mechanical ventilation. An invasive bacterial co-infection was present in 12% of hospitalized cases. Antiviral treatment was prescribed for 93% of cases. Overall, 93% of adult and 47% of pediatric cases had at least one chronic medical condition that would have put them at increased risk for influenza disease.
There were 2 pediatric influenza-associated deaths, 1 positive for influenza A (H3), and 1 positive for influenza B/Victoria lineage.
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. 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 September 30, 2018–May 18, 2019, laboratories reported data on 44,297 influenza molecular tests, 6,032 (14%) of which were positive for influenza. Of these, 121 (2%) were positive for influenza A (H3), 333 (6%) were positive for influenza A (H1N1)pdm09, 5,430 (90%) were positive for influenza A-not subtyped, and 148 (2%) 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 17 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 January 6-12, 2019 at 4.7%.
Influenza Incidence Surveillance
MDH was one of 12 nationwide sites to participate in Optional Influenza Surveillance Enhancements. Four 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 September 30, 2018–May 18, 2019, these clinics saw 1,448 ILI patients. They submitted 236 specimens for influenza testing; 33 (14%) were positive for influenza. Of those, 10 (30%) were positive for influenza A (H3), 14 (42%) was positive for influenza A (H1N1)pdm09, 1 (3%) were positive for influenza A-type unspecified, 1 (3%) were positive for influenza B/Yamagata lineage, 3 (9%) were positive for influenza B/ Victoria lineage, and 3 (9%) were positive for influenza C.
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.
Three hundred eighty-three schools in 74 counties reported ILI outbreaks during the 2018-2019 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. Sixty facilities in 37 counties reported confirmed outbreaks during the 2018-2019 influenza season. The number of LTCFs reporting outbreaks ranged from a low of three in 2008- 2009 to a high of 212 in 2017-2018.
- For up to date information see>> Influenza (Flu)
- Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2018