The 2018-2019 influenza season was an unusually long season in Utah, with elevated influenza activity running 22 consecutive weeks from late November to late April. Influenza illness largely fell into in two distinct waves. The first wave was predominantly driven by influenza A H1N1 while the second started in mid-February 2019 and was driven by influenza A H3N2. Very few cases of influenza B were reported. In total, over 1,800 people were hospitalized and six people under the age of 18 died with influenza associated illnesses during the 2018-19 influenza season. This high severity season highlights the need for ongoing influenza prevention like getting a flu shot each season; preventing the spread of flu by staying home when sick, covering coughs, and washing hands; and calling your doctor if you think you make be sick with the flu.
UDOH estimates influenza severity by comparing the rates of hospitalizations, influenza-like illness, and positive laboratory results to historic influenza seasons. The 2018-19 seasonal severity estimate is High.
Indicators that are below the IT50 threshold are shown in green in the gauges and plots below, and are considered “Low” severity. Indicators between IT50 and it IT90 (yellow) are considered “Moderate” severity, indicators between IT90 and IT98 (red) are considered “High” severity, and indicators above IT98 are considered “Very High” severity (purple). The Utah season severity estimate is then set by comparing across all three indicators. This methodology was published by CDC researchers in the American Journal of Epidemiology in October 2017.
Influenza-associated hospitalization (IAH) is a reportable condition in Utah. People meet the case definition for influenza hospitalization if they are hospitalized for any length of time and have a positive influenza laboratory test. Public health in Utah gathers a variety of data on influenza hospitalizations, including clinical features, course of illness, risk and protective factors, and influenza type and subtype. Data from influenza hospitalizations allow public health officials in Utah to better understand the groups of people most severely affected by influenza and help guide prevention messages and interventions. Data collection for the 2018/2019 influenza season began on September 30, 2018.
The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) is a national system that conducts surveillance for ILI in outpatient healthcare facilities, including emergency departments. For this system, ILI is defined as fever (temperature of 100°F or greater) and a cough and/or sore throat. These data provide an indication of ILI circulating in the community. During the 2018-2019 influenza season, ILI reached the second highest peak seen in the previous 5 years, and was above seasonal baseline for 22 consecutive weeks.
Due to technical issues, ILI in Utah was significantly underreported during the 2017-2018 season. ILI rates were recalculated in the summer of 2018 and the updated data are shown below.
| Count | Percentage | |
|---|---|---|
| Influenza A | 8346 | 94.5% |
| Influenza A (H3N2) | 998 | 12.0% |
| Influenza A (H1N1) | 1307 | 15.7% |
| Influenza A, unsubtyped | 6035 | 72.3% |
| Influenza B | 483 | 5.5% |
Influenza A viruses can be typed into different strains. The most common subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses. In the spring of 2009, a new influenza A (H1N1) virus emerged that caused severe illness in humans. This new virus was very different from the human influenza A (H1N1) viruses circulating at that time and caused the first influenza pandemic in more than 40 years. That virus (often called “2009 H1N1”) has now replaced the H1N1 virus that was previously circulating in humans. Some influenza tests are unable to differentiate between influenza A strains, so many positive influenza A results reported to UDOH are unsubtyped.
School-age children are at high risk for respiratory viral infections, including influenza. Absenteeism data due to all causes is collected weekly from over 350 schools throughout Utah. These data are analyzed to identify elevated absenteeism rates that may indicate the circulation of influenza in school-age children.
Each week the total number of death certificates received and the number of those for which pneumonia or influenza was listed as an underlying or contributing cause of death is collected. The percentage of deaths due to pneumonia and influenza are compared with a seasonal baseline and predicted epidemic threshold. These data are used to monitor the severity of influenza illness in the community. Utah did not cross the epidemic threshold during the 2018-19 influenza season.