A contagious respiratory virus in young children is often misdiagnosed and is more common than thought, researchers in the U.S. say.

In Thursday's New England Journal of Medicine, Dr. Caroline Breese Hall of the University of Rochester Medical Center in New York and her colleagues reported on the burden that respiratory syncytial virus or RSV among children under the age of five.

RSV is the most common cause of bronchiolitis and pneumonia among children under one year of age. The least severe form leads to the sniffles, but it can also infect the small airways in the lungs and may be fatal in babies and small children.

Over four years, from November through April, the researchers said the virus was responsible for:

• 20 per cent of hospitalizations.
• 18 per cent of visits to emergency rooms.
• 15 per cent of office visits for respiratory infections.

That's as many trips to the doctor each year as influenza, and RSV causes three times as many hospitalizations, the researchers said.

The study looked at 5,067 children in three counties in the U.S.

Vaccine needed

Most of the children studied, 78 per cent, were older than one year of age, and most had no other medical conditions that would place them at high risk.

Of those who were not hospitalized, only three per cent received a diagnosis of RSV.

"The rates of RSV infections requiring medical attention are high not only during infancy but throughout the first five years of life," the study's authors wrote.

"This factor underscores the as-yet-unmet need for an effective vaccine."

Outbreaks of RSV are more common in the winter months. The symptoms resemble respiratory diseases caused by bacterial infections.

The infection is most often spread through coughing and sneezing. Most otherwise healthy people recover from RSV infection in a week or two, according to the U.S. Centers for Disease Control.

Several of the study's authors reported receiving grants or fees from various pharmaceutical companies.