This question pertains to the following abstract33:

Acute influenza infection may be transiently associated with the risk of cardiovascular disease. We examined the association between influenza vaccination and incident myocardial infarction (MI) and stroke in a population-based case-control study. Case subjects were members of Group Health Cooperative (GHC) with incident MI or ischemic stroke during “flu season” (November-March) of 1992–1998. Control subjects were GHC members without history of MI or stroke who were frequency matched to case subjects by age, sex, and calendar year.

The medical records of 584 case subjects with MI, 269 case subjects with ischemic stroke, and 1,415 controls were reviewed. Receipt of each year’s influenza vaccine was not associated with risk of incident MI (odds ratio [OR] = 0.95, 95% confidence interval [CI]: 0.77, 1.17) or ischemic stroke (OR = 1.20, 95% CI: 0.91, 1.60) during the period of expected influenza activity. This study suggests that . . . influenza vaccination is not associated with a reduction in risk of first MI or ischemic stroke.  

By restricting cases of MI and stroke to those that occurred during November March, the investigators obtained a sample size that was considerably smaller than the one that would have included persons diagnosed with these illnesses in other months as well. What do you believe to have been the primary compensating advantage of the choice they made?