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2015-2016 Flu Season in Review

by Anna Zink, Data Engineer, athenaResearch

After five weeks of declining influenza-like-illness (or “ILI”) rates, it appears that the 2015-2016 flu season may finally be coming to a close. National flu rates still remain slightly higher than they were at the same time last year but are dropping at a rate of about 10-20% each week. It has been the latest-breaking flu season we’ve seen in the last three years and an interesting one to follow.

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In early October, the athenaResearch team began its annual monitoring of national and regional ILI. People prepared for this flu season as usual with their flu shots - about 18-20% of outpatient visits to primary care physicians resulted in a flu vaccination in mid to late October, which was similar to last year. (Note: this vaccination rate largely reflects vaccinations administered in primary care offices rather than convenient care clinics, an option selected by an increasing number of adults).

A late rise for flu


Flu rates have been known to peak any month from October through March, but most frequently peak in February. Last year the flu began to rise in early December, finally peaking late in December/early January. But this year, December came and went with no signs of an increase in ILI rates. By January ILI rates began to rise each week, increasing for eight weeks until finally peaking in early March at 3.4% nationally and 6.6% for visits to pediatricians. This was a modest peak compared to last year, which was a particularly severe season with an ILI peak of 6.2% for all visits and 10.0% for visits to pediatricians. The CDC reported a 60% effectiveness in this year’s vaccine versus 19% reported last year.1,2 

A lighter year for older people

We find that the distribution of age by flu season varies year over year. Last year was more severe for all patients, but in particular for patients ages 65 or older who comprised 8.3% of all flu cases compared to 5.1% in 2013-2014 and 4.8% this year. During more moderate seasons, even during the peak, less than 1% of visits for patients over 65 have a report of ILI, but in 2014-2015 the peak ILI rate for patients ages 65 and older was 3.3%. In general, children ages 0-17 make up the largest share of ILI visits each year: approximately 60% of all ILI visits are from this age group. For those with the appetite, Table 1 below shows the age distribution over the last 3 flu seasons.

West sees the first hints of flu

We observed some regional variation this season as well. The West peaked earlier than the other regions, hitting an ILI rate of 3.5% in late February before falling again. The ILI rate in the West is now at 1.9%, lower than it was at the same point last year. The other three regions (East, Midwest, and South) peaked a few weeks later during the second week of March and are continuing to climb downwards as of the week ending April 16th, 2016.

Up Next

We will continue to monitor the flu each week in case of any upticks. In the meantime, we have started researching provider prescribing patterns of antivirals for patients with ILI. We welcome any comments on our flu surveillance work.

About the Flu Trends Report

athenahealth monitors flu severity by measuring the share of outpatient visits to primary care and emergency care providers with influenza-like-illness or viral symptoms with data collected from over 20,000 providers. Thanks to the near real-time access of our cloud-based network, we are able to report on flu activity with a 24-48 hour delay. We share this data with public health departments and academic researchers to help them monitor local and regional flu activity as well as advance flu surveillance techniques. The data for this post is collected from health care providers live on athenahealth since 2013 so that we can compare flu seasons for the same cohort of providers. Our sample consists of about 690,000 visits per week to all providers, 120,000 of which are to pediatricians.

1 http://www.cdc.gov/media/releases/2016/flu-vaccine-60-percent.html

2 http://www.cdc.gov/flu/news/updated-vaccine-effectiveness-2014-15.htm

athenaResearch intern Jameson Williams conducted much of the analysis for this report  

                                 

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