The New York-based company's Prevnar 13 protects against 13 strains of pneumococcal disease, which can cause painful children's ear infections, pneumonia and life-threatening bloodstream infections.
The study, known by the acronym CAPiTA, included about 85,000 patients aged 65 or older.
It found that compared to study participants getting a dummy shot, those getting the vaccine had about 46 percent fewer cases of pneumonia and about 75 per cent fewer cases of invasive pneumococcal disease such as bloodstream infections.
In the US, it's approved for children from six weeks to 17 years old and for adults aged 50 and older. In 2011, the Food and Drug Administration granted accelerated approval to use the vaccine in the 65-and-up group, but required a large study to verify it actually prevented illness in them.
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Senior citizens are particularly vulnerable to infections by pneumococcal bacteria, and potentially life-threatening complications, because their immune systems don't work as well as when they were younger. For the same reason, vaccines generally are only about 50 per cent effective in patients 65 and older.
Pfizer's results were presented yesterday at a medical conference on pneumococcal diseases in India.
In a report to investors, UBS analyst Marc Goodman called the results "very promising.