What Is Coronavirus' R Number and How Contagious Is COVID-19?

The COVID-19 pandemic has thrown up myriad questions, including how contagious the coronavirus which causes the disease is. One common way of expressing this is what experts who study the spread of disease call the reproduction number, referred to as the "R0" (pronounced "R-naught") or the "R number."

The reproduction number describes how many people a single person will pass a bug on to when they are infectious, Kit Yates, senior lecturer in mathematical biology at the University of Bath, explained to Newsweek.

"It's important to appreciate that R0 is not a fundamental property of the disease," said Yates, the author of The Math of Life and Death.

The R0 changes depending on how a person behaves, social structures of a population, the bug's ability to infect people, and how long the sick are infectious, he said.

If the R0 is less than one at the start of an outbreak, this means a virus is no longer able to spread as it is being passed on to less than one other person, on average.

"But if R0 is larger than one then the outbreak will grow exponentially," Yates said.

"As the disease progresses, the reproduction number changes as there are fewer susceptible people to infect," Yates continued. "It also changes due to the interventions we put in place to slow the spread of the disease. Once the disease is in full flow we tend to call it R, the 'effective reproduction number' or just the 'reproduction number.'"

Reproduction numbers can typically be broken down into three components, according to Yates: the size of the population, the rate at which susceptible people become infected (often known as the force of infection), and the rate of recovery or death from the disease. When the first two increase, so does the reproduction number. When the recovery rates rises, it drops.

"The bigger the population and the faster the disease spreads between individuals, the larger the outbreak is likely to be," said Yates. "The quicker individuals recover, the less time they have to pass on the disease to others and, consequently, the easier it will be to bring an outbreak under control."

Currently, over 2 million people have been diagnosed with COVID-19, according to Johns Hopkins University. Since the pandemic started in Wuhan, a city in central China and the capital of Hubei province, the coronavirus has spread to every continent except Antarctica, as shown in the Statista map below. More than 137,000 people are known to have died, and almost 517,000 recovered.

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A map showing COVID-19 cases worldwide as of April 15, 2020. Statista

At present, it is not possible to accurately estimate the worldwide reproduction number of the coronavirus, Jeremy Rossman, honorary senior lecturer in virology at the University of Kent told Newsweek.

Firstly, different models and datasets are used to calculate it, he said. Secondly, the reproduction values are "highly location and time dependent."

Experts estimate the figure is currently between R2 to R3, said Russman, but can vary from 1.1 to 11 in unique environments like the confined space of the Diamond Princess cruise ship, said Rossman.

"For example, in Wuhan the estimated R0 was likely around three and then rose to 3.82 in January before falling significantly after lockdown measures were taken," he said.

The number dropped below 1 in the city on February 6, and to 0.3 on March 1, according to a study Rossman cited which was published in the journal JAMA.

"In addition, the measures of R0 varied for many cities around China, ranging from 5.9 in Shenzhen to 0.9 in Beijing. In each of these cities, the public health interventions significantly decreased" the reproduction number.

A further "significant issue" with accurately calculating the R0 is the lack of widespread testing, said Rossman, meaning the majority of mild COVID-19 cases are missed. Complicating matters further is potentially a significant number of infected people who show no or few symptoms, as well as those who have atypical effects like digestive problems, said Rossman.

As a result, similarly to how death rates somewhat misleadingly vary between countries, "estimates of R0 will vary from location to location in ways that do not actually reflect differences in transmission but is just reporting of cases," Rossman explained.

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A motorist rides past a graffiti painted on a road to raise awareness about COVID-19 in Chennai, India, on April 13, 2020. ARUN SANKAR/AFP via Getty Images

As China relaxes some lockdown measures and other countries hope to soon follow suit, calculations like the reproduction number can help policymakers decide how to proceed.

"Understanding that our aim is to bring the basic reproduction number below one suggests some sensible strategies for controlling the spread of an infectious disease," said Yates.

"When it comes to easing social distancing measures, if we don't want a second peak or flare up then we have to be careful to ensure the reproduction number stays below one," said Yates. "This might mean relaxing certain social distancing measures but not others.

"Alternatively it might mean reducing the number of new cases to nearly zero and then aggressively isolating any new cases and quarantining anyone they have found to be in contact with (contact tracing).

"An alternative strategy might be to completely release all restrictions but to understand that they will need to be brought back as soon as certain trigger points in cases or ICU beds are hit."

Rossman believes we will need some level of continued physical distancing as well as to "significantly scale up our testing and contact tracing.

"These have been shown to be most effective, but we are not routinely performing testing or contact tracing for all that might be infected."

Rossman warned: "Without this measure I think it will be very difficult to ease restrictions and maintain control over the pandemic."

Centers for Disease Control and Prevention Advice on Using Face Coverings to Slow Spread of COVID-19

  • CDC recommends wearing a cloth face covering in public where social distancing measures are difficult to maintain.
  • A simple cloth face covering can help slow the spread of the virus by those infected and by those who do not exhibit symptoms.
  • Cloth face coverings can be fashioned from household items. Guides are offered by the CDC. (https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/diy-cloth-face-coverings.html)
  • Cloth face coverings should be washed regularly. A washing machine will suffice.
  • Practice safe removal of face coverings by not touching eyes, nose, and mouth, and wash hands immediately after removing the covering.

World Health Organization advice for avoiding spread of coronavirus disease (COVID-19)

Hygiene advice

  • Clean hands frequently with soap and water, or alcohol-based hand rub.
  • Wash hands after coughing or sneezing; when caring for the sick; before, during and after food preparation; before eating; after using the toilet; when hands are visibly dirty; and after handling animals or waste.
  • Maintain at least 1 meter (3 feet) distance from anyone who is coughing or sneezing.
  • Avoid touching your hands, nose and mouth. Do not spit in public.
  • Cover your mouth and nose with a tissue or bent elbow when coughing or sneezing. Discard the tissue immediately and clean your hands.

Medical advice

  • Avoid close contact with others if you have any symptoms.
  • Stay at home if you feel unwell, even with mild symptoms such as headache and runny nose, to avoid potential spread of the disease to medical facilities and other people.
  • If you develop serious symptoms (fever, cough, difficulty breathing) seek medical care early and contact local health authorities in advance.
  • Note any recent contact with others and travel details to provide to authorities who can trace and prevent spread of the disease.
  • Stay up to date on COVID-19 developments issued by health authorities and follow their guidance.

Mask and glove usage

  • Healthy individuals only need to wear a mask if taking care of a sick person.
  • Wear a mask if you are coughing or sneezing.
  • Masks are effective when used in combination with frequent hand cleaning.
  • Do not touch the mask while wearing it. Clean hands if you touch the mask.
  • Learn how to properly put on, remove and dispose of masks. Clean hands after disposing of the mask.
  • Do not reuse single-use masks.
  • Regularly washing bare hands is more effective against catching COVID-19 than wearing rubber gloves.
  • The COVID-19 virus can still be picked up on rubber gloves and transmitted by touching your face.