Effect of pneumococcal capsular polysaccharide vaccine on pneumococcal bacteremia was studied in three-week-old Sprague-Dawley rats. Seventy two animals were divided into the following six equal groups. I-sham operation plus saline administration II-sham operation plus type 6 pneumococcal vaccine III-sham operation plus killed type 6 pneumococci IV-splenectomy plus saline administration V-splenectomy plus type 6 pneumococcal vaccine VI-splenectomy plus killed type 6 pneumococci The intravascular clearance of type 6 Streptococcus pneumoniae and the number of pneumococci in the liver and spleen were compared among the six groups. The serum opsonic activity of all animals after vaccination was also measured before bacterial challenge. Bacteremia was induced by intravascular injection of 10^5 type 6 S. pneumoniae. Splenectomy produced significant impairment of intravascular clearance of pneumococci compared to clearance in sham-operated animals. Administration of type 6 pneumococcal vaccine and killed type 6 pneumococci significantly improved pneumococcal clearance compared to the non-vaccinated groups in both sham-operated and splenectomized rats. Significantly less number of pneumococci were cultured in the liver of vaccinated rats than in the liver of controls. The serum opsonic activity in vaccinated rats was significantly higher than in non-vaccinated animals, and this higher opsonic activity seemed to contribute to improvement of pneumococcal clearance by vaccination. These results suggest that pneumococcal capsular polysaccharide vaccine could provide protection against pneumococcal sepsis in splenectomized children.
