Sars is less likely to cause as many deaths worldwide as flu. Credit: © WHO/P.Virot

Nearly half of the elderly people admitted to hospital with severe acute respiratory syndrome (SARS) die from the disease, according to the most thorough analysis of the outbreak so far1.

This sounds bad, but it is comparable to death rates from influenza. Among those under 60 years of age, the mortality rate is far higher than for flu: some 13% of those with SARS die from it.

"It's a relatively high fatality rate," says one of the study's authors, epidemiologist Azra Ghani of Imperial College, London. But this doesn't mean that SARS is more dangerous than flu, she says: "It's much less transmissible than flu and is therefore much less likely to cause as many deaths worldwide."

The study, based on data from Hong Kong hospitals, also provides estimates of the incubation period and severity of SARS.

It takes an average of 6.4 days for a person infected with SARS to show symptoms, the researchers found. Three to five days passed between the onset of symptoms and admission to hospital. This period shortened as awareness of the disease spread.

These figures are in line with previous estimates made by public-health workers and the World Health Organization (WHO). They also suggest that the strategies so far used to contain SARS have been the right ones.

"Based on these data, the control efforts are exactly what they should be," says epidemiologist Ira Longini of Emory University in Atlanta.

The strategy has been to detect SARS cases as early as possible and isolate them from the population. Reducing the time between detection and isolation is currently the best way to curb the epidemic.

Unanswered questions

Crucial questions about SARS remain, some of which could change the study's conclusions.

"The big question is what proportion of infected people get only a mild infection and don't go to hospital," says Longini. If the number is high, SARS is less deadly than the new results suggest. It also means that the true number of SARS cases - and therefore how difficult it will be to contain the epidemic - remains unknown.

The question is how many people get a mild infection and don't go to hospital Ira Longini , Emory University

The only way to find out is to develop a rapid test for the virus and to screen those in epidemic areas. "I'd put extreme emphasis on that activity," says Longini. The WHO and its partners are working towards a SARS test.

Another key question is for how long virus carriers can infect others for before they develop SARS. "At the moment we don't know about virus shedding in mild or sub-clinical cases," says virologist Albert Osterhaus at Erasmus University in Rotterdam, the Netherlands. Osterhaus is working on this question using a SARS-like virus in animals.

Important new insights into the disease's spread may come from studies of a high-rise housing block in Hong Kong that was quarantined during the epidemic. But these data will not emerge for several months, says Longini.