The federal government will maintain the current swine flu threat level for the foreseeable future, warning it is still too early to say whether Australia will avoid an outbreak.

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The head of the World Health Organisation (WHO) also issued a warning, saying a second wave of the potentially deadly virus could strike “with a vengeance”.

There are still no confirmed cases of H1N1 influenza in Australia with 399 of 434 people tested having been cleared of the disease. Another 35 people, including some who were added to the list of suspected cases on Monday, are yet to receive their test results.

Three Australians living in the United Kingdom have tested positive for swine flu, while a fourth Australian has been placed in quarantine in Hong Kong along with 300 other passengers who arrived on a flight from Mexico.

Health Minister Nicola Roxon said that despite there being no evidence of an outbreak in Australia, and signs infection rates in Mexico, the epicentre of the disease, had stabilised, the government remained concerned about the threat of an outbreak.

“We\’re still in a situation where it\’s a novel disease, no one is entirely sure how it will develop into the future and in the coming months,” she told reporters in Canberra.

“The data is still showing a much broader spread to more countries, it\’s showing human-to-human transmission and we are still at the early stages of the disease.

“We intend to keep making sure that we err on the side of caution rather than complacency for a disease that can still present serious risks in the community.”

Ms Roxon\’s comments came as the WHO warned it could soon raise its global pandemic alert level to a maximum of six and stressed that any fresh outbreaks of the virus in pigs must be contained and monitored.

The WHO pointed to the risk of further spread in humans after a herd of pigs in Canada was infected with (A)H1N1 strain, in what is believed to be the first instance of human to swine transmission.

The head of the WHO, Margaret Chan, told Britain\’s Financial Times that declining mortality rates did not mean the worst was over and that a second wave of the potentially deadly virus could strike “with a vengeance”.

“We hope the virus fizzles out, because if it doesn\’t we are heading for a big outbreak,” she said.

Australia\’s chief medical officer, Jim Bishop, said it was good news the situation in Mexico appeared to be stabilising.

“There are still more countries coming on board with evidence of infection, but I\’m happy to say also that there\’s been no deaths, apart from the one child in the United States, outside Mexico,” Professor Bishop said. “We\’re starting to see a better picture about what the disease will do over time and I think that will play out over the next couple of days.”

Thermal screening will continue at eight airports around the country while health declaration cards, which become mandatory from May 6, have been delivered to all international airlines flying to Australia.

“It\’s too early for us to think about lessening those precautions,” Ms Roxon said. “We\’re still in the phase of making sure that we are being vigilant but not alarmed.”

A total of 23 people have died, 22 of them in Mexico, and the WHO estimates nearly 900 cases of infections worldwide.