How this flu season stacks up to past seasons: 5 key metrics

While flu season inevitably comes every year, each season is different, with flu strains and severity differing each time.

 

It appears as though the 2016-17 flu season has not yet peaked. "Influenza activity in the United States began to increase in mid-December, remained elevated through February 4, 2017, and is expected to continue for several more weeks," according to a CDC report issued Friday.

Using data from the CDC's FluView and athenaInsight's Flu Dashboard, here are five key points on how the 2016-17 flu season compares so far to the 2014-15 and 2015-16 flu seasons.

1. Outpatient flu visits. It doesn't appear as though this season has peaked yet in terms of outpatient visits for influenza-like illness. As of the week ending Feb. 10, 5.2 percent of outpatient visits were due to influenza-like illness, according to the CDC.

This has already outpaced the peak of the 2015-16 season, which was milder overall. That season's outpatient flu visits peaked during the week of March 6, when 3.6 percent of overall outpatient visits were for influenza-like illness, according to the CDC. "This is one of the later season peaks on record," according to a CDC summary of the season. Just three seasons have peaked during March: 2015-16, 2011-12 and 2005-06.

The 2014-15 flu season's peak occurred the week of Dec. 21, when 6 percent of outpatient visits were for influenza-like illness.

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2. Flu hospitalizations. According to CDC FluView data for the week ending Feb. 10, the cumulative flu hospitalization rate is 29.4 per 100,000 people so far this season.

During the comparable week in the 2015-16 season, the flu hospitalization rate was 5.1 per 100,000 people, and the cumulative hospitalization rate by the end of the season was 31.4 per 100,000 people.

For the comparable week in the 2014-15 season, the flu hospitalization rate was 52.8 per 100,000. By the end of the season it was 64.1 per 100,000 people.

3. Dominant flu strain. This season, H3N2 has been the dominant strain, which usually means higher rates of hospitalizations and more deaths, according to the CDC.

That was also the dominant strain in the 2014-15 season, but it had "drifted" from H3N2 strain in that season's vaccine, which contributed to making that season more severe. Additionally, a second wave of influenza B came in early March that season.

The 2015-16 season saw H1N1 dominate. However, H3N2 strains also circulated, mostly from October through early December, with influenza B viruses becoming more common from mid-April through mid-May.

4. Vaccine efficacy. The CDC released a midseason flu vaccine efficacy report for the 2016-17 season on Friday, reporting the vaccine is 48 percent effective so far.

This is comparable to the 2015-16 season, when the vaccine was 47 percent effective. It is much better compared to the 2014-15 season, when the most common virus strain drifted from the virus in the vaccine, rendering the vaccination just 19 percent effective.

5. Effect on children. The current flu season has been especially rough on children, according to athenaInsight data. For the week ending Feb. 11, 8.29 percent of pediatric visits to pediatricians resulted in an influenza-like illness diagnosis, which is higher than any percentage recorded in 2015-16.

Per CDC data, each year flu-associated pediatric deaths have ranged from 37 to 171 (excluding the pandemic of 2009). So far this season, 29 pediatric flu deaths have been reported. In 2015-16, 89 pediatric flu deaths were reported through the whole season, and 148 pediatric flu deaths were reported in 2014-15.

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