Effectiveness of the 23-Valent Polysaccharide Vaccine against Invasive Pneumococcal Disease in Navajo Adults

Invasive pneumococcal disease occurs 2–3-fold more often among Navajo adults than among adults in the general United States population. The objective of this observational study was to determine the effectiveness of the 23-valent pneumococcal polysaccharide vaccine (PPV23) among Navajo adults. Active surveillance identified cases of invasive pneumococcal disease during 1996–1997. Three control patients per case patient were matched according to underlying medical conditions, sex, age, and location of medical care. Effectiveness was calculated by regression analysis of case-control sets and by indirect cohort methodology. Diabetes and alcoholism occurred in 41% and 43% of 108 case patients, respectively; 62% of case patients and 64% of control patients were immunized. Overall vaccine effectiveness was 26% (95% confidence interval [CI], −29% to 58%); 15% (95% CI, −116% to 67%) for patients with diabetes and −5% (95% CI, −141% to 54%) for patients with alcoholism. Overall vaccine effectiveness, as determined by use of the indirect cohort methodology, was 35% (95% CI, −33% to 69%). PPV23 was not significantly effective among Navajo adults and may be inadequate to prevent serious pneumococcal disease in this population.