While early reports of the deadliness of human avian influenza suggested that about 90 per cent of the victims died, there are growing signs that the disease's true death rate is much lower ' although still high enough to kill many millions of people if the worst fears about its spread come to fruition.
Few acute infectious diseases have death rates exceeding 5 to 10 per cent. Exceptions are rabies, which is nearly always fatal, and Ebola and Lassa fever, with reported death rates ranging from 25 to 90 per cent. The death rate for garden-variety flu for children, the elderly or the immuno-compromised is less than 1 per cent in developed countries. About 20 million people worldwide died in the 1918 influenza pandemic, with an estimated death rate of 2 per cent.
As of March 14, the death rate from A(H5N1) avian influenza in Southeast Asia was 67 per cent: 46 deaths among 69 confirmed cases reported from Cambodia (one), Thailand (17) and Vietnam (51), according to the World Health Organisation.
The death rate for bird flu is dwindling because it is easier to count people who die than those who may become infected and have minor symptoms, or none at all. This phenomenon of subclinical disease ' a mild case of the bird flu, as it were ' seems to be occurring with more frequency than previously appreciated. For instance, the virus was detected in a healthy 81-year-old man in Vietnam and in a few others who barely knew they had been ill. If mild or symptomless cases are missed, the death rate will be skewed to falsely high levels.
Other findings, however, indicate that human bird flu infections may be more widespread than initially suspected and possibly transmitted by faecal contamination. The virus was found in a child with severe diarrhoea and encephalitis, but no respiratory symptoms, leading health officials to ask doctors to consider testing feces for the A(H5N1) virus more often.
The bottom line is that more human bird flu may be around than once suspected, but fewer people who get it become gravely ill and die than initially predicted. That, however, is an oversimplification. Establishing a death rate for a disease like bird flu is a moving target for medical detectives. Determining an accurate death rate depends on a variety of factors including the ability of people to get health care and the quality of the laboratory testing for the A(H5N1) virus.
Each step in the process must go right to get an accurate death rate, and that is difficult enough to attain in the US. The problems are multiplied in poor countries with too few trained technicians and inadequate laboratory facilities.
Also, a true death rate depends on governments' honesty to overcome fears about losses from decreased tourism and trade to report all cases. Health officials credit journalists in Vietnam for reporting more and timelier information about bird flu than the government.
Medical history teaches that high death rates first reported in outbreaks of newly recognised infectious agents usually drop as epidemiologists interview contacts and test for mild infection. Until that is done, death rates usually are limited to people who get to a hospital, who are often the sickest. Even the beginning of the process of determining death rates is difficult. As doctors examine patients with respiratory symptoms, they must overcome practical hurdles like having enough swabs and other standard equipment to take samples from patients' noses. They must also know how to obtain the specimens properly.
| Missed bugs: Labs are not proficient enough to pin down the virus
Few laboratories in the developing countries can test for the virus. Also, sending specimens to laboratories can be nearly impossible because of the lack of good roads and express mail systems. Specimens must be packed to stay cold for about 12 hours to preserve any virus present. But dry ice, widely available in developed countries, is nonexistent in some affected countries.
Dr Klaus Stoehr, the WHO's top influenza expert, said his team had to fly dry ice from Bangkok ,Thailand, to Cambodia last week to keep specimens cool during shipment to Paris for testing at the Pasteur Institute.
In Southeast Asia, some scientists can test for A(H5N1) virus only under trying conditions. Stoehr cited one laboratory where scientists can work on influenza for only two hours a day because they share the space and equipment with colleagues who study different infectious agents. Under such circumstances, contamination of tests can be a serious problem.
Japanese experts are retesting about 100 specimens that a Vietnamese laboratory reported as negative for A(H5N1).
'Of the first 30 so-called negative, 11 were found positive, suggesting that the laboratory was missing about one-third of cases,' Stoehr said.
It is not known how many cases other laboratories are failing to detect. To find out, the WHO is studying ways to improve the varying skills of the scientists working with A(H5N1) virus.
One possibility is to send specimens with known amounts of virus to rate laboratory proficiency in detecting the amount, a costly and demanding exercise. Scarcity of epidemiologists also delays the medical detective work to trace how patients became infected and whether they spread the virus to contacts. In addition, scientists need the viruses isolated from new bird flu cases to monitor for mutations and genetic changes.
A major change could make useless the current pilot human avian influenza vaccine that is about to undergo its first tests in people.
The WHO is planning an international meeting in Vietnam this spring where doctors who have treated one or two cases ' too few to report in medical journals ' can relate their experience. Then other scientists can learn more about the range of patients' symptoms, the effectiveness of different treatment regimens and tests of patients' contacts to get a truer death rate.