Rethinking number-needed-to-vaccinate for pneumococcal conjugate vaccines in older adults: Current and future implications

Abstract

Background: Number-needed-to-vaccinate (NNV) is increasingly used to inform decisions about vaccine use, but it is not calculated uniformly across studies. This study compared two methodologies for calculating NNV with 13-valent pneumococcal conjugate vaccine (PCV13) to prevent one case of community-acquired pneumonia (CAP) among US adults aged ≥65years: (i) using one-year absolute rate differences as was originally performed by the Centers for Disease Control and Prevention (CDC) and (ii) using absolute risk reduction over 5years.

Methods: We constructed a hypothetical fixed cohort of 200,000 adults aged ≥65years equally separated into PCV13-vaccinated and PCV13-unvaccinated groups. We incorporated the same conservative assumptions used by CDC in 2014 regarding annual incidence of hospitalized (1375 per 100,000) and outpatient (2010 per 100,000) CAP, the initial (2014) proportion of adult PCV13-type CAP (10%), and PCV13 efficacy against vaccine-type CAP (45%). To model PCV13 impact over time, we assumed annual mortality was 5% for both groups, the percentage of adult PCV13-type CAP declined annually due to pediatric herd effects, and PCV13 efficacy did not wane over 5years.

Results: Among adults aged ≥65years, NNV with PCV13 to prevent one hospitalized and one outpatient case of CAP as originally calculated by CDC in 2014 were 1620 and 1110, respectively. Accounting for 5-year cumulative effects, NNV with PCV13 to prevent one hospitalized and one outpatient case of CAP over 5years were 576 and 394, respectively. These revised NNV estimates are roughly one third of initial estimates in which cumulative effects were ignored. NNV to prevent any CAP (inpatient or outpatient) over 5years with one PCV13 dose was 234.

Conclusion: Accounting for cumulative preventive effects of PCV13 vaccination over time is critical. Failing to do so, even when using conservative disease burden parameters, can grossly underestimate the public health impact of adult PCV13 use.

Keywords: 13-Valent pneumococcal conjugate vaccine (PCV13); Absolute risk reduction; Adult; Cumulative incidence; Number-needed-to-vaccinate (NNV); Public health impact.