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Flu Trends Report: Has the 2015-2016 Season Finally Peaked?

by Anna Zink, Data Engineer, athenaResearch

Has the 2015-2016 flu peaked? It’s too early to tell. But after nine consecutive weeks of increasing influenza-like-illness (“ILI”) rates, athenaResearch is gratified to see a decrease in what has been an unusually late flu season.

National

Flu rates appear to have peaked across the nation as of the week ending March 19, 2016. This season started off mildly compared to the past two flu seasons, but in early January ILI rates began to climb, more than doubling well into March and peaking at 3.4% nationally (6.6% for pediatricians) in the week ending March 12th. Now, a week later, ILI rates are down to 2.9% nationally and 5.4% for pediatricians, signaling that the flu season may have peaked.

Flu rates are higher than they were at the same point last year: 2.9% this year versus 1.5% last year for all visits and 5.4% versus 3.5% for visits to pediatricians. The previous two seasons peaked in late December-early January, and ILI rates were already well in their decline by March. The most typical peak month for flu is February. Since 1982, there have only been five other recorded seasons with March as the peak month of flu activity1.

fig 1

Regional

Regionally, ILI rates have also decreased. The Midwest, Northeast, and South show the first signs of declining ILI rates as of the week ending March 19, 2016. The sharpest decrease occurred in the South, where the flu rate for pediatricians has dropped 2.0%, from a peak of 7.3% to 5.3% since last week. ILI rates in the South for all visits are down 0.7% (from 3.6% to 2.9%). If this is truly the peak in the South, this season does not begin to compare to the last in terms of magnitude. Last year, ILI rates for visits to pediatricians hit 13.8% in late December, almost double the rate of this year’s first observed peak.

Decreases in other regions have been more moderate. Overall ILI rates in the Midwest have gone down by 0.7% since last week, while visits to pediatricians are down 1.1%. In the Northeast, visits to all providers as well as pediatricians are down 0.3%. Flu rates in the West have been declining for weeks now; our data shows a peak of 3.5% in mid to late-February for all visits and 6.3% for pediatricians. ILI rates in the West have dropped by about 1.0% since this time.

Regional Influenza-Like Illness Rate Through Week Ending March 19, 2016

Summary

Although the initial data show a decline in flu rates, it is too soon to predict whether we are seeing the beginning of the end of the 2015-2016 flu season. Flu rates can fluctuate week to week, and despite regional declines, some states such as New York do not yet show decreases in ILI rates. athenaResearch will continue to monitor flu until the end of the season. Josh Gray, Vice President of Research at athenahealth, will be tweeting out the latest data each week on Twitter (@JoshGray_hit). For more flu surveillance and forecast estimates, go to Healthmap, a website from Boston Children’s Hospital and Harvard Medical School which reports on the flu based on a myriad of sources, including athenahealth.

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. Our data, collected from over 20,000 providers, is comparable to the CDC’s own ILINet.  Thanks to the near real-time access of our cloud-based network, we are able to report flu activity in advance of many other surveillance groups. 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 are 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.

1http://www.cdc.gov/flu/about/season/flu-season.htm

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