UK swine flu toll is really 30,000, says leading scientist

True extent of the outbreak is claimed to be 300 times worse than government agency admits

Thirty thousand people in Britain are likely to have been already infected by swine flu, one of the country's leading authorities has told The Independent on Sunday. This would mean that the virus is 300 times more widespread than the Health Protection Agency (HPA) admits.

The startling estimate by top virologist Professor John Oxford comes as leading scientists are warning that the agency's announcements on the spread of the disease are "meaningless" and hiding its true extent. And it tallies with official estimates made in the United States.

Yet the World Health Organisation (WHO) late last week changed its rules in order to avoid declaring that the flu has become a pandemic after pleas from governments, led by Britain.

Professor Oxford, of Queen Mary, University of London, believes that thousands of people have caught the virus and suffered only the most minor symptoms, or none at all, over the past weeks, as the new strain of H1N1 has spread nationwide – welcome testimony to the mildness of the epidemic to date.

He also thinks that some 100,000 people will have been infected in the US – the same number as is being privately estimated by experts at the US Centers for Disease Control and Prevention – and another 30,000 in Japan, where swine flu was spreading rapidly last week.

This contrasts with the number of officially tested and confirmed cases recorded by the WHO, which now stands at 11,168 in 42 countries on every continent except Africa. So far just 86 deaths have been ascribed to the disease, all of them in North and Central America, including 75 in Mexico.

Britain has the most official cases in Europe, at 122, with another 169 under investigation. But the figure, put out by the HPA, is increasingly coming to be seen as unrealistic. And it is certainly rising extraordinarily slowly, merely increasing from 82 a week ago and 39 the week before that, while normally the numbers affected by an infectious disease multiply rapidly.

In Japan – which now has the most confirmed cases outside North America – the official numbers have shot up from four to 294 in a week.

Professor Andrew Pekosz, of John Hopkins University in Baltimore, in the US, told the Associated Press late last week: "It's odd that we have not seen more cases in Britain," adding that after the first ones "a relatively wide outbreak" would be expected.

"The numbers in Britain are not really telling the story," added Professor Michael Osterholm, one of the world's top flu experts and an adviser to the US government. He called the official figures "meaningless" and said that, while the authorities were not hiding cases, they were not hunting very hard for H1N1.

The agency says that the numbers are low because it has been successful in containing the spread of the disease by giving the antiviral drug Tamiflu to all those in contact with anyone found to have it.

But Professor Osterholm says that the disease spreads too fast for this to work, adding: "It's like trying to maintain the integrity of a submarine with screen doors."

Britain may be missing cases because it is testing people with flu-like symptoms only if they have been in Mexico and the US in the previous week or have been in contact with a "probable or confirmed case" of the disease.

The spread of H1N1 in Japan, after the Americas, would normally have been enough to justify the WHO declaring a pandemic, under criteria established in April this year. But Britain, Japan and other countries last week persuaded it not to do so, in order to prevent panic, border closures and disruption to trade.

However, the UN body continues to warn that the disease may come back in a much more deadly form in the autumn.