GENEVA — The United Nations health agency urged medical authorities worldwide on Tuesday to launch an influenza vaccination campaign, saying it would help stop confusion in future outbreaks of SARS.

The World Health Organization said that flu symptoms — such as a high temperature — are often confused with those of SARS, or severe acute respiratory syndrome. Immunizing people against flu will slash the number of cases of the illness and make it easier for doctors to decide whether someone is suffering from flu or SARS.

Many respiratory diseases occur every winter, but flu is one of the most severe, infecting up to 20 percent of the world's population, causing up to 5 million cases of severe illness and at least 250,000-500,000 deaths each year worldwide. While some vaccinated people may still contract mild flu, the vaccine does protect from the most dangerous consequence of the disease, pneumonia.

"Many experts are concerned that SARS might be a seasonal disease and return in the next few months," said WHO. After surfacing in southern China in November, SARS infected more than 8,400 people worldwide and killed more than 800 people, mostly in Asia, before subsiding in June.

"The influenza vaccine does not prevent other respiratory diseases and, importantly, it does not provide protection from SARS," said WHO.

However, the agency said, "high vaccination coverage may reduce the number of pneumonia cases caused by influenza (and) reducing pneumonia cases may also lower the possibility of misdiagnosing influenza as SARS."

Flu vaccination should focus on the elderly and those who are already sick from other illnesses, as well as health workers.

"Suspected SARS cases can result in considerable disruption of health services as well as costly precautionary measures and investigations," said WHO.

"Decreasing the number of pneumonia cases, through influenza vaccination, can help in the early identification of a true SARS outbreak — should the disease recur. Early detection is essential to keep the disease contained."