Swine flu could affect third of world's population, says study

The swine flu virus will infect a third of the world's population if it continues to spread at its current rate, scientists warned today, as three more cases were confirmed in the UK.

In what the journal Science described as the "first quick and dirty analysis" of swine flu, a study by researchers at Imperial College London predicted the virus was likely to cause an epidemic in the northern hemisphere in the autumn.

One of the authors, the epidemiologist and disease modeller Neil Ferguson, who sits on the World Health Organisation's emergency committee for the outbreak, said the virus had "full pandemic potential".

Speaking on BBC Radio 4's Today programme, he said: "It is likely to spread around the world in the next six to nine months, and when it does so, it will affect about one-third of the world's population.

"To put that into context, normal seasonal flu probably affects around 10% of the world's population every year, so we are heading for a flu season which is perhaps three times worse than usual – not allowing for whether this virus is more severe than normal seasonal flu viruses."

The Health Protection Agency announced three more confirmed cases of the virus in the UK, bringing the total to 68. The three patients – two children and one adult from London – all had close contact with previously confirmed cases.

Today's study estimated the contagiousness of the disease by analysing the number of people travelling to Mexico who became infected, and comparing that with a study of a Mexican village where the disease has spread. The research also examined how the virus was mutating.

It estimated that swine flu had killed between 0.4% and 1.4% of its victims in Mexico. The report's lead author, Christophe Fraser, said it was too early to predict what the death rate was likely to be outside Mexico. "My hunch is that the death rate will be lower elsewhere – Mexico has underlying issues with respiratory disease," he said.

The researchers said this H1N1 virus appeared to be about equal in severity to the flu of 1957, and less severe than the deadly 1918 version.

"At the moment the virus is not spreading fast in the northern hemisphere because we are outside the normal flu season, but come the autumn it is likely to cause a really major epidemic," Ferguson said.

He declined to put an estimate on the number of deaths that may occur. "We have some assessment, but the uncertainty is still quite broad," he said.

"We can say it is not going to be as catastrophic as the Spanish flu pandemic of 1918, it is milder than that. But it is still possible it could be [like] 1957 – where about three to four out of 1,000 people who were infected died and overall about 3 million to 4 million people died that year because of the pandemic worldwide – or it could be even milder than that, like the 1968 pandemic which was barely worse than a normal seasonal flu year.

"I am not predicting three million to four million [deaths]. That was what happened in 1957. The world is a very different place today. There are more people in the world, but there is also a much better healthcare system. We have drugs and vaccines, particularly in developed countries, which should markedly reduce the burdens of the disease."

Ferguson said the findings of the study confirmed that decisions must be taken swiftly on vaccine production.

"One of the key decisions which has to be made this week by the world community is how much do we switch over current vaccine production for seasonal flu to make a vaccine against this particular virus."

Today Cuba and Thailand confirmed their first case of the virus. The patient in Cuba is a Mexican student at a medical school.

Fidel Castro accused Mexico of failing to disclose the spread of swine flu until after Barack Obama, the US president, had visited the country on 16 April. In a newspaper column, the former Cuban leader wrote: "Mexican authorities did not inform the world of the presence [of swine flu], while they waited for Obama's visit."

At least 61 people have been killed by swine flu around the world, and the WHO has confirmed about 4,800 cases.