Monday, January 27, 2020

The Epidemiology of the New Corona Virus

With China's spreading outbreak of a new refrain of the coronavirus (termed 2019-nCoV), one has to wonder how fast this latest threat to human life can spread, particularly given the high density of China's urban areas and the vast numbers of Chinese citizens that travel throughout the world.  A recent study by Jonathan M. Read et al examines the rate of spread of this virus and its implications to China's population.  

Let's start with a basic concept, that of R0 or Rnaught.  R0 is defined as follows:

"R0 is a mathematical term that indicates how contagious an infectious disease is. It’s also referred to as the reproduction number. As an infection spreads to new people, it reproduces itself.

R0 tells you the average number of people who will catch a disease from one contagious person. It specifically applies to a population of people who were previously free of infection and haven’t been vaccinated. If a disease has an R0 of 18, a person who has the disease will transmit it to an average of 18 other people, as long as no one has been vaccinated against it or is already immune to it in their community.

Here are some examples:

1.) If R0 is less than 1, each existing infection causes less than one new infection. In this case, the disease will decline and eventually die out.

2.) If R0 equals 1, each existing infection causes one new infection. The disease will stay alive and stable, but there won’t be an outbreak or an epidemic.

3.) If R0 is more than 1, each existing infection causes more than one new infection. The disease will spread between people, and there may be an outbreak or epidemic.

R0 only applies when everyone in a population of people is equally and completely vulnerable to the disease.  This means that no one is vaccinated against the disease, no one has had the disease before and there is no way to control the spread of the disease.

In 1918, there was a global outbreak of the swine influenza that killed 50 million people.  In this case, R0 was estimated to be between 1.4 and 2.8.  In a more recent example, the swine influenza H1N1 that came into existence in 2009 had an R0 of between 1.4 and 1.6.

Now, let's go back to the study by Read et al.  In this case, based on current information, the authors of the study estimate the R0 to be 3.8 (with a 95 percent confidence interval of 3.6 and 4.0) which is higher than the average of SARS estimated from outbreaks back in 2003.  This means that between 72 percent and 75 percent of transmissions must be prevented by control measures to prevent the number of infections from increasing.  The high R0 value suggests that sustained human-to-human transmission is occurring in China.

The authors then looked forward in time.  They predict that, should the epidemic continue unabated in Wuhan, the origin of the outbreak, the number of infected will grow very rapidly. By February 4, 2020, the authors estimated that there will be between 132,751 and 273,649 infected individuals with a 95 percent certainly that there will be 191,529 infections.

While the Chinese government has taken significant steps to restrict travel to and from Wuhan, the authors project that a reduction in air travel will have little impact on reducing the number of infections as follows:

- 50 percent travel reduction - 12.6 percent infection reduction

- 80 percent travel reduction - 20.1 percent infection reduction

- 90 percent travel reduction - 22.6 percent infection reduction

- 99 percent travel reduction - 24.9 percent infection reduction

As you can see, while the reduction in air travel into and out of Wuhan will have some impact on the number of people that are infected by 2019 nCoV, there will still be a very rapidly growing number of newly infected people in Wuhan and the surrounding district.  Additionally, there will be a growing number of infected people in other cities throughout China and the surrounding nations as the days pass, thanks to travellers moving throughout the region.

Let's close with this video from the Event 201 pandemic exercise hosted by The Johns Hopkins Center for Health Security held in October 2019 which looked at the preparedness efforts for a severe pandemic:

In this mock exercise, the participants estimated that a novel coronavirus could result in the deaths of 65 million people.

While we are still in the early phases of this outbreak, the conclusions of this study would suggest that the current outbreak of this virus could worsen significantly over the coming days and weeks.    With the disease being spread during the incubation phase, it is quite likely that it has spread far further than is currently believed.  Once all of the pieces fall into place, it is only then that we will know whether this version of the coronavirus will be responsible for a global pandemic.

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