Influenza  
Annual Summary of Reportable Diseases

Several influenza surveillance methods are employed. Data are summarized by influenza season (generally October-May) rather than calendar year. This year’s newsletter includes data from the 2021-2022 and the 2022-2023 influenza seasons. 

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 and expanded to include adults for the 2005-2006 influenza season. Surveillance was expanded statewide for the 2008-2009 season. 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 MDH for influenza testing. For the 2014-2015 season, influenza B subtyping was added. 

During the 2021-2022 influenza season (October 3, 2021 – May 20, 2022; MMWR weeks 40-20), there were 904 laboratory-confirmed hospitalized cases reported (15.98 cases per 100,000 persons) compared to 0.62 cases per 100,000 in 2020-2021 and 71.3 cases per 100,000 in 2019-2020. Cases included 889 influenza A (168 [19%] H3 and 721 [80%] unknown A type), 13 (1.4%) influenza B (all of unknown lineage), and two (0.2%) coinfections of influenza A and B. Among the cases, 16% were 0-18, 19% were 19-49, 17% were 50-64, and 47% were 65 years of age and older. Median age was 64 years. Residents of the metropolitan area made up 52% of cases. 

During the 2022-2023 influenza season (October 2, 2022 – May 21, 2023; MMWR weeks 40-20), there were 3,338 laboratory-confirmed hospitalized cases (58.49 cases per 100,000 persons) reported, compared to 15.98 cases per 100,000 in 2021- 2022. Cases included 3,248 (97.3%) influenza A (785 [24%] H3, 110 [3%] [H1N1]pdm09 and 2,353 [70%] unknown A type), 86 (2.6%) influenza B (7 [0.2%] of Victoria lineage and 78 [2.3%] of unknown lineage), and three (0.09%) coinfections of influenza A and B. Among the cases, 17% were 0-18, 16% were 19-49, 17% were 50-64, and 50% were 65 years of age and older. Median age was 64 years. Residents of the metropolitan area made up 60% of cases. 

Case report forms were completed on 100% of the 467 metropolitan area cases for the 2021-2022 season. Of these, 24% were diagnosed with pneumonia, 11% required admission into an intensive care unit, and 5% were placed on mechanical ventilation. An invasive bacterial co-infection was present in 13% of hospitalized cases. Antiviral treatment was prescribed for 70% of cases. Overall, 95% of adult and 58% of pediatric cases had at least one chronic medical condition that would have put them at increased risk for influenza disease. 

Case report forms were completed on 100% of the 1,955 metropolitan area cases for the 2022-2023 season. Of these, 28% were diagnosed with pneumonia, 14% required admission into an intensive care unit, and 6% were placed on mechanical ventilation. An invasive bacterial co-infection was present in 16% of hospitalized cases. Antiviral treatment was prescribed for 78% of cases. Overall, 96% of adult and 50% of pediatric cases had at least one chronic medical condition that would have put them at increased risk for influenza disease. 

Pediatric Deaths 

There were two pediatric influenza-associated deaths in the 2021-2022 season (one positive for influenza A H3 and one positive for influenza A no subtype). There were two pediatric influenza-associated deaths in the 2022-2023 season (both positive for influenza A [H1N1]pdm09). 

Laboratory Data 

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. Some laboratories perform PCR testing for influenza, and others also perform PCR testing for other respiratory viruses. The MDH Public Health Lab (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. 

From October 3, 2021–May 21, 2022, laboratories reported data on 174,423 influenza molecular tests, 9,486 (5%) of which were positive for influenza. Of these, 230 (2.4%) were positive for influenza A (H3), one (≤0.1%) was positive for influenza A (H1N1)pdm09, 9,197 (97%) were positive for influenza A-not subtyped, and 58 (0.6%) were positive for influenza B. From October 2, 2022– May 20, 2023, laboratories reported data on 135,209 influenza molecular tests, 17,440 (13%) of which were positive for influenza. Of these, 305 (1.7%) were positive for influenza A (H3), 43 (0.3%) were positive for influenza A (H1N1)pdm09, 16,530 (94.8%) were positive for influenza A-not subtyped, and 562 (3.2%) were positive for influenza B. 

Sentinel Surveillance 

MDH conducts sentinel surveillance for ILI (fever >100° F, and cough, and/ or sore throat) through outpatient medical providers, including those in private practice, public health clinics, urgent care centers, emergency rooms, and university student health centers. During the 2021- 2022 and 2022-2023 seasons, there were approximately 72 sentinel surveillance sites in 34 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, 25-49 years, 50-64 years, ≥65 years). In the 2021-2022 season, the percentage of ILI peaked during the week beginning December 26, 2021, at 6.6%. In the 2022-2023 season, the percentage of ILI peaked during the week November 22, 2022, at 6.1%. 

Influenza Incidence Surveillance 

MDH continued to participate in Optional Influenza Surveillance Enhancements during the 2021-2022 and 2022-2023 influenza seasons. Each week, clinic sites reported the number of ILI patients divided by the total patients seen by the following age groups: 0-4 years, 5-24 years, 25- 49 years, 50-64 years, and ≥65 years. Clinical specimens were collected on the first 10 patients with acute respiratory illness for PCR testing performed by the PHL for influenza, SARS-CoV-2, and 13 other respiratory pathogens. 

Minimal demographic information and clinical data were provided with each specimen. From October 3, 2021–May 21, 2022, these clinics saw 837 ILI patients. They submitted 662 specimens for influenza testing; 74 (11%) were positive for influenza. From October 2, 2022–May 20, 2023, these clinics saw 321 ILI patients. They submitted 443 specimens for influenza testing; 58 (13%) were positive for influenza. Note that some sites were able to submit specimens but not report ILI visits during the COVID-19 pandemic period. 

ILI Outbreaks in Schools and Long-term Care Facilities 

Since 2009, schools report outbreaks when the number of students absent with ILI reaches 5% of total enrollment, or when three or more students with ILI were absent from the same elementary classroom. During the 2021-2022 school year, 346 schools in 60 counties reported ILI outbreaks; during the 2022-2023 school year, 1,021 schools in 85 counties reported ILI outbreaks. The number of schools reporting ILI outbreaks since the 2009-2010 school year ranged from a low of 6 in 2020-2021 to a high of 1,302 in 2009-2010. 

An influenza-like illness outbreak in a long-term care facility (LTCF) is defined as at least two cases of laboratory-confirmed influenza (or RSV) identified within 72 hours of each other in residents on the same unit. During the 2021-2022 influenza season, 49 facilities in 33 counties reported confirmed outbreaks; during the 2022-2023 influenza season, 107 facilities in 54 counties reported confirmed outbreaks. 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)

Several influenza surveillance methods are employed. 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 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 Public Health Lab (PHL) for influenza testing. For the 2014-2015 season, influenza B subtyping was added. 

During the 2020-2021 influenza season (October 1, 2020 – April 30, 2021), there were 35 laboratory-confirmed hospitalized cases reported. This represents a rate of 0.62 cases per 100,000 persons, compared to 71.3 cases per 100,000 in 2019-2020 and 44.6 cases per 100,000 in 2018-2019. Cases included 11 influenza A (all unknown A type) and 24 influenza B (all unknown lineage). Among the cases, 9% were 0-18, 17% were 19-49, 14% were 50-64, and 60% were 65 years of age and older. Median age was 69 years. Residents of the metropolitan area made up 57% of cases. 

Case report forms have been completed on 100% of the 20 metropolitan area cases that were selected for review. Of these, 28% were diagnosed with pneumonia, 17% required admission into an intensive care unit, and 11% were placed on mechanical ventilation. An invasive bacterial co-infection was present in 22% of hospitalized cases.

Antiviral treatment was prescribed for 67% of cases. Overall, 82% of adult and 0% of pediatric cases had at least one chronic medical condition that would have put them at increased risk for influenza disease.

Pediatric Deaths

There were 0 pediatric influenza-associated deaths during this season.

Laboratory Data 

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. Some laboratories perform PCR testing for influenza, and others 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 4, 2020–May 22, 2021, laboratories reported data on 54,981 influenza molecular tests, 24 (<1%) of which were positive for influenza. Of these, 0 were positive for influenza A (H3), 0 were positive for influenza A (H1N1)pdm09, 10 (42%) were positive for influenza A-not subtyped, and 14 (58%) were positive for influenza B. 

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. There were 85 sites in 39 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). The percentage of ILI peaked during the week October 4-10, 2020, at 0.9%. 

Influenza Incidence Surveillance 

MDH continued to participate in Optional Influenza Surveillance Enhancements during the 2020-2021 influenza season. Each week, nine clinic sites reported the number of ILI patients divided by the total patients seen by the following age groups: 0-4 years, 5-24 years, 25-49 years, 50-64 years, and ≥65 years. Clinical specimens were collected on the first 10 patients with acute respiratory illness for PCR testing performed by the PHL for influenza, SARS-CoV-2, and 13 other respiratory pathogens. Minimal demographic information and clinical data were provided with each specimen. From October 4, 2020–May 22, 2021, these clinics saw 1,777 ILI patients. They submitted 984 specimens for influenza testing; none were positive for influenza. 

ILI Outbreaks in Schools and Long-term Care Facilities 

Since 2009, schools report outbreaks when the number of students absent with ILI reaches 5% of total enrollment, or when three or more students with ILI were absent from the same elementary classroom. Six schools in 3 counties reported ILI outbreaks during the 2020-2021 school year. The number of schools reporting ILI outbreaks since the 2009-2010 school year ranged from a low of 6 in 2020-2021 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. Six facilities in 5 counties reported confirmed outbreaks during the 2020- 2021 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)

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

Hospitalized Cases 

Since the 2008-2009 season, statewide surveillance for pediatric and adult cases has occurred. 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 2019-2020 influenza season (October 1, 2019 – April 30, 2020), there were 4,022 laboratory-confirmed hospitalized cases (71.3 cases per 100,000 persons compared to 44.6 cases per 100,000 in 2018-2019, and 112.8 cases per 100,000 in 2017- 2018) reported. Cases included 3,029 influenza A (1,003 A[H1N1]pdm09, 24 H3, and 2,002 unknown A type), 986 influenza B (17 of Yamagata lineage and 285 of Victoria lineage), 2 positive for both influenza A and B, and 4 of unknown influenza types. Among the cases, 15% were 0-18, 19% were 19-49, 23% were 50-64, and 43% were 65 years of age and older. Median age was 61 years. Residents of the metropolitan area made up 59% of cases. 

Case report forms have been completed on all 760 metropolitan area cases that were selected for review. Of these, 32% were diagnosed with pneumonia, 20% required admission into an intensive care unit, and 8% were placed on mechanical ventilation. An invasive bacterial co-infection was present in 10% of hospitalized cases. Antiviral treatment was prescribed for 92% of cases. Overall, 92% of adult and 45% of pediatric cases had at least one chronic medical condition that would have put them at increased risk for influenza disease. 

Pediatric Deaths 

There were 3 pediatric influenza-associated deaths, 2 positive for influenza B (no genotype), and 1 positive for influenza B/Victoria lineage. 

Laboratory Data 

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 29, 2019–May 16, 2020, laboratories reported data on 74,599 influenza molecular tests, 16,293 (22%) of which were positive for influenza. Of these, 12 (<0.1%) were positive for influenza A (H3), 597 (4%) were positive for influenza A (H1N1)pdm09, 8,043 (49%) were positive for influenza A-not subtyped, and 7,641 (47%) were positive for influenza B. 

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. There were 33 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 December 22-28, 2019 at 9.7%. 

Influenza Incidence Surveillance 

MDH was one of 12 nationwide sites to participate in Optional Influenza Surveillance Enhancements. Nine 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 29, 2019–May 16, 2020, these clinics saw 4,926 ILI patients. They submitted 175 specimens for influenza testing; 42 (24%) were positive for influenza. Of those, 1 (2%) was positive for influenza A (H3), 18 (43%) were positive for influenza A (H1N1)pdm09, 1 (2%) was positive for influenza A-type unspecified, 3 (7%) were positive for influenza B/Yamagata lineage, 18 (43%) were positive for influenza B/ Victoria lineage, and 1 (2%) was positive for influenzaB/Unknown lineage. 

ILI Outbreaks in 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. Nine hundred three schools in 79 counties reported ILI outbreaks during the 2019-2020 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. One hundred seven LTCFs in 43 counties reported confirmed outbreaks during the 2018-2019 influenza season. The number of LTCFs reporting outbreaks ranged from a low of 3 in 2008- 2009 to a high of 212 in 2017-2018.

• For up to date information see: Influenza (Flu)

Several influenza surveillance methods are employed. 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 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 2019-2020 influenza season (October 1, 2019 – April 30, 2020), there were 4,022 laboratory-confirmed hospitalized cases (71.3 cases per 100,000 persons compared to 44.6 cases per 100,000 in 2018-2019 and 112.8 cases per 100,000 in 2017- 2018) reported. Cases included 3,029 influenza A (1,003 A[H1N1]pdm09, 24 H3, and 2,002 unknown A type), 986 influenza B (17 of Yamagata lineage and 285 of Victoria lineage), 2 positive for both influenza A and B, and 4 of unknown influenza types. Among the cases, 15% were 0-18, 19% were 19-49, 23% were 50-64, and 43% were 65 years of age and older. Median age was 61 years. Residents of the metropolitan area made up 59% of cases. Case report forms have been completed on all of the 760 metropolitan area cases that were selected for review. Of these, 32% were diagnosed with pneumonia, 20% required admission into an intensive care unit, and 8% were placed on mechanical ventilation. An invasive bacterial co-infection was present in 10% of hospitalized cases. Antiviral treatment was prescribed for 92% of cases. Overall, 92% of adult and 45% of pediatric cases had at least one chronic medical condition that would have put them at increased risk for influenza disease.

Pediatric Deaths

There were 3 pediatric influenza-associated deaths, 2 positive for influenza B (no genotype), and 1 for influenza B/Victoria lineage.

Laboratory Data

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 29, 2019–May 16, 2020, laboratories reported data on 74,599 influenza molecular tests, 16,293(22%) of which were positive for influenza. Of these, 12 (<0.1%) were positive for influenza A (H3), 597 (4%) were positive for influenza A (H1N1)pdm09, 8,043 (49%) were positive for influenza A-not subtyped, and 7,641 (47%) were positive for influenza B.

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. There were 33 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 December 22-28, 2019 at 9.7%.

Influenza Incidence Surveillance

MDH was one of 12 nationwide sites to participate in Optional Influenza Surveillance Enhancements. Nine 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 29, 2019–May 16, 2020, these clinics saw 4,926 ILI patients. They submitted 175 specimens for influenza testing; 42 (24%) were positive for influenza. Of those, 1 (2%) was positive for influenza A (H3), 18 (43%) were positive for influenza A (H1N1)pdm09, 1 (2%) was positive for influenza A-type unspecified, 3 (7%) were positive for influenza B/Yamagata lineage, 18 (43%) were positive for influenza B/ Victoria lineage, and 1 (2%) was positive for influenzaB/Unknown 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. Nine hundred three schools in 79 counties reported ILI outbreaks during the 2019-2020 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. One hundred seven LTCFs in 43 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.

• Find up to date information: Influenza (Flu)

Several influenza surveillance methods are employed. 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 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 laboratory-confirmed 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.

Pediatric Deaths

There were 2 pediatric influenza-associated deaths, 1 positive for influenza A (H3), and 1 positive for influenza B/Victoria lineage.

Laboratory Data

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.

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. 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)

• Archive of Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health
  Archive of past summaries (years prior to 2023 are available as PDFs).