As influenza cases have been rising recently in China, experts said the surge is attributed to the virus' low circulation during the COVID-19 epidemic and the resulting low level of immunity among the population.

They emphasized early diagnosis and intervention for vulnerable groups as health authorities have stepped up training at hospitals on diagnosing and treating the disease.

From Feb 20 to 26, the country reported 390 flu outbreaks caused by two influenza A strains known as H1N1 and H3N2, up from 108 recorded during the previous seven-day period, according to a flu surveillance update published on Thursday by the Chinese Center for Disease Control and Prevention.

The flu positivity rate nationwide rose to 23.9 percent during the Feb 20-26 period, compared with 7.8 percent during the previous week.

The China CDC said that the percentage of influenza-like illnesses among hospital visits rose both in southern and northern regions, and the rates were either equal to or exceeded those seen since 2020.

Sun Jimin, a researcher at the Zhejiang Provincial Center for Disease Control and Prevention, said that COVID-19 control measures had also suppressed the spread of the flu virus in the past three years. With optimization of COVID-19 response measures and increasing mobility and gatherings, the flu virus has spread more easily.

"Many people are susceptible to infection, because it had not been transmitted widely for three consecutive years and the overall flu vaccination rate is low," he said.

However, Sun added that this winter's flu virus outbreak appears stronger than the past three years but remains milder than 2019 and 2018, and it is expected to decline after two weeks.

Hu Yang, a respiratory doctor at Shanghai Pulmonary Hospital, said it is difficult to make predictions on the spread of flu cases due to the unpredictable nature of the virus' circulation. "The epidemic in the past three years has also changed some traits of flu virus transmission," he said.

Hu said that many people had not caught a cold in the past three years and lost immunity to the influenza virus, causing a new wave of flu outbreaks this winter.

Most patients infected with the influenza A virus can recover without special treatment in about a week, but the public is advised to delay travel plans until this wave subsides, he said.

The National Health Commission's medical emergency response department said on Thursday that it has launched training at all medical institutions for treating the influenza virus and the norovirus — an infectious disease causing stomach pain and diarrhea.

The department stressed the need to enhance emergency response and staffing at emergency care, fever and intestinal departments and pay special attention to high-risk groups.

Guo Yanhong, an official at the commission, said that some vulnerable patients, including the elderly, children, pregnant women and people with existing illnesses, can develop pneumonia or other complications.