After neighboring Zhejiang Province discovered a small influenza outbreak, the city's health authorities encouraged residents to get flu shots, especially children and the elderly.
Since last month, Zhejiang has detected a total of more than 40 flu cases in Ningbo, Jiaxing and Wenzhou.
"We haven't found any problem in the city, but we always monitor the flu situation closely," said Feng Xiaogang, an official at Shanghai Disease Prevention and Control Center.
Medical experts said there were sporadic cases of flu in Shanghai every year and that a slim possibility of a big outbreak does exist.
Experts said the best defense is vaccination.
Zhang Liesheng, vice director of Shanghai Institute of Biological Products, said not enough people get flu shots in the country.
Beijing is the city with the highest vaccination rate, which is 5 percent, Zhang noted. "While it is about 30 percent in the West."
"Disease prevention and control centers in Shanghai only bought 1 million vials of flu vaccine this year. But there are more than 17 million people in the city," Zhang added.
He said the best time for a flu shot is September or October.
The peak influenza season runs from December through May.