Effectiveness and Serotype Coverage: Key Criteria for Pneumococcal Vaccines for Adults

Pneumococcal polysaccharide vaccines (PPSVs) received their proof-of-principle affirmation over 65 years ago, in the pivotal trial led by Colin MacLeod and Michael Heidelberger at a military technical school in the 1940s [1]. Four cases of vaccine-type pneumococcal pneumonia developed among 8586 vaccine recipients, in contrast to 26 cases among 8449 controls injected with saline as placebo. All 4 cases among vaccinees developed within 2 weeks of vaccination. Cases of non–vaccine-type pneumococcal pneumonia occurred at comparable rates in each group of young, working adults. Interestingly, the pneumococcal carriage rate in vaccine recipients was significantly lower than in the control group, similar to findings from South Africa in the 1970s [2]. Robert Austrian cited the MacLeod-Heidelberger study, among others, in concluding that PPSVs prevent nonbacteremic pneumococcal pneumonia [3]. Subsequent randomized clinical trials and observational studies, overall and in strata of older populations (discussed below), provide further evidence for PPSV efficacy in preventing invasive pneumococcal disease (IPD) and pneumococcal pneumonia [4–6]. The 14-valent PPSV14 was licensed in 1977 for people 50 years and older or those 2 years and older with certain underlying health conditions, targeting 70%– 80% of IPD cases [7–9]. Expansion to 23 serotypes (PPSV23) was based on a broader global survey of invasive pneumococcal isolates coordinated by multiple national governments and the World Health Organization. Thus, PPSV23 was introduced in 1983 with serotypes covering about 87% of bacteremic pneumococcal disease in the United States [9, 10]. Although the technology for conjugate vaccines dates back to the 1920s [11, 12], that technique was applied to useful pneumococcal conjugate vaccines (PCVs) only since 2000. The most recent development has been licensing of PCV13 for adults in industrialized countries since late 2011. Now that 2 forms of pneumococcal vaccine are available for adults, how should clinicians choose between them? Clinical efficacy measures (eg, reduction in disease incidence, safety) are obvious choices, as is the proportion of disease burden a vaccine can target. Except for a 2-dose PCV7 study among adults infected with human immunodeficiency virus (HIV) in Malawi [13], the randomized clinical trial results for preventing pneumococcal disease in adults currently available assess the performance of PPSVs against clinical endpoints [4–6]. As discussed at the February 2012 meeting of the US Advisory Committee on Immunization Practices (ACIP), the recent licensure of PCV13 for adults is based on noninferiority of antibody concentrations (typically at day 30 after vaccination) in relation to PPSV23 serotypes in common. Until now, clinicaloutcome studies with PCVs have been conducted in infants and young children, whose response to vaccination varies substantially from that of adults (who respond robustly to both PPSVs and PCVs). PCV13 is the subject of a randomized clinical trial in Netherlands adults that eventually will compare its clinical performance to a saline placebo; key endpoints are not expected until the latter part of 2013 [14, 15]. Received 28 December 2011; accepted 29 February 2012; electronically published 11 April 2012. Correspondence: John D. Grabenstein, RPh, PhD, Merck Vaccines, 770 Sumneytown Pike, WP97-B364, West Point, PA 19426 ( john_grabenstein@merck.com). Clinical Infectious Diseases 2012;55(2):255–8 © The Author 2012. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com. DOI: 10.1093/cid/cis354