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Understanding the Science Behind Idaho's Mild 2015-2016 Flu Season


  • Adam Rosenlund

This time last year, health care providers who reported flu cases to the Idaho Department of Health and Welfare said 6 percent of patient visits were flu-related—down from an 8 percent high at the end of December 2014. By the end of January 2016, just under 1 percent of visits were flu-related. The 2015-2016 flu season is—knock on wood—one of the mildest in years. 

"Compared to the last three flu seasons, we still haven't seen much of a rise in flu activity at all," said IDHW Influenza Surveillance Coordinator Dr. Leslie Tengelsen. "Normally flu seasons pick up after the Christmas holiday and in the last three years, they've really been hammering mid- to late-December and into January."

Every year, 3 million-5 million people contract the flu and between 250,000-500,000 people die, according to the World Health Organization. In Idaho, dozens of people may die from complications. 

Tengelsen files weekly reports on Idaho influenza activity during the flu season using data pulled from medical providers across the state. Flu isn't one of the diseases on the state's reportable database—which includes diseases like Ebola, Colorado fever and HIV/AIDS—but labs across the state provide her with data about what strains of influenza are active in Idaho at any given time. She also reports on fatalities from the disease.

- Idaho flu-related fatalities by season. -  - HEALTHANDWELFARE.IDAHO.GOV
  • Idaho flu-related fatalities by season.
There has been a single reported flu fatality in Idaho during the current flu season, but Jan. 8-22, 2015, there was an average of one flu-related death per day. During that season, 32 people died.

The exact reasons why some flu seasons are deadly and others comparatively mild continue to baffle researchers, but they're aware of a few factors that contribute to a season's flu severity. These include the incidence of exotic strains of the virus that migrate from animal to human populations and whether available vaccines are a good match for the strains of flu circulating in a given year. 

"This year the vaccine was considered a very good match to the flu viruses that are circulating out there. It's just a quieter year and we're thankful for that," Tengelsen said. 

Every year, scientists around the world gather data and study circulating viruses to come up with vaccines applicable to their communities. They generate huge amounts of information and have access to laboratories, sophisticated modeling computers and research tools, but having enough doses of vaccines means health officials prepare for flu seasons months and sometimes years in advance. That means taking a gamble. Scientists must project what viruses will be most prevalent and develop vaccines accordingly—sometimes they project wrong. That's what happened in late 2014 and early 2015, when the "match" between the vaccine and active viruses was poor.

"Last year was a terrible match and we had a lot of deaths," Tengelsen said.

Regardless of whether a flu vaccine is well matched to a given year's strain, Tengelsen said getting a flu shot can reduce the severity of illness in individuals and protect people with less immunity, like the immunocompromised, the young and the elderly.

Unlike many other vaccines, such as MMR or polio, which are effective against illness for years, the flu vaccine is most effective the year in which it was administered. The other reason vaccines are important is herd immunity: The principle that someone who is unable to get a flu shot is protected by an inoculated community's immunity.

Despite what Tenglesen and other public health officials say about the value of vaccinations, the Gem State has one of the highest childhood vaccine exemption rates in the U.S., with many Idahoans objecting to vaccines on philosophical grounds. During the 2014-2015 school year, Idaho's childhood vaccination exemption rate was 6.5 percent, or 1,432 students. Bonner County, in the Idaho panhandle, reported a 20.6 percent exemption rate and several other counties have exemption rates well above 10 percent.

"Vaccination is really important for you, the individual, who receives vaccines," Tengelsen said. "It may prevent you from getting ill or reduce the severity in you. The other reason people should get vaccinated is you might spend time with your grandmother who, if you carry the flu virus around, might get so sick that she dies. It provides protection in the community; it prevents other people from getting sick."