RT Journal
A1 Fisman DN
T1 TO rule out confounding, observational studies of influenza vaccine need to include analyses during the “preinfluenza period”—reply
JF Archives of Internal Medicine
JO Archives of Internal Medicine
YR 2007
FD July 23
VO 167
IS 14
SP 1554
OP 1555
DO 10.1001/archinte.167.14.1554
UL http://dx.doi.org/10.1001/archinte.167.14.1554
AB 
We appreciate the comments of Jackson and colleagues regarding possible residual confounding by health status as an explanation for the protection provided by influenza vaccination against death in our study.1 Confounding and biases represent ever-present threats to validity for even the best thought-out observational research.2 We controlled for numerous potential confounders, including health status, nursing home residence, and severity of illness on admission in multivariable models, and the observed protective effect of influenza vaccination remained robust. Furthermore, we included only hospitalized individuals with pneumonia. The implicit exclusion of vaccinated individuals at low risk of pneumonia-related hospitalization makes a “healthy vaccine effect” unlikely.