id	author	title	date	pages	extension	mime	words	sentences	flesch	summary	cache	txt
cord-011754-lumzp1ca	Jackson, Michael L.	Further Evidence for Bias in Observational Studies of Influenza Vaccine Effectiveness: The 2009 Influenza A(H1N1) Pandemic	2013-10-15		.txt	text/plain	2800	127	37	Numerous observational studies have estimated that influenza vaccine reduces the risk of all-cause mortality among seniors during the winter months by 40% or more and have concluded that influenza vaccine is highly effective (4, (6) (7) (8) . Strong evidence for confounding comes from studies that have estimated the association between influenza vaccination and risk of death during time periods before, during, and after the seasonal circulation of influenza. If the same trends in apparent associations between vaccination and mortality were observed during a year when there could be no true vaccine effect, the claim that confounding differs between time periods would be refuted. We do not believe there was a true difference between associations in these two years, as the hazard ratio estimates were further from the null during the 2007/2008 control time periods ( preinfluenza and summer, when any apparent association is the result of uncontrolled confounding) as well as during influenza season.	./cache/cord-011754-lumzp1ca.txt	./txt/cord-011754-lumzp1ca.txt