Feb. 13 (HealthDay News) -- It may be possible to develop a vaccine to control noroviruses -- a common cause of stomach flu -- but it's likely a such a vaccine would have to be changed every year because the viruses evolve quickly to avoid attacks by the immune system, new research suggests.

Noroviruses infect cells after attaching to histo-blood group antigens (HBGA), molecules located on the surface of cells. HBGAs are a family of complex sugar molecules that exist in great variety among humans, according to background information in the study in the journal PLoS Medicine.

Ralph Baric, of the University of North Carolina at Chapel Hill, and his colleagues found this large variety of HBGAs contributes to recurrence of norovirus outbreaks, even among people who have previously been exposed to the virus and have developed antibodies against it.

The researchers analyzed noroviruses that caused several outbreaks and found that the viruses evolved to avoid attack by immune system antibodies.

Over time, some of the norovirus strains developed a shape that enabled them to bind other forms of HBGA. These viruses were then resistant to previously existing antibodies and able to infect cells carrying that particular form of HBGA. These viruses could then cause a new outbreak, with the cycle repeating itself, the researchers said.

Noroviruses, which can cause nausea, vomiting and diarrhea, are highly contagious. Most people recover within a few days, but the viruses can cause severe problems in very young and in old people. There is no specific treatment for norovirus infection.

SOURCE: PLoS Medicine, news release, Feb. 11, 2008