Ministers urged people not to panic yesterday as the first case of human-to-human transmission of swine flu within the UK was confirmed in a 24-year-old man, only hours before it was followed by a second case involving a 42-year-old from Gloucestershire.
Graeme Pacitti, an NHS worker at Falkirk Royal Infirmary, tested positive for the virus after coming into contact with football team-mate Iain Askham, who fell ill with his wife after their honeymoon in Mexico. The total number of swine flu cases stands at 13, with two now believed to be have been transmitted by person-to-person.
"It is significant, it is concerning. I do want to stress, however, that it doesn't mean that there is more reason for people to worry," said Scottish Health Secretary Nicola Sturgeon. The positive test result came as scientists reported that preliminary analysis of the virus suggested it was a mild strain but could still cause death on a wide scale.
Scientists at the National Institute for Medical Research in Mill Hill, north London, said initial work on samples of virus sent from the US suggested the H1N1 swine flu was similar to H1N1 seasonal flu and there was no immediate cause for concern. However, even a mild virus could cause millions of deaths if it spread widely. Dr John McCauley of the institute cited the instance of the 1957 pandemic which had a mortality rate of 0.1 per cent but which infected between 30 per cent and 70 per cent of the population, causing about two million deaths.
Further mutations to the virus could occur, making it more virulent. In Mexico, US scientists suggested an explanation for the higher number of deaths – that most infections are mild but only the severe ones that sought treatment in hospitals have been seen so far.
The US Centres for Disease Control and Prevention said in a report released yesterday that Mexico's epidemic of the virus may not be as severe as it looked at first, because many mild cases were not immediately noticed. In the UK, the tally of swine flu cases rose to 13 last night, 11 of whom contracted it abroad.
Alan Hay, head of the World Health Organisation's Collaboration Centre, said the genetic make-up of the virus suggested it would probably result in a mild pandemic. "We've not identified any features to give us cause for concern but that does not mean these might not emerge," he said.
Dr Hay added that the virus was similar to the swine virus that has been circulating in pigs in the US for eight to 10 years, but it also contained genes found in viruses infecting pigs in Europe and Asia. "It's mutating constantly. The question is whether a mutation will appear that will affect the ability of the virus to spread," Dr Hay said.
Professor Wendy Barclay, chair of influenza virology at Imperial College London, said initial indications suggested that nothing about the virus's genetic make-up made it an unusual threat. "It is binding to the upper respiratory tract rather than deep in the lungs," she said. "If a virus binds further down in the lungs, it tends to cause more severe illness, as in the case of the H5N1 avian flu virus."
In Scotland, Mr Pacitti, a clerical assistant for the NHS, said he was "gutted" on hearing he had tested positive for the virus. He met his friend Mr Askham last Thursday and began to show symptoms on Saturday. He has been at his Falkirk flat since he was contacted by a doctor and tested. "I've been feeling a lot better today. I've still got an upset stomach and a sore throat. Earlier in the week I had headaches and felt feverish," he said.
Yesterday Gordon Brown urged people with swine flu symptoms to seek advice while reiterating that Britain was well placed to deal with the outbreak. He said the Government was increasing the number of face masks available to the NHS and looking at vaccines, while saying the Tamiflu anti-viral drug would deal with the bug. "This is happening in every country, but we are better prepared," Mr Brown added.
Dr Alan McNally, senior lecturer and influenza diagnostics researcher at Nottingham Trent University, said human-to-human transmission within the UK would not be a significant development.
He said: "We know that it is transmitted from human to human. It has happened in other parts of the world and we know it will happen here. I know there will be interest in it because members of the public will see that they don't need to have been to Mexico to get it."
He said the majority of the UK's 230 possible cases were likely to have originated from contact with other infected people. The latest countries to confirm cases were Denmark, Hong Kong and France, bringing the total to 17.
The US, which has the greatest number of confirmed cases outside Mexico, reported 143 cases across 19 states yesterday. Almost all infections outside Mexico have been mild, and only a handful of patients have required hospital treatment.
The World Health Organisation said there was no immediate likelihood of its phase 5 alert being raised to a full phase 6 pandemic warning.