Epidemiologists recognize that for vector borne diseases like malaria and sexually transmitted diseases like AIDS, the 20/80 rule applys, that is 20% of infected individuals provide 80% of subsequent infections. This has important treatment implications; containment or eradication programs must target that 20% of the population in order to be successful. By comparison, the consequences of outbreaks of directly transmitted infectious diseases like measles, smallpox, SARS, and ebola virus are usually predicted by including many variables to determine an average "homogenous" infection rate across a population. Public health measures are then planned based on these calculations.
A team of researchers led by UC Berkeley's James Lloyd-Smith suggest that a heterogeneous model of infection occurs for these diseases too. Along with co-authors Sebastian Schreiber, associate professor of mathematics at the College of William and Mary in Virginia; and P. Ekkehard Kopp, professor of mathematics at the University of Hull in Great Britain, the group analyzed data from eight disease outbreaks from the last sixty years.
The research supports the fact that individual behaviors can strongly influence the progression of the disease. Some people choose not to seek treatment, while some doctors misdiagnose disease. If someone is immunocompromised or has multiple infections they can help the disease spread. Also environment, age, occupation and pathogen virulence all affect an individual's potential to infect others or become a superspreader.
The study also supports the common conclusions of almost any study of this sort: hospitals need to be vigilant and capable of ramping up during infections. In order to understand and affectively contain infectious epidemics, the authors suggest that we pay attention to the unique characteristics of superspreaders for each epidemic. Public health efforts control infections by containing an infection with blanket measures, but they should also should focus on identifying potential superspreaders. The research was published in Nature (438, no. 7066: 355-359) last week.
Of course the research invites many questions. One is, how does the research translate to different public health actions? Its fairly easy to identify high risk populations for sexually transmitted diseases and target these populations for treatment. It is also fairly easy to identify potential risky populations for a directly transmitted disease like viral influenza -- public schools, public transportation riders, elderly, health care workers, maybe food preparers, the list goes on. But since these people represent such a large swath of the population how would treating these people individually differ from quarantine and/or vaccination plans now in place?
