On Jun. 30, 1983, the U.S. Food and Drug Administration (FDA) licensed two enhanced pneumococcal polysaccharide vaccines (Pneumovax 23 by Merck on Jun. 30 and Pnu-Imune 23 by Lederle on July 11). These vaccines included 23 purified capsular polysaccharide antigens of Streptococcus pneumoniae and replaced the 14-valent polysaccharide vaccine licensed in 1977.

Using a case–control design, showed that protection of young adults by PPSV23 at 3 years exceeded 90% and remained at > 85% for 5 years of observation. However, with each decade of increase in age of the subjects, protection declined, such that in the oldest group, no protection was observed five years post-vaccination.

While there has been enormous progress in the development of pneumococcal vaccines during the past century, attempts to develop a vaccine that will retain its efficacy for most pneumococcal serotypes were ongoing.