As the new coronavirus pandemic leads to lock-downs, cancelled events, and overwhelmed hospitals, officials are scrambling to get ahead of the pathogen.
To reign in the outbreak, experts need to know how contagious it is. That comes from one crucial metric: the R0 (pronounced R-naught).
R0 refers to the average number of people that one sick person goes on to infect, among a group that has no immunity to the virus. 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 an interview hosted by the Journal of the American Medical Association on February 6.
The R0 of the new 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.
Here’s how R0 works: 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.
COVID-19 seems more contagious than flu, but less than SARS
Still, the likelihood of one person getting someone else sick depends on a lot of different factors that can be tough to estimate, and that can vary based on circumstances.
Those can include: 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.
For example, research has shown that the R0 for measles could be as low as 3.7 or as high as 203. The R0 of seasonal influenza is about 1.3.
Even when the range of possible Ros is narrow, 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).
It’s still unclear what R0 means for this new pandemic
How well a virus spreads can impact how deadly it is.
“Infections, particularly respiratory infections — the more efficiency they gain in spreading, the less case fatality they have,” Fauci said.
That’s why the H1N1 flu outbreak in 2009 “wasn’t dubbed as a particularly serious pandemic,” Fauci said. “That spread very, very well, but the fatality rate was quite low.”
There are some exceptions to this, Fauci said, like the 1918 flu outbreak that was both highly contagious and deadly.
So far, the fatality rate for COVID-19 is hovering around 3.4%, but experts think it could drop to about 1% as more mild cases are diagnosed. That’s roughly ten times the death rate of seasonal flu.
Though research on the new coronavirus is still in its early stages, and the world looks nothing like it did in 1918, the numbers have started to ring alarm bells.
“WHO has been assessing this outbreak around the clock, and we’re deeply concerned both by the alarming levels of spread and severity and by the alarming levels of inaction,” the organization’s director-general, Tedros Adhanom Ghebreyesus, told reporters.
Control measures can lower a virus’s R0
We can take action to stop a virus’s spread. R0 isn’t fate.
Take Middle Eastern Respiratory Syndrome, or MERS, a particularly deadly coronavirus.
MERS first 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 show it reached as high as 8, especially in hospitals.
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 July WHO report said.
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