The number of flu patients has been steadily rising since late last month, health officials said Sunday, stirring concerns over the further spread of the influenza virus.

According to the Korea Centers for Disease Control and Prevention, the ratio of flu-suspected patients per 1,000 outpatients in the country reached 10.6 from Dec. 28 to Jan. 2. This is higher by 1.6 from the previous week and about 3 points higher on-year.

The disease control agency conducted an influenza sample test at 200 medical institutions on patients who showed symptoms of coughing, sore throat and fever of over 38 degrees Celsius.

Unlike the common cold, flu patients have fever, chills, headache and muscle pain along with respiratory symptoms such as coughing and a sore throat.

Based on the test results, the agency set the threshold of flu epidemic this winter at 11.3 patients per 1,000 individuals. If the ratio exceeds 11.3, it will issue a flu epidemic alert, the authorities said.

Moreover, the ratio of flu-suspected patients aged between 7 and 18 already soared to 16.8, up by 28 percent in a week. Among those aged 20 to 40, the ratio reached 12.4.

While the incidence of flu is currently limited to Seoul, Gyeonggi Province and Gangwon Province, it is likely to further spread to other regions as the influenza A (H1N1) pdm09 virus was officially detected last week at a hospital in South Jeolla Province, they added.

Doctors normally recommend that people get vaccinated around October and November, a month before the flu season.

The elderly aged 65 or older can get free vaccines at public health centers.

By Lee Hyun-jeong (rene@heraldcorp.com)