Vaccines are the only available means of preventing pneumococcal disease.
The polysaccharide capsule is the pneumococcus' main virulence factor. The capsule provides protection against ingestion by human immune system cells, and antibodies against specific types of pneumococcal capsular polysaccharide provide protection against disease. Researchers during much of the 20 th century have focused on using collections of polysaccharide antigens to create vaccines. A vaccine consisting of six of the more than 90 pneumococcal polysaccharide serotypes was the first licensed pneumococcal vaccine in the United States. This vaccine, licensed in the 1940's, was viewed as unnecessary when penicillin became widely available and was taken off the market.