Researchers in a Midwestern county tracked flu cases requiring hospitalization in those residents aged 65
and older during a two-month period one winter. They matched each case with 2 controls by sex and age (150 cases,
300 controls). They used medical records to determine whether cases and controls had received a flu vaccine shot
and whether they had underlying lung disease. They wanted to know whether flu vaccination prevents hospitalization
for flu (severe cases of flu). Underlying lung disease is a potential confounder.