* More than 20 new infections confirmed in past week
* Winter flu season means strain circulating more widely
* Experts concerned virus may mutate, pose greater threat (Updates with China health ministry statement)
By Kate Kelland, Health and Science Correspondent
LONDON, Jan 17 (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 Organisation (WHO), a big increase on the two cases and one death reported for the four-month period of June to September.
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.
Several clusters of cases in people who had close contact with an infected person have been reported in China. 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
The WHO's stance, based on its Dec. 20 assessment, is that five small family clusters have been reported but "evidence does not currently support sustained human-to-human transmission of this virus. The current likelihood of community-level spread ... is considered to be low."
That 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".
MIX AND MINGLE
Flu viruses, however, often put on their biggest show of strength in the northern hemisphere's cold winter months of January and February.
And with more of the virus circulating in wild birds, poultry and in the larger numbers of people infected, the new strain has more opportunity to adapt and mix with other strains that may give it pandemic potential.
Peter Openshaw, director of the Centre for Respiratory Infection at Imperial College London, said the rising toll of infections and deaths is "a signal for concern" because "historically what has happened in major outbreaks is there are occasional, sporadic cases and then it starts to build".
"But whether it means that there is any change in the virus' behaviour is another important question. If it were changing the way it is behaving, that would be more alarming," he said.
Early gene analysis work on the emerging H7N9 virus in April last year found it had been circulating widely but went undetected and had acquired significant genetic diversity.
Scientists warned then that its genetic diversity showed the H7N9 virus has an ability to mutate repeatedly and was likely to continue doing so.
Hartl said: "Mutations happen all the time. While yes, the more virus there is, the more mutations could happen, it is also true that almost all of these mutations are benign." (Reporting by Kate Kelland; Editing by Tom Heneghan and Janet Lawrence)