Sars threat officially declared over - at least for now

For now, the threat from Sars, the first serious new disease of the 21st century, seems to be over. The disease that came out of China and spread round the world, infecting thousands and killing hundreds, has been put back in its box.

Yesterday, the World Health Organisation lifted its warning against travel to Beijing, the last place that was subject to the alert, and said the most effective weapon against the disease had been the humble thermometer. By checking the temperatures of people with symptoms, and using old fashioned methods of isolating those with a fever and quarantining their contacts, the spread of the disease had been halted.

"Today is a milestone in the fight against Sars, not only in China but in the world," Shigeru Omi, the WHO regional director for the Western Pacific, said in Beijing. More than 20 days had elapsed since a new case of severe acute respiratory syndrome had been recorded in the Chinese capital, the city worst hit by the disease with 2,521 cases and 191 deaths. The face masks, which came to symbolise tainted cities everywhere, have disappeared.

Only Toronto and Taiwan remain on the WHO list of Sars-affected areas, even though the warning on travel to both destinations was lifted weeks ago. They have yet to go 20 days without recording a new case, the WHO criterion for declaring them disease-free.

The achievement in lifting the last travel alert was being celebrated yesterday by WHO officials who insisted it was not due to any natural change in the virulence or infectivity of the virus. New diseases often burn out quickly of their own accord but Sars was only defeated thanks to the "monumental efforts" of governments, doctors and nurses and a "well-informed and co-operative public".

Declaring Beijing open to visitors yesterday, Gro Harlem Brundtland, the director general of the WHO said: "This is very good news and shows the great progress the world has made against Sars."

But it is not the end of the story. By Monday, the official tally was 8,459 cases of Sars and 805 deaths worldwide, recorded in 104 days since the organisation issued its first global warning about a new disease on 12 March. In addition to devastating families, the virus has caused billions of pounds of damage to Far East economies, driven hotels and airlines to the brink of bankruptcy and caused panic around the world.

The virus has proved to be one of the fastest moving in history. From the 55 cases recorded on the day of the first warning, the disease exploded within a month to cause more than 3,000 cases and more than 100 deaths in 20 countries on all continents. There was no vaccine, no effective treatment, a high death rate and no way of knowing how far it would go. The epidemic highlighted, for the first time, the vulnerability of a globalised society linked by air-travel to the emergence of lethal new pathogens.

The disease originated in southern China, where the first cases occurred in November 2002, and was carried into Hong Kong by a professor of respiratory medicine who had been treating patients with Sars. He spent a night on the ninth floor of the Metropole hotel in Hong Kong in late February where he infected at least 16 other people who spread the disease internationally. One cough in the lift lobby may have been all it took to trigger the epidemic.

An important factor in the early spread of the disease was the reluctance of the Chinese authorities to admit it was happening. China only started daily reporting of cases in early April, and, by then, it had gained a foothold from which it was hard to dislodge. It had also transmitted the infection to neighbouring Hong Kong, one of the world's great travel hubs.

The disease continued to accelerate in March and April, peaking in early May. Toronto, the first and only Western city to have deaths from Sars, was hit by a WHO travel ban in April that had to be reimposed 12 days after it was lifted last month, when the city suffered a renewed outbreak. In Britain, only four cases were recorded and all recovered.

From early March, the WHO's global network of 11 laboratories worked round the clock to identify the virus and investigate how it was transmitted. By early April, it was established as a member of the coronavirus family, a cause of the common cold, and soon afterwards it was traced to a species of the civet cat, a delicacy in southern China. However, it remains uncertain whether the civet cat is the original host for the virus or had picked it up from another animal or even a human keeper.

The biggest unknown about the virus is: Will it recur? Toronto's recent experience of a second outbreak has proved that even in a sophisticated Western city, the virus has the capacity to evade modern methods of surveillance. Continued vigilance will be needed for at least a year, according to the WHO, to ensure the chain of person to person transmission is broken. Even then, Sars may have found an animal host or other place in nature in which to hide, as the Ebola virus does, and from which it could return when conditions are right.

The most urgent challenge for scientists is to develop an accurate, quick diagnostic test so that if a new outbreak should occur it can be rapidly identified and contained. Otherwise, Sars could spread mayhem through the world again.