The World Health Organisation (WHO) said on Monday (April 1st) there was no sign of human-to-human transmission, after two people died in China from a strain of avian influenza that had never been passed to humans before.

But in a media briefing in Beijing, the WHO's China representative Michael O'Leary said vigilance was still needed in case transmission between people did occur.

"Again, there's been no human-to-human transmission that we're aware of, and over 80 close contacts have been tested so far and found to be negative. So at this point this is a good sign. But it of course requires careful follow up to see if there should be any possibility of human-to-human transmission," he said.

Two men in Shanghai, aged 87 and 27, fell sick in late February. A woman in Anhui province also contracted the virus in early March and is in critical condition.

China's National Health and Family Planning Commission confirmed on Sunday that the three cases were the H7N9 virus, which had not previously been known to infect humans.

It is unclear how the three victims were infected.

In 2003, China's cover up of the SARS virus shook public confidence in the health authorities and drew strong criticism over a lack of transparency.

But O'Leary said this time around, the government had made a timely announcement.

"No transparency issues that we feel have been evident. We know that the virus was confirmed in the laboratory as H7N9 on Friday and the report to WHO was on Sunday, so that's an appropriate timeframe. The patients were sick earlier but, as I say, there's quite a lengthy investigation that has to take place, especially since there was no connection between the patients, there was not, it didn't seem to be spreading to anyone else and so on. These were sick individuals and there are lots of people in that situation. So they were being investigated case by case, and many things had to be ruled out," he said.

There are no known vaccines against the H7N9 virus.