A jump in the number of people in China infected with bird flu this year is raising concern among experts, who say a previously circulating strain appears to have changed and may be more infectious to people.

China has reported 21 human infections with the H5N6 subtype of avian influenza in 2021 to the World Health Organization (WHO), compared with only five last year, it said.

Though the numbers are much lower than the hundreds infected with H7N9 in 2017, the infections are serious, leaving many critically ill, and at least six dead.

"The increase in human cases in China this year is of concern. It's a virus that causes high mortality," said Thijs Kuiken, professor of comparative pathology at Erasmus University Medical Centre in Rotterdam.

Most of the cases had come into contact with poultry, and there are no confirmed cases of human-to-human transmission, said the WHO, which highlighted the rise in cases in a statement on Oct. 4.

It said further investigation was "urgently" required to understand the risk and the increase in spill over to people.

Since then, a 60-year-old woman in Hunan province was admitted to hospital in a critical condition with H5N6 influenza on Oct. 13, according to a Hong Kong government statement.

While human H5N6 cases have been reported, no outbreaks of H5N6 have been reported in poultry in China since February 2020.

China is the world's biggest poultry producer and top producer of ducks, which act as a reservoir for flu viruses.

The Chinese Center for Disease Control and Prevention (CDC) could not be reached for comment on the rise in H5N6 human cases. However, a study published on its website last month said the "increasing genetic diversity and geographical distribution of H5N6 pose a serious threat to the poultry industry and human health".

Avian influenza viruses constantly circulate in domestic and wild birds, but rarely infect people. However, the evolution of the viruses, which have increased as poultry populations grow, is a major concern because they could change into a virus that spreads easily between people and cause a pandemic.

The largest number of H5N6 infections have been in southwestern Sichuan province, though cases have also been reported in neighbouring Chongqing and Guangxi, as well as Guangdong, Anhui and Hunan provinces.

At least 10 were caused by viruses genetically very similar to the H5N8 virus that ravaged poultry farms across Europe last winter and also killed wild birds in China. That suggests the latest H5N6 infections in China may be a new variant.

"It could be that this variant is a little more infectious (to people)...or there could be more of this virus in poultry at the moment and that's why more people are getting infected," said Kuiken.

Four of the Sichuan cases raised poultry at home and had been in contact with dead birds, said a September report by China's CDC. Another had bought a duck from a live poultry market a week before developing symptoms.

China vaccinates poultry against avian influenza but the vaccine used last year may only partially protect against emerging viruses, preventing large outbreaks but allowing the virus to keep circulating, said Filip Claes, Regional Laboratory Coordinator at the Emergency Centre for Transboundary Animal Diseases at the Food and Agriculture Organization.

The Ministry of Agriculture and Rural Affairs did not respond to a request for comment.

Backyard farms in China are common and many people still prefer to buy live chickens at markets.

Guilin city in Guangxi region, which had two human cases in August, said last month it had suspended trading of live poultry in 13 urban markets and would abolish the trade within a year.