The threat of a killer flu pandemic is greater than ever because of the spread of the bird flu virus in south-east Asia, the World Health Organisation said yesterday.
Avian influenza is still spreading, despite countermeasures, and the possibility of a global epidemic that could kill millions is said to be more likely than not.
More than 50 people have died from the H5N1 virus in south-east Asia, most of them in Vietnam, where tens of thousands raise poultry in small back-yard farms.
In its current form, the virus is difficult for humans to contract, but if it changes its genetic make-up, it could spread rapidly.
If the virus infects a person who has a human flu virus, the two may shuffle their genes to create a deadly mix.
A new strain could combine the lethal effects of the bird virus with the ability to spread easily among people supplied by the human virus.
The World Health Organisation (WHO) computer models suggest that a pandemic of H5N1 would kill between 2 million and 7.4 million within 12 months in a “best case scenario” based on the “mild” pandemic of 1968.
But if it were similar to the 1918 pandemic, the models estimate a much higher total.
Peter Horby, the WHO medical epidemiologist in Hanoi, said that while the mortality rate from H5N1 infections was falling, the virus appeared to be adapting to human hosts, which was taken to be an indication that transmission between humans would become easier.
“The fact that it’s been around for a year and we haven’t seen a pandemic is no reason to be complacent,” he said.
“I’m more concerned than I was a year ago.” He estimated the probability of a pandemic at “more than 50 per cent”.
Dr Klaus Stohr, the WHO global influenza programme chief, reported an increase in the number of cluster cases reported recently, with the biggest a family of five cases.
There have been seven cluster cases in Vietnam, all within single families, most recently in the northern province of Haiphong.
No relative has been proved to have passed the disease to another, but Dr Horby said: “The onset dates could be consistent with human to human transmission.”
The WHO expects to complete the genetic analysis of virus samples from Cambodia and Vietnam within a few days to see if there has been a significant change in its make-up.
To date, there has been human-to-human transmission but none beyond “one or two links in the chain”, following close contact with a terminally ill patient.
“The moment we have the slightest suspicion of a change in the epidemiology, we would react very aggressively,” Dr Stohr said.
Based on historical patterns, influenza pandemics can be expected to occur, on average, three to four times each century when new virus sub-types emerge and are readily transmitted from person to person.
In the 20th century, the flu pandemic of 1918-1919, which killed up to 50 million, was followed by pandemics in 1957-1958 and 1968-1969.
The viruses that caused these pandemics are all thought to have derived, at least in part, from avian strains. Most influenza experts agree that the prompt culling of Hong Kong’s entire 1.5 million poultry population in 1997 probably averted a pandemic.
Britain is among 12 countries that have ordered stocks of a new drug, oseltamivir (Tamiflu), which is effective against H5N1.
Sebastien Berger and Roger Highfield, The Telegraph