WHO revises estimates of fatality ratio in SARS cases

The United Nations health agency today revised its initial estimates of the case fatality ratio of Severe Acute Respiratory Syndrome (SARS), saying that the proportion of people who contract illness, and then are likely to die from it, depends on factors such as the age group affected.

The World Health Organization's revision is based on an analysis of the latest data from Canada, China, Hong Kong, Singapore and Viet Nam, and results in new case fatality ratios that range from 0 to 50 per cent, with an overall estimate of case fatality of 14 to 15 per cent.

The agency said the likelihood of dying from SARS in a given area has been shown to depend on the profile of the cases, including the age group most affected, and the presence of underlying disease. Based on data received by WHO to date, the case fatality ratio is estimated to be less than 1 per cent in persons aged 24 years or younger, 6 per cent in persons aged 25 to 44 years, 15 per cent in persons aged 45 to 64 years, and greater than 50 per cent in persons aged 65 years and older.

A case fatality ratio measures the proportion of all people with a disease who will die from the disease. "In other words, it measures the likelihood that a disease will kill its host, and is thus an important indicator of the severity of a disease and its significance as a public health problem," WHO said, noting that the likelihood that a person will die of SARS could be influenced by factors related to the SARS virus, the route of exposure and amount of virus, personal factors such as age or the presence of another disease, and access to prompt medical care.

Meanwhile, WHO has also reviewed estimates of the incubation period of SARS, using individual case data. It concluded that the current best estimate of the virus' maximum incubation period continues to be 10 days.