A New Epidemic Model

stat INFOGRAPHIC /l Beyond the Ro Value Transmission rates were once assumed to approximate a bell curve. However. some super-spreaders can be linked to dozens of cases, while the majority infected few or none. The phenomenon of super-spreading is as important to epidemiologists as the overall R, A New Epidemic Model Bending the Rules SARS does not appear to be the only infectious disease upending the R, model. Researcher James Lloyd-Smith at UCLA has examined outbreaks of measles, smallpox, plague and monkeypox, finding a skewed distribution of cases in all of them. BY MICHELE COHEN MARILL // INFOGRAPHIC BY FLYING CHILLI When an infectious disease outbreak occurs, public health authorities are keen to determine its R, value: the reproductive number of the pathogen, or how many people will become infected from contact with one individual. Yet with SARS and other respiratory diseases, it turns out that the R, value tells only part of the story about a disease's spread. 0.8 ISARS HMeasles Smallpox HMonkeypox 1.0 0.6 0.8 This bell curve represents the rate of transmission that might be expected: one or two people infecting just one person. What Makes an Epidemic? If R, is less than one-meaning that, on average, most people aren't infecting anyone else-the disease will eventually die out on its own. .But in reality, the vast majority infected no one else. Pneumonic plague 0.6 0.4 0.4- -Homogeneous population .while a handful of patients infected an astounding number of people. 0.2 0.2 0.4 1.0 0.6 Proportion of infectious cases (ranked) 0.2 0.8 10 15 25 How to Stop the Spread? Does this mean we should be on the hunt for modern-day Typhoid Marys? Number of secondary cases Case Study: Beijing SARS Outbreak 2003 If R, is more than one-on average, each case leads to more cases-public health authorities must contend with an epidemic. A woman entered a Beijing hospital in February 2003 to be treated for complications of diabetes. No one knows of any contact she had with a SARS patient, but the hospital did treat someone with SARS in March. On April 5, the diabetic woman developed a fever, headache and fluid on her lungs. On April 12, the day she died, eight of her relatives were diagnosed with probable SARS. She infected 33 of 74 people who had contact with her in the hospital; the chain of transmission ultimately included 77 SARS cases and 15 deaths. Targeting infection control strategies-such as isolation or adminis- tering scarce vaccines-on the people who are most likely to spread disease would be an optimal way to halt an outbreak. But that raises both practical and ethical issues Source and transmission of SARS cases Beijing 2003 Several factors can affect the rate of Generation infection of a disease. Pros Cons Someone who coughs on a crowded airplane or train is more likely to infect other people. Chain of transmission of the disease It releases from isolation those (O Super-spreaders are usually identified only after they patients who aren't likely to spread disease. have infected People who produce lots of infectious body fluids pose higher risk of infection. many people. DNA sequencing Identification of an Source R, varies based on the traits of the pathogen and environmental factors. can help epidemiologists trace back to an asymptomatic carrier could harm that person's career. 33 infected 10 individual. Super-spreader Immunity because of vaccination or previous illness can have an effect. Infects multiple people Research hopes to find factors linked to increased risk of transmission. There may be as yet unknown factors, such as a genetic propensity to be more infectious. Co-infection A Genetic mutation Infected with disease Behaviors protomag.com // WINTER 14