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Understanding COVID-19: Episode 9 – Disease Reproduction

By Bob Arnot, M.D., sponsored by DeVry University

April 2, 2020
4 min read

Dr. Arnot discusses some of the key characteristics about COVID-19 and dives into the importance of the R naught value (basic reproduction number) and how this compares to previous pandemics. The doctor explains what R naught value we should aim for in order to suppress the virus and what measures we can take as a society to slow and possibly end the speed in which the virus is infecting the population. The video also illustrates what outcomes we might expect depending on what measures we take as a society.

Video Transcription

So, with his terrifying worldwide spread, let's take a step back and look at some of the key characteristics of the virus, beginning with its base reproduction rate. This is reflected in a value called R naught, which stands for the basic reproduction number. This number is critical to the infectiousness of any illness.

Let's look at R for the most infectious of diseases:

  • Measles

    With measles, one person may affect 12 to 18 others.

  • Polio and Mumps

    Polio and mumps are also highly contagious, infecting up to seven other people.

  • Coronavirus

    Now let's look at corona. This is spread by airborne droplets and the R value may be between 1.4 and 3.9, although the standard value used is 2.2.

As we will see, this number can change depending on the control measures used. The idea is to get it below one.

Basic Reproduction Number
Now let's look at a bit more detail. The base reproduction number describes the contagiousness or transmissibility of infections like corona. Fortunately, this is not biologically constant as we have seen. The R naught value describes the secondary cases one would expect to see in a completely susceptible population with no immunity. For every case detected, however, there may be as many as 10 undetected cases, explaining why corona has spread so far and wide.

To decrease the R value social distancing has the greatest impact. Although corona does live on surfaces, there aren't any proven cases, and it's believed that most transmission is airborne when an individual coughs or sneezes who has the virus. The only way to overcome the epidemic is by suppressing it through isolation.

So, to review, R naught is the average number of people that are infected. And if the R is less than one, the epidemic will die out. If R is one, there will be an outbreak, but the disease will stay alive and stable, which worries many public health experts.

So with corona, an R of one would keep the virus in the population without an increase in numbers unless the virus mutates or dies out. When R naught is greater than one, each existing infection causes more than one new infection, leading to an outbreak or epidemic. You can see on the right how R values increase and become more and more infectious from Ebola and hepatitis on the low end to measles at the upper end.

Control Measures
Control measures are necessary to decrease the R value. With no control, you'd see a massive increase in infections across the US in this New York Times map. With some in the middle, you can see more moderate spread. And finally with severe measures like those used in China and Wuhan, you could see a dramatic containment of new cases.

Here is how this would translate into new cases. The red spike reflecting no measure, the yellow with some measures, and the blue was severe measures. The chart reflects new cases every day. Over time, the New York Times shows how much spread to expect by April, May and June for each of the three control measures: no intervention, some control and severe measures. The key focus of dampening the number of new cases is keeping the case load low and within the confines of a healthcare system's ability to treat all patients who might survive.

So the strategies are mitigation, which slows but doesn't stop the epidemic, or suppression where the R value is reduced to less than one. Again, that means that each infected individual infects less than one other person. There's still a lot to be learned about the transmission of corona, especially with a large number of undetected cases. Schools double the per capita contact, and so most school systems have closed in many affected areas.

Disclaimer: The opinions expressed are those of the author. The content is intended to provide general information on the nature of the pandemic, potential exposures, and is not intended to provide medical advice or address medical concerns or specific risk circumstances. Always seek the advice of your doctor or other qualified health provider regarding a medical condition. Neither DeVry University nor its employees or business partners, nor any contributor to this content, makes any representations, express or implied, with respect to the information provided herein or to its use.

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