This is because infants who were too young to receive the vaccine also showed a reduce in the infection rate. But in the present study that was conducted by Dr. Katherine A. Poehling, from Vanderbilt University School of Medicine in Nashville, and colleagues it showed something different. She assessed the pneumonia rates among infants, 0 to 90 days of age, before and after the introduction of the pneumonia vaccine.
In the eight states studied, 89 cases of invasive pneumococcal disease occurred before introduction and 57 occurred after introduction of the vaccine. With the introduction of the vaccine, the average rate of pneumococcal disease among young infants fell 40 %. The reduction was most pronounced for black infants. In case of white infants the decline was not statistically significant.
In conclusion the authors says that the pneumonia vaccine has resulted in herd immunity because neonates and young infants have had a significant decrease in pneumococcal disease rates, although they are too young to receive a full series of the vaccine.