Ro & AIDS-- Epidemiological Inaccuracy

The basic reproductive number of disease, R0, is generally used to estimate the spread of an epidemic. It is used by epidemiologists and public planners to estimate the spread of an outbreak, and the level of response that will be necessary to quash it. In our reading for this week, HIV's R0 is estimated at 4, and I would like to take a closer look at what this means. In its most basic form, R0 expresses the disease's factor of contagion-- if one person has the virus, this 'patient zero' is likely to spread it to 4 people in a susceptible population. In order to be considered infectious, R0 for a pathogen must be greater than 1 (or else the disease will die out and not spread).

However, there are other important factors when considering the spread of infectious diseases. In a formula measuring an epidemic's potential impact, the timing of the incubation and contagious periods must be taken into account. Additionally, the 'susceptibility' of the population in question may vary. In today's society in which the movement of people and goods takes place at hyperspeed and on a massive scale, R0 values are not always a good indicator of a disease's infectiousness.

A virus like HIV, additionally, has several confounding factors that makes it extremely difficult to obtain a standard value. Most basic reproductive numbers are based solely on past epidemic statistics, not on any biological data. With a disease like AIDS, which manifests itself with a wide variety of symptoms and can take decades to fully develop, good statistics are difficult to come by. R0 values also assume that the potentially affected population is homogeneous. It is abundantly clear that in the case of HIV, some people are far more likely to be infected than others, and transmission generally requires a specific network of behaviors. For airborne diseases like influenza, only the most fleeting of contact is necessary to spread the virus, but with HIV, people engaged in specific behaviors are at infinitely higher risk of infection. Yet IV drug users, gay men, and sex workers do not fit into a homogeneous population model, and do not share its epidemic trends.

Thus far, epidemiologists have been unable to develop a disease model that reflects the real world.

Katie Nelson
9/23