Background: The 23-valent polysaccharide pneumococcal vaccine (PPV) has been recommended for high-risk adult patients (e.g., nursing home residents, older patients, patients with chronic pulmonary and nonpulmonary conditions) for more than 20 years. However, vaccination rates remain well below target goals of 80 to 90 percent for eligible populations. This may be because randomized trials have demonstrated that PPV can prevent invasive pneumococcal disease but not community-acquired pneumonia (CAP). Some studies have suggested that PPV might reduce the severity of CAP through a partial immunity mechanism, but they have been hampered by small sample sizes or a lack of detailed clinical data. Johnstone and colleagues prospectively analyzed the effect of previous PPV vaccination on outcomes in patients who developed CAP.

The Study: A population-based cohort of Canadian adults hospitalized for CAP received standardized admission orders related to choice and routes of antibiotics, supplemental oxygen therapy, hydration, and thromboembolic prophylaxis. Patients were excluded from the study if they had tuberculosis or cystic fibrosis, had used immunosuppressants for more than one month, had human immunodeficiency virus infection with a CD4 cell count of less than 250 cells per mm³ (250 × 10⁹ per L), or were pregnant.

A pneumonia severity index (PSI) was calculated for all patients to assess the severity of illness. PPV vaccination status was determined at admission by research nurses who were blinded to post-admission outcomes. The nurses obtained vaccination history by reviewing patient history and medical records and contacting patients' primary care offices. Patients were monitored for the duration of their hospital stay. The primary outcome measured was patients' in-hospital mortality or admission to the intensive care unit (ICU).

Results: The study monitored 3,145 patients with a median age of 75 years who were hospitalized with CAP. Nineteen percent of the patients were from nursing homes, and 31 percent had chronic obstructive pulmonary disease. Sixty-two percent were classified as having severe pneumonia (PSI class IV or V). Twenty-two percent of the patients had been immunized previously with PPV; older patients, those with more comorbidities, and those who were more frail were more likely to have been vaccinated.

Overall, 624 patients (18 percent) were either admitted to the ICU or died in the hospital. The likelihood of death or ICU admission was significantly less for patients who had been vaccinated previously (odds ratio = 0.61; P = .02). Similar findings were observed when vaccinated and unvaccinated patients were matched for age and comorbidities, and in sub-analyses restricted to patients 65 years or older and patients in nursing homes. These findings were primarily due to a decreased need for ICU admission among the PPV-vaccinated patients. Once a patient was admitted to the ICU, mortality rates were equivalent between the two groups.

Pneumococcal bacteremia was also significantly less common in the previously vaccinated group compared with those who were not vaccinated (2 percent versus 5 percent, respectively). Additionally, there were no deaths or ICU admissions among vaccinated patients with confirmed pneumococcal pneumonia compared with 27 of 85 (32 percent) unvaccinated patients.

Conclusion: Patients hospitalized for CAP who had been vaccinated previously with PPV had a 40 percent reduction in overall mortality or need for ICU admission when compared with nonvaccinated patients. These results emphasize the importance of encouraging patients to adhere to current adult pneumococcal vaccination guidelines.