Health officials in protective suits transport sacks of poultry as part of preventive measures against the H7N9 bird flu at a poultry market in Zhuji, Zhejiang province, China, on Jan. 6, 2014. Reuters

A wave of H7N9 bird flu cases and deaths in China since the start of 2014 shows emerging flu strains need constant surveillance if the world is not to be caught off guard by a deadly pandemic.

At least 24 H7N9 flu infections and three deaths have been confirmed in the past week by the World Health Organization (WHO), a big increase from the two cases and one death reported for the four-month period of June to September, which was the last time a cluster had been reported.

China's National Health and Family Planning Commission said on Friday it had seen 28 confirmed human cases of H7N9 in five provinces across the country since the start of the year.

"There's now a clear second wave of this virus," said Jake Dunning, a researcher at Imperial College London who has been monitoring the outbreak.

While the winter flu season means an increase in infections is not unexpected, it raises the risk of the virus mutating and perhaps getting a chance to acquire genetic changes that may allow it to spread easily from one person to another.

The H7N9 bird flu virus emerged in March last year and has so far infected at least 170 people in China, Taiwan and Hong Kong, killing around 50 of them.

Many, but not all, of those infected had contact with poultry or other birds, so for now this virus has apparently not adapted to easy human-to-human transmission – one of the main features keeping a pandemic emergency response on hold.

China's health commission said experts had concluded that H7N9 transmission "is still from poultry to humans".

Yet the strain does have some worrying features, including a limited capability to spread from one person to another. An analysis of probable H7N9 transmission from person to person, published last August, gave the best proof yet that it can sporadically jump between people.

A separate team of researchers in the United States said in December while it is not impossible that H7N9 could become easily transmissible from person to person, it would need to undergo multiple mutations.

Another alarm was sounded last month when scientists said they had found that a mutation in the virus can render it resistant to a first-line treatment drug without limiting its ability to spread in mammals.

WHO chief spokesman Gregory Hartl said the United Nations health agency had noted the rapid increase in infections in the past few weeks and was keeping a watchful eye.

"So far we haven't seen anything that causes us to change our risk assessment," he said from WHO's Geneva headquarters

His sentiment was echoed by the Chinese health commission, which said, "up to now, inspections have found no mutations of the virus that are of significance to public health."

Al Jazeera and wire serivces