In issuing the warning, Shigeru Omi, director for WHO Western Pacific region where 95 percent of SARS cases were reported during the recent outbreak, said SARS-hit countries should maintain their surveillance systems for at least one more year.
More than 100 people mostly in Asia were still recovering from the illness and it was too soon to give the all-clear signal, he added.
He feared some patients could be carrying the coronavirus causing Severe Acute Respiratory Syndrome (SARS) although they seemingly had recovered from the pneumonia-like disease.
Those infected might also not display any SARS symptoms, making it impossible to detect them, he added.
WHO announced at the weekend that SARS had been contained worldwide, with Taiwan the last to be declared free of the disease.
It had killed more than 800 people and infected more than 8,400 in some 30 countries since it first emerged in the Chinese province of Guangdong in November.
"In addition to those factors (pointing to a possible reemergence of the disease), one thing that makes us more cautious is the possibility of SARS returning this winter" in the northern hemisphere, Omi said.
"Even if this virus does not come back, certainly there will be other diseases with similar symptoms coming up – influenza or common cold which represent similar symptoms as SARS."
He said that unless a more sensitive diagnostic kit was developed before winter, "there is going to be a lot of confusion with so many possible cases overwhelming health systems."
Diagnostic kits currently available can catch only about 70 percent of SARS cases.
"In my view developing a more sensitive diagnostic kit is the most important priority in addition to being vigilant. Otherwise, we will have to, for example, quarantine all the influenza cases, which may be a very high number," Omi said.