A huge study of Pfizer's blockbuster vaccine against pneumonia and other bacterial infections in adults 65 and older found it protected most of them against dangerous infections in the bloodstream and other sterile body areas.

That age group is particularly vulnerable, because normal weakening of the immune system as people age makes them more likely to get life-threatening pneumonia and other infections caused by the bacteria.

Half of the 84,496 study participants got a dose of Prevnar 13; the other half received a dummy shot.

Overall, Prevnar reduced community-acquired pneumococcal infections — those contracted outside of hospitals and nursing homes — by about 45.6 percent, the Pfizer-funded study found.

The New York-based drugmaker reported that less-common, but more-dangerous bloodstream and other invasive infections occurred in 75 percent fewer patients getting Prevnar shots, compared with the group getting a placebo shot.

The study was published Wednesday in The New England Journal of Medicine.

Prevnar 13, called Prevenar in some foreign countries, is the world's most widely used vaccine against pneumococcal infections, with more than 750 million doses distributed worldwide.

It's also the most lucrative vaccine in history, generating $4.5 billion in sales last year. It's the No. 2 product for Pfizer, which also makes Viagra and Lyrica for fibromyalgia and other pain.

Prevnar 13 protects against 13 strains of pneumococcal disease, the most common bacterial cause of pneumonia and a top cause of death and hospitalization worldwide. The bacteria also cause children's ear infections and other illnesses.

Pfizer was required to do the study, known as CAPITA for Community-Acquired Pneumonia Immunization Trial in Adults, by regulatory bodies in some countries.

Most recently, the vaccine was approved in February in the European Union for people aged 18 and older. It's approved in more than 120 countries. In the U.S., it's approved for children from six weeks through 17 years old and adults over 49.