Just out from NIH:

Reuters Health Information
Vaccine Cuts Bacterial Disease in US
Thursday, September 15, 2005

NEW YORK (Reuters Health) - In 2003, routine immunization of US children with a vaccine that protects against bacterial infection prevented nearly 30,000 cases of severe disease, researchers from the Centers for Disease Control and Prevention report.

The 7-valent pneumococcal conjugate vaccine (PCV7) vaccine protects against a variety of infections caused by Streptococcus pneumoniae, many of which, such as meningitis or pneumonia, can be fatal.

"Routine use of PCV7 in young children has reduced the (rate of severe pneumococcal infections) in children and adults, and these reductions have increased since 2001," Dr. Deron C. Burton and colleagues note in the CDC's Morbidity and Morality Weekly Report. The use of the vaccine was tied to a drop in all pneumococcal infections, not just those that the vaccine was specifically designed to combat, the so-called vaccine-type infections.

In 2000, PCV7 was licensed for routine use in children under 5 years of age. In the years preceding licensure, the rate of vaccine-type infections among young children was 80 cases per 100,000 population. By 2003, this rate had dropped 94 percent to just 4.6 cases per 100,000 population.

However, it is not just the direct protective effects that make PCV7 important from a public health standpoint, but also the "herd immunity" that occurs.

Herd immunity occurs when a large proportion of the population is vaccinated against a particular microbe, which, in turns, makes the odds of infection among unvaccinated individuals low. In the present analysis, the herd immunity effect was actually twice as large as the direct protective effect.

Of the 29,599 vaccine-type infections prevented by PCV7 in 2003, just 9140 were through direct protective effects. The rest of the prevented infections were the result of herd immunity.

Outside of the target population, the greatest declines in both vaccine-type and overall pneumococcal infections occurred in people who were at least 65 years of age, the report indicates.

SOURCE: Morbidity and Mortality Weekly Report, September 16, 2005.