Pharmaceutical giant Pfizer on Thursday announced positive results in the elderly for a vaccine against respiratory syncytial virus (RSV), for which there are currently no authorized shots.

The virus causes bronchiolitis, a respiratory disease that mainly affects infants, but can also be dangerous for the elderly, who can develop pneumonia.

More than 177,000 elderly people are hospitalized each year in the United States because of RSV infection, and about 14,000 die, according to US health authorities.

The vaccine is being tested on people 60 years and older in an ongoing clinical trial. According to a preliminary analysis, it was about 85 percent effective at preventing severe cases -- those involving three or more symptoms.

Pfizer says it plans to submit an application for the vaccine to the US Food and Drug Administration, as well as submissions to other regulatory agencies in the coming months.

"Scientists and researchers have worked to develop RSV vaccines with little success for over half a century. These findings are an important step in our effort to help protect against RSV disease," Annaliesa Anderson, Pfizer's chief scientific officer responsible for vaccine research and development, said in a statement.

The vaccine was well tolerated and did not raise any safety concerns, according to the company, which said the results would be submitted for publication in a scientific journal.

About 37,000 participants have already been enrolled in the clinical trial, out of an expected total of 40,000. Half of them are receiving the vaccine, which targets two strains of the virus, and the others a placebo.

Other companies, including Moderna, are also working to develop an RSV vaccine.