- Calculating how the new coronavirus will continue to spread involves one of the most important numbers in epidemiology: the basic reproduction number, also known as R0 (pronounced R-naught).
- R0 represents how many people an average patient spreads the disease to.
- Scientists around the world have calculated different R0 values for the new coronavirus, mostly between 2 and 4.
- The number can inform public-health measures, such as isolation and quarantines.
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Scientists are racing to answer one important question about the coronavirus outbreak: When will it be over?
Any potential answer hinges on one of the most important numbers in epidemiology – the basic reproduction number, also known as R0 (pronounced R-naught).
R0 refers to the average number of people that one infected person goes on to infect. Experts use it to predict how far and how fast a disease will spread, and the number can also inform policy decisions about how to most effectively contain an outbreak.
“R0 is a population-based determination that helps you to decide, is the outbreak taking off, leveling off, or diminishing?”Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in a conversation with Howard Bauchner, editor of the Journal of the American Medical Association.
In the case of this novel coronavirus, scientists' preliminary calculations are offering a range of different R0 numbers, from 1.4 to 5.47. The bad news is that all of the possible values suggest the outbreak isn't slowing yet.
Confusion around the R0 of the novel coronavirus
It's been just over a month since the novel coronavirus, whose scientific name is 2019-nCov, was identified in Wuhan, China. It has spread to 26 countries, sickening more than 28,000 people and killing 565. (For the latest case total, death toll, travel information, and guidance about how to protect yourself, see Business Insider's live updates here.)
A lower R0 means an outbreak is slowing or stopping, while a higher one means it's swelling. An R0 value of 1 means the average person who gets that disease will transmit it to one other person; in that case, the disease is spreading but at a stable rate. A value of less than 1 means a disease is in decline and could die out. An R0 of more than 1 means the disease is continuing to spread among people exponentially.
The World Health Organization has estimated the R0 of the coronavirus to be between 1.4 to 2.5. A group of researchers at Lancaster University suggested higher value of 3.8 last week, but revised it this week to 2.5 as new data became available. A group of Chinese doctors, meanwhile, calculated the R0 to be between 3.3 and 5.47 in research published in the International Journal of Infectious Diseases on January 29.
The differences in these estimates reflect a variety of lingering questions about the coronavirus, Kim Leslie, an emergency-department nursing director at Swedish Hospital in Chicago, told Business Insider.
"The fear is, even among healthcare providers, is the unknown about this," Leslie said. "Is it shed before symptoms? What is the incubation? Does it need to be treated? The ones who have gotten ill and died - why them? What else did they have going on?"
Why R0 is tough to calculate
The R0 of the seasonal flu, for comparison's sake, is about 1.3. Researchers calculated the R0 of SARS to be between 2 and 4 - it varied from location to location based on transmission dynamics. The R0 of measles, which is extremely contagious, is between 12 and 16.
"There are a lot of caveats with R0s," Fauci said.
How likely an infected patient is to transmit a disease depends on a variety of intertwined and specific factors, including the way it gets transmitted (through the air or in bodily fluids); whether a pathogen is contagious during its incubation period; how long that incubation period lasts; and how many people the average patient has contact with.
Factors like contact rate vary from patient to patient - a person who gets sick might stay inside for the next week, or they might continue to take public transportation to their job, socialize with friends and family, and visit a new location (perhaps even before they show symptoms).
Another challenge in calculating transmission rates is that some patients are "super-spreaders" that can infect many people they come into contact with. Doctors believe this might be true of some Wuhan coronavirus patients.
In addition, R0 can depend on how far a pathogen can travel without a host and in what form. The Wuhan coronavirus spreads in droplets - usually saliva or phlegm from coughs and sneezes - which on average can travel 6 feet. So far, it does not seem able to survive for more than a few hours on surfaces.
By comparison, measles can travel 100 feet in the air and stay alive on surfaces for hours.
The availability of good data also informs the accuracy of R0 calculations. Because doctors in China have been so busy treating patients, they might not always have time to collect comprehensive data from each one. So the data set scientists are using to estimate averages so far is limited.
Plus, as Health and Homeland Security secretary Alex Azar said last week, some patients with milder symptoms might not even seek medical treatment so may not be represented in the data at all.
"The cases that have been identified skew to the severe," Azar said during a briefing last week. "How does this skew our understanding of the virus?"
Public health measures based on R0
R0 is a key indicator about whether an infectious disease is causing an epidemic or pandemic. Those determinations tell policy-makers what kinds of rules to implement and what guidance or instruction to give the public.
When outbreaks grow, public-health agencies rely on strategies like isolation and quarantines to lower the transmission potential. That proved effective during the SARS pandemic, and China is taking a similar, though more aggressive, approach to the novel coronavirus.
Authorities instituted a quarantine for the city of Wuhan on January 23, then soon expanded it to 15 other cities - the orders affect more than 50 million people.
In China and other countries with coronavirus cases, R0 estimates are also helping doctors compare the new virus to better understood outbreaks of the past.
"We're certainly using lessons learned in flu and tuberculosis," Dr. Kathleen Jordan, an infectious-disease specialist at Saint Francis Memorial Hospital in San Francisco, told Business Insider. "But because we don't know as much about the coronavirus, we sort of err on the side of caution and we're using a higher level of protective equipment."
Morgan McFall-Johnsen contributed reporting for this story.