A study of the efficacy of pneumococcal vaccination in the elderly was described as follows 22 : We.

A study of the efficacy of pneumococcal
vaccination in the elderly was described as follows22:

We conducted a 2-year retrospective cohort
study among all elderly members of a staff-model managed care organization who
had a baseline diagnosis of chronic lung disease. The study outcomes were
assessed over 2 years, from November 15, 1993, through November 14, 1995, and
included hospitalizations for pneumonia and influenza.

Of 1898 subjects, 1280 (67%) had received
pneumococcal vaccination. This included 843 (44%) who were vaccinated prior to
November 15, 1993 and an additional 437 (23%)

A study of the efficacy of pneumococcal
vaccination in the elderly was described as follows22:

We conducted a 2-year retrospective cohort
study among all elderly members of a staff-model managed care organization who
had a baseline diagnosis of chronic lung disease. The study outcomes were
assessed over 2 years, from November 15, 1993, through November 14, 1995, and
included hospitalizations for pneumonia and influenza.

Of 1898 subjects, 1280 (67%) had received
pneumococcal vaccination. This included 843 (44%) who were vaccinated prior to
November 15, 1993 and an additional 437 (23%) vaccinated after that date.
During the follow-up period there were 174 hospitalizations for pneumonia and
influenza. The observed cumulative incidence was 138 per 1000 in the 618
unvaccinated persons and 70 per 1000 in the 1280 persons following their
receipt of vaccination. This represents a 49% reduction in hospitalization for
pneumonia and influenza.

Even if there were neither
misclassification nor confounding in this study, the estimate of benefit
associated with pneumococcal vaccination in elderly patients with chronic lung
disease must be biased.

a. Why? In which direction?

b. How could the analysis be conducted to
remove this source of bias?

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