China's influenza activity has rapidly increased in recent weeks but remains within the expected winter range, with no novel or unknown variants being detected, health authorities said recently.

Latest data from the Chinese Center for Disease Control and Prevention shows that flu levels are increasing in both northern and southern regions. This surge led to 955 reported infection clusters in the week beginning Nov 10.

The H3N2 subtype of influenza A is currently dominant, accounting for over 99 percent of samples from flu-like cases, with the remainder being the H1N1 subtype of influenza A and the influenza B strain.

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"The nation has entered a period of rapidly increasing flu cases, with the majority of regions seeing a medium to high level of virus circulation," said Peng Zhibin, a researcher at the China CDC, at a news conference on Friday.

Peng also noted a significant rise in outbreaks at schools and nurseries, with the flu positivity rate among children and adolescents aged 5 to 14 markedly higher than in other age groups.

Last winter's flu season was primarily driven by the H1N1 strain, while the current dominant strain is the H3N2 strain.

A specific H3N2 subclade, known as K, has also driven up infections in parts of the northern hemisphere, including the United Kingdom, Canada and Japan, fueling concerns that this signals a faster evolving or more severe virus.

In response, Peng said that the flu virus is prone to minor mutations. The current flu activity still remains at the normal, seasonal epidemic level and does not indicate that the virus is becoming increasingly fierce.

"So far, all detected cases are common and known pathogens, with no unknown or newly emerging pathogens," she said.

Wang Dayan, director of the Chinese National Influenza Center, said during an earlier news conference that the composition of each year's flu vaccine is updated to best match the strains expected to circulate, including this year's dose.

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"The matching level for the H3N2 virus has improved compared to previous years, while the matches for H1N1 and influenza B are even higher," she said. "Over 95 percent of circulating strains are antigenically similar to the vaccine strains."

Experts said that the optimized period to get vaccinated is before the start of the flu season, so as to reduce the possibility of severe symptoms and prevent infection clusters.

However, Peng said that even individuals who have already been infected can benefit from the shot, as the trivalent vaccine contains all three circulating strains and can provide protection against those they have not contracted.

Contact the writers at wangxiaoyu@chinadaily.com.cn