Influenza-like-illness or “ILI” affects on average about 5% to 20%1 of the US population each year. The CDC collects ILI data from all over the country in order to monitor ILI, which can spread quickly and result in mild to severe illness. At athenahealth we provide timely regional and national data on flu trends from our cloud-based network of providers. For this analysis, we looked at flu activity from a continuous sample of practices on athenahealth since at least 2013 so we could compare regional and national ILI rates across flu seasons. We measure ILI by the proportion of primary care visits with a diagnosis of ILI, referred to as the ILI rate.
So far the 2015-2016 flu season has been fairly modest compared to the last two seasons (Figure 1). The spike in late December/early January which we observed in previous years did not materialize. At the same time last year, flu had already peaked for the second time and was declining, but this year the national ILI rate as of the week ending February 6, 2016 is the highest it has been all season at 0.8%. This is half last year’s ILI rate at the same time, which was 1.7% with a peak of 4.5% during the first week of January. Similarly for pediatrics, last year ILI rates were higher but declining at this point in time, whereas this year the ILI rate is low but increasing, up from 1.0% to 1.7% since the beginning of January. This season is not considered unusual as flu seasons are highly variable, and last season was considered to be severe especially for people 65 and older.2
We are seeing the most ILI activity in the West for Pediatrics where we track approximately 480 pediatricians with an average of 17,000 visits each week. The rate of ILI has more than doubled since early January, from 1.4% to 3.3% and is higher than it was at the same time last year (Figure 2). The ILI rate for all visits has also been increasing, but at a slower rate. In other regions, Pediatric ILI rates are the highest they’ve been so far this season: the Pediatric ILI rate is 1.3% in the Midwest, 1.2% in the Northeast, and 1.8% in the South – an increase of 0.3-0.4% in the last four weeks. ILI rates for all visits in these regions remains low (Figure 3).
Flu season is not over. According to the CDC, flu often peaks in February and we may be looking at such a season. athenaResearch will continue to monitor ILI in the coming weeks; in particular at the regional level where we are starting to see rates pick up for Pediatrics. Look to the athenahealth blog CloudView for further updates on this flu season. For more information on flu, you can visit the CDC website or track flu trends at HealthMap, a website providing real-time and forecast estimation of flu activity in the US using athenahealth data in concert with other data sources.