Nancy Cox, director of the US Centre for Disease Controlâ€™s influenza unit, says it is impossible to predict the consequences of a mutation of H5N1 bird flu virus. “We might have a relatively mild pandemic like we did in 1968,” Dr Cox says. “Alternatively, we could have a relatively severe pandemic as occurred in 1918 or perhaps even worse.”
The possibility of something worse is very real. In the 1918 flu pandemic the case fatality rate was in the single digits, around 5%. Today H5N1 has a case fatality rate that is more than ten times higher. Although that high rate could come down as H5N1 becomes more efficient at human-to-human transmission, case fatality rates in the single digits would have severe consequences.
H5N1 has been passing from index case to family members for over a year. The clusters were bimodal, but the transmission chains were short, involving one or two family members.
However, at the beginning of this year the number of familial clusters began to grow. Data are lacking in southern Vietnam, which has a case fatality rate of 100% this year on confirmed cases. The lack of timely information is cause for concern because of the high fatality rate, the high concentrations of H5N1 positive poultry, and references to human cases in southern Vietnam and Cambodia as being complex.
More information has been available in the north, but the data reveals both geographical and familial clustering in Thai Binh. Last week a nurse was confirmed to be H5N1 positive, which was the first reported case in a health care worker infected in the current outbreak. That report was followed by the hospitalisation of a second health care worker. The second case has not been confirmed, but the first case extended the transmission chain, suggesting more efficient human-to-human transmissions. The bird flu cases in the north have been milder and several patients have been discharged. There are also reports of milder atypical cases, but the details have not been released.
False negative results in the north and south have clouded the H5N1 picture. Lack of testing due to a narrow case definition has also limited an understanding of the extent of the spread of H5N1. H5N1 has been associated with a variety of conditions, in addition to respiratory disease. There have been gastro-intestinal and neurological components, which dictate widespread screening and prompt reporting.
The expanded clusters in the north and the high case fatality rate in the south are causes for concern. Aggressive screening, contract tracing, and viral isolation are important aspects of managing this evolving pandemic situation.