- A crucial metric called R0, pronounced R-naught, represents how many people an average person with a virus infects.
- The coronavirus has an R0 of roughly 2 to 2.5, meaning that each new person spreads the disease to about 2.2 people on average.
- That makes COVID-19 more contagious than the seasonal flu.
- But a disease’s R0 isn’t fixed – it can decrease with the right preventive measures. Bringing it below 1 would end the pandemic.
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A crucial step in reigning in the coronavirus pandemic is determining exactly how contagious it is. That comes down to one crucial metric: the R0 (pronounced R-naught).
R0 refers to the average number of people that one sick person goes on to infect in a group that has no immunity. 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 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 February interview hosted by the Journal of the American Medical Association.
The R0 of the coronavirus so far seems to hover around 2 to 2.5, according to the World Health Organization. A study of the poorly contained outbreak on the Diamond Princess cruise ship revealed an R0 consistent with those estimates: 2.2.
That means it's more contagious than the seasonal flu, but less contagious than measles.
"It is a virus that is quite good at transmitting from one person to another," Fauci said.
How R0 works
A given pathogen's R0 value changes with place and time.
"There is no R0 - there is an R0 in a population," Elizabeth Halloran, a biostatistician at Fred Hutchinson Cancer Research Center and University of Washington, told Business Insider. A recent CDC study found that the coronavirus's R0 was as high as 5.7 in the early days of its Wuhan outbreak.
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 at a stable rate. An R0 of more than 1 means the disease spreads exponentially.
When experts strategize about how to end the pandemic, their goal is to bring the R0 below 1, which would put the coronavirus in decline until it dies out.
Controlling one key factor - social interaction - is crucial in lowering the R0 of any disease.
"Even measles, if it's out in the country, doesn't have a very high R0 because you have to interact with a certain number of people to be able to infect them," Halloran said. "Part of the transmission involves social contacts between people, and the R0 depends on how fast people are interacting with each other."
Control measures can lower a virus's R0
The likelihood that an infected person gets someone else sick depends on many factors, which can be tough to estimate and vary based on circumstances.
Those can include: the way the virus gets transmitted (through the air or in bodily fluids); whether a virus is contagious during its incubation period; how long that incubation period lasts; and how many people the average patient has contact with. (That varies widely from patient to patient, of course: 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).
That's why R0s are often discussed in ranges. Research has shown that the R0 for measles, for example, could be as low as 3.7 or as high as 203. The R0 of seasonal influenza is more set, at about 1.3.
Reducing an R0 typically requires social distancing, among other measures.
"We here in Seattle have changed our R0 drastically because people aren't interacting with one another," Halloran said. "A lot of the estimates came in between 2 and 2.4 in the early days. Now we've gotten to the point in Seattle where we've flattened the curve and it's starting to drop."
Similar efforts helped curb the spread of Middle Eastern Respiratory Syndrome (MERS), a particularly deadly coronavirus. MERS emerged in Saudi Arabia in 2012. There, its R0 was less than 1. But when it reached South Korea in 2015, the MERS R0 skyrocketed. Some estimates suggest it got as high as 8, especially in hospitals.
But through quarantine measures, contact tracing, and limiting unnecessary contact between healthcare workers and patients, South Korea brought that outbreak under control within months. The R0 of MERS is now below 1, according to the WHO.
"With early isolation of cases and adequate infection prevention and control measures R0 can be brought to <1," a WHO report published in July 2019 said.
Herd immunity is still far away
Once lockdowns lift, experts fear a resurgence of new coronavirus cases. To prevent the next wave of COVID-19 patients from overwhelming hospitals again, a group of infectious-disease researchers at Harvard have suggested that on-and-off social distancing could be necessary into 2022.
Eventually - most likely via a vaccine - communities or countries could achieve "herd immunity," the point at which enough of a population has become immune to a virus to prevent it from spreading and keep its R0 below 1.
But for now, experts agree that we're not close to a point of herd immunity for this coronavirus.
"That [R0] is similar to pandemic flu of 1918, and it implies that the end of this epidemic is going to require nearly 50% of the population to be immune, either from a vaccine, which is not on the immediate horizon, or from natural infection," Harvard University epidemiologist Marc Lipsitch told a gathering of experts last month.