Flu rates

This graph shows rates of “influence-like illness” tracked at sentinel monitoring sites in Indiana in recent years. While the 2020-21 saw historically low flu activity for the entire season, flu activity usually rises in December and runs higher through March or April each year.

INDIANAPOLIS — It’s October, which means it’s time for the Indiana State Department of Health to start monitoring seasonal influenza.

The state is coming off one of its mildest flu seasons ever in 2020-21, as mitigation efforts put in place to stymie the spread of COVID-19 also had widely beneficial effects in reducing flu and other communicable illnesses last fall and winter.

What will transpire this year, only time will tell, but with a majority of COVID mitigation efforts pulled back, it would seem likely the state could be in for a more typical flu season.

Seasonal flus typically resurface each year as the weather turns colder, which pushes more people indoors and the cold weather taxes people’s immune systems.

Activity usually starts to pick up in December and can stay high through March or April, although every year is different.

Indiana begins flu monitoring the first week of October and then runs 20 weeks into the new year, taking weekly flu surveillance through May.

The state tracks “influenza-like illness,” by receiving reports from sentinel outpatient providers participating monitoring. A positive test for influenza from a lab or rapid test isn’t required in order to be counted in that trend, making flu counts more broad than other data collection like COVID-19 cases, which are based only on positive lab tests.

In recent years — excluding 2020-21 because activity was so low — flu reports typically run under a 2% baseline, but rises upward of 3% during the winter months, ranging as high about 8% of illness reports during the harsh 2017-18 season.

The state also collects and reports data from hospital emergency department chief complaint records through the Indiana Public Health Emergency Surveillance System, which captures the percentage of cases coming in to emergency rooms and urgent care clinics where flu is the primary cause of the visit.

Like the general sentinel data, chief complaint rates usually run below 2% baseline but can rise up in the 3-7% during peak periods.

The state also logs annual death counts attributable to flu, with Indiana seeing about 103 deaths per year average over the past nine years.

Indiana logged just seven deaths from flu in the 2020-21 season, which was unusually low, but not even the lowest in recent history, as the state recorded only three deaths in 2011-12.

The 2017-18 season was particularly bad, with 336 flu deaths, but other years in the nine-year lookback period have run in the 70-150 range.

Flu deaths are not incorrectly classified as COVID-19 deaths, as some have claimed, because in order to be counted as a COVID-19 death, a patient must have tested positive for that virus. Flu and COVID-19 are not the same illness and have separate confirmatory tests, so a person who tests positive for flu — or at least doesn’t test positive for COVID-19 — wouldn’t be counted as a COVID death.

Beyond that, while Indiana typically averages around 100 flu deaths per year, the state has seen more than 15,000 deaths attributable to COVID-19.

Also in weekly monitoring, Indiana tracks lab testing data when samples are sent for analysis to determine a particular strain of flu, to help identify which strains might be most dominant during a particular season.

Last flu season was an outlier on the extreme low end not just in Indiana but nationally.

“Flu activity was unusually low throughout the 2020-2021 flu season both in the United States and globally, despite high levels of testing,” the Centers for Disease Control and Prevention noted in its season summary. “The low level of flu activity during this past season contributed to dramatically fewer flu illnesses, hospitalizations, and deaths compared with previous flu seasons.”

Health officials attributed the low number of flu cases last season primarily to mitigation efforts in place that were aimed at stopping COVID-19, but also helped reduce transmission of flu and other viruses.

“COVID-19 mitigation measures such as wearing face masks, staying home, hand washing, school closures, reduced travel, increased ventilation of indoor spaces, and physical distancing, likely contributed to the decline in 2020-2021 flu incidence, hospitalizations and deaths,” the CDC said.

Annual flu shots also can help reduce transmission year to year and the U.S. had a record number of flu vaccines — 193.8 million doses — distributed in 2020-21.

Typically about 50% of adults get a flu shot each year, which immunization rates in children typically run about 60%.

Flu shots are typically a cocktail of antibody-spurring formulas, aimed at reducing impact of strains expected to become widespread during the year. Effectiveness of flu shots can vary from year to year, primarily dependent on whether vaccine developers did a good job forecasting which virus strains are likely to hit, with bad years coming when a strain outside the formula becomes more widespread.

Seasonal flu has a lower mortality rate than COVID-19, which has similar symptoms and spreads in a similar fashion. Like COVID-19, while younger, healthy people are likely to be ill for a few days and recover, flu can be serious and even deadly to some people.

The elderly and immuno-compromised patients are at higher risk of severe illness of death from flu. Unlike COVID-19, which generally causes few issues for the very young, influenza is also dangerous to children under 5 years old and especially those younger than 6 months old.

Pregnant women and women within two weeks postpartum are also be at elevated risk from flu.

Weekly flu reports from the state can be found online at in.gov/health/erc/infectious-disease-epidemiology/diseases-and-conditions-resource-page/weekly-influenza-reports. Reports are typically uploaded each Friday.

Editor’s Note: As was done last year, KPC Media Group will round up weekly flu surveillance reports in Indiana. Those stories will usually post to kpcnews.com on Mondays and appear in print on Tuesdays, barring delays.

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