Modeling Epidemics

Researchers published articles in the journals Science and Nature this week, that attempt to predict the outcome of a potential avian influenza A (H5N1) virus pandemic and containment response in South East Asia. The two groups were Ira M. Longini et al. of Emory University; who wrote, "Containing Pandemic Influenza at the Source", for this week's August 3, 2005 online issue of Nature (subscription), and Neil M Ferguson et al. who published, "Strategies for containing an Emerging Influenza Pandemic in Southeast Asia", in this week's August 5, 2005 issue of Science(subscription).

Both groups had similiar results. They used mathematical and statistical models to predict disease containment outcomes based on various estimates of how quickly the virus would reproduce and how fast and effective the response with anti-viral prophalactics would need to be to contain the disease. The basic reproductive number "R0" (the zero should be subscript), quantifies the transmissibility of a pathogen. It is the number of secondary cases generated by a primary case if the entire population is susceptible. The goal of containment is to reduce the spread of the disease to R0<1. If R0>1 then the disease can spread.

Disease chains can be eliminated by decreasing the social contact rates, reducing the spread via infected individuals, and reducing the susceptibility of uninfected individuals by vaccination or antiviral prophylaxis. Successful hypothetical containment in this research necessitated effective and accurate public health information and response, potential quarantines, anti-virals and vaccinations if possible.

The Nature team used the population of Thailand (85 million) to model the epidemic, while the Science group also looked at Thailand but focused their study. The groups stressed that the models would only succeed if they were based on accurate assumptions, for instance R0 needs to be accurately determined, which is challenging during real time epidemics. As well, a treatment plan needs to be in action when the infected group is still very small (like 40 people). The treatment radius around the initial outbreak needs to be correctly assessed for the antiviral distribution to be effective. So if a 5 mile radius of individuals is treated but there are a significant number of undiagnosed cases in a 20 mile radius, the strategy will fail. The rapid response that is required is arguably easier to affect in a rural area as opposed to a large urban population.

Other important parameters include getting accurate initial surveillance to identify infection agent and the relative virulence of the strain- different strains can cause vastly different physiological responses. Considerations like the relative ease and lower cost of social containment, a strategy that provides antivirals only to those who come into contact with infected individuals, need to be weighed against the risks of limiting the treatments to these individuals.

These are only some of many conditions and assumptions amoung many directions that an outbreak could take. Both teams churned through hundreds of scenarios to arrive at their predictions. The Science group estimated that if R0 were <1.6 then 100,000 to 1 million vaccinations would be needed to contain an epidemic. The Nature group estimated that 3 million drug courses should be available to contain an outbreak as if the basic reproductive number (R0) were <2.

Under certain conditions both groups predicted that a human epidemic could be contained, however many scientists are highly skeptical. The clear concern is that the best case scenario that accomodates all the conditions that the researchers predict nessecitates very precise estimates and circumstances during an actual outbreak. Otherwise public health officials will not get accurate enough information to take the appropriate response.

Generating data to predict public health measures is valuable, which is why the studies have recieved so much attention. In addition to the biological data though, a lot is also dependent on cooperation and trust within and between the political systems of the countries effected, as well as their ability to communicate honestly, openly and frankly with global resources such as the WHO and CDC. SARS (Severe Accute Respiratory Response) and H5N1 both initiated in China. The international public health community criticized the country's response to SARS, that allegedly included cover-up, obfuscation and false assurances by the government. Of course, reportedly, China thinks differently. China has also been questioned about certain aspects of its response to the The H5N1 virus, which is suspected to have initiated in China in 1996. One troubling response was the the widespread and inappropriate use of antivirals recommended by the Chinese government that were then used as prophalactics in an attempt to contain the virus. An action widely suspected to have increased the resistance of the virus.

The pig infection that is currently active in China is linked to Streptococcus suis II has also prompted less than transparent reactions, including reports of banned media access. As well, the outbreak has followed an unpredictable infection course for this bacterium, that has infectious disease scientists concerned that the bacteria has either mutated or mixed with another bacteria or virus to acquire increased lethality.

Each of these diseases have been fairly well contained, but variables like those illustrated in these examples can make all the difference in the success of containment. Tamiflu, is manufactured by Roche Laboratories and is the current anti-viral that would be used in such a scenario. The World Health Organization (WHO) currently has about 195,000 courses of the drug and is negotiating to obtain a total of one million.

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