Bird-flu danger: Have we underestimated risk?

13 Dec 2005, The Straits Times

Question

Name of the Person: Lee Tze Chuen

Bird-flu danger: Have we underestimated risk?

I REFER to the article, 'A bird flu pandemic could kill 1,900 in Singapore' (ST, Dec 10). 

The World Health Organisation mentioned in its FAQ page on the Web that the mortality rate in poultry could approach 100 per cent. 

In medical circles, the estimated mortality rate is 75 per cent, and one of the most prestigious scientific journals, Nature, observed an apparent 80 per cent mortality for the H5N1 virus. 

How is it then that our estimate is based on a totally different strain of influenza virus (H1 strain for the 1918 Spanish flu pandemic) with a mortality rate of 2 to 3 per cent? 

An underestimate of the disease's mortality rate can bring about complacency and unpreparedness among the public which might worsen the situation when the disease strikes. The worst situation is when people are misled by the low mortality rate stated in the report and not care less about the disease and go on with life as usual. 

Should the mortality rate of the H5N1 virus be maintained at its current level and the virus mutates into a form that can be spread from human to human, just a few hundred infected people can increase the spread of the disease tremendously.

Reply

Reply from MOH 

Flu Pandemic - Various factors affect mortality rate 

I refer to Lee Tze Chuen's letter, "Bird-flu danger: Have we underestimated risk?" published in the Forum in the Straits Times on 13 Dec 2005. 

As of 9 Dec 2005, the World Health Organization (WHO) reported a total of 137 confirmed human cases of avian influenza A/H5N1 with 70 deaths. This represents a case fatality rate of slightly over 50%. The actual fatality rate may be much lower as mild infections usually go undetected. Human infections without any symptoms can also occur and this was found in the A/H5N1 outbreak in Hong Kong in 1997. 

So far these human cases have been associated with close contact with infected poultry and except for one probable instance in Thailand in October 2004, there has been no human-to-human transmission of H5N1 avian influenza. The scientific consensus is that if avian influenza A/H5N1 mutates into a human disease, it will adopt characteristics of human influenza virus strains and the fatality rate will most probably come down very significantly. It is useful to look at past influenza pandemics as these provide a guide on the severity of a future pandemic. The actual severity will only be known during a pandemic. The mortality rate would depend on various factors such as the virulence of the novel virus, host response and the access to healthcare. The most severe influenza pandemic on record was the 1918-1919 pandemic. The influenza virus A/H1N1 strain that caused this pandemic was thought to have originated from birds. The case fatality rate of that pandemic in the United States was about 2.5%. As a percentage of the entire US population at that time, the mortality rate was 0.6%. In Singapore, the estimated number of excess deaths from all-causes in 1918 was 2,400 (also 0.6% of the population at that time). Pandemics in 1957 and 1968 were relatively milder, with case fatality rates below 0.5%. The estimated number of excess deaths in Singapore in those two pandemics is estimated at about 500 or less. 

The Ministry of Health's projection of 1,900 deaths is derived from the US CDC's FluAID model which is based on data from the last pandemic in 1968. The figure is a conservative estimate as the 1968 pandemic was relatively mild compared to the 1918 one. If we apply a 1918 scenario into the model, the number of fatalities would be greater - about 15,000. Another 900,000 persons or so would fall ill and survive. The majority of persons who fall ill would not need to be hospitalized. Whatever numbers turn out eventually will be a challenge to the healthcare system and we are thus making preparations to meet these challenges. 

The Ministry of Health will continue to work with other government ministries and agencies to improve our preparedness for an influenza pandemic. For the plan to be effective, we would also need the cooperation and involvement of all Singaporeans for example in observing good personal and environmental hygiene. For more information, the public may wish to visit our website at www.moh.gov.sg.