great article in the Winnipeg Free Press today exploring the reasons why we might be having such a late and mild flu season. Possible reasons include a mild winter and perhaps high vaccine uptake.
Branswell quotes one of our favorite Canadian's Allison McGeer who said "there's lots of evidence that it's not as simple as temperature. People have been looking for temperature and humidity indications for a long time — and there may well be some contribution — but if it is, it's subtle and complex. It's very clear that it's not just having a milder than usual winter that makes a difference." The article also mentions that vaccination rates in Canada
and the United States are around 35-40%, which should not be high enough to explain the magnitude of annual influenza epidemics. Of course vaccination rates are only relevant if the vaccine is effective.
In the most recent issue of Archives of Internal Medicine, Kenny Wong and colleagues from the University of Toronto studied the effectiveness of influenza vaccination in a large cohort of community-dwelling Ontario residents over 65yo. Using administrative data from 2000 to 2009 and controlling for selection bias using an instrumental variable approach, they report minimal effectiveness in reducing all-cause mortality. Looking at a composite outcome of mortality plus admissions for pneumonia, they did find vaccine to be protective. For an excellent review of this paper, prior studies and the use of instrumental variables, see this most excellent post by Robert Roos over at CIDRAP. The summary of all of this is: with current vaccines, those over 65 can have their shot and get the flu too.
1) Wong K, et al. Arch Intern Med. 27 Feb 2012
2) Branswell H, Winnipeg Free Press. 6 Mar 2012
3) Roos R, CIDRAP. 1 Mar 2012