Things To Know About The R-Nought Number

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If you’ve paid any attention to the worldwide pandemic of COVID-19 or watched movies like Contagion, , you’ve heard the term “R-nought”.

Alfred Lotka

The R-nought (or reproduction number) is the 100-year-old brainchild of a public health expert in demographics named Alfred Lotka. A disease’s R-Nought, he said, is the number of cases that will occur in a population if an infected person is placed in the middle of it. Not just any population, however; one that hasn’t been exposed to the infection in the past.

In the 1950s, epidemiologist George MacDonald used it to describe the contagious potential of malaria. He suggested that, if the R-nought is less than 1, the infectious person will transmit to fewer than one other person and an outbreak will eventually peter out. On the other hand, if the R-Nought is greater than 1, the disease will spread. Seasonal flu carries an R-Nought of 1.28, while the current COVID-19 is probably closer to 3.

Probably? Certainly, the R-Nought represents important data regarding an infectious disease. Why, then, probably? Because different sources may report different R-Noughts for the same disease based on a number of factors. It’s not just the nature of the virus itself.

Estimation of the R-nought primarily relates to 3 parameters:

  1. how long a person is contagious
  2. the likelihood that contact with a susceptible person will end in transmission of the disease
  3. the frequency of contact between the infected individual and the susceptible population.

Let’s take them one-by-one. The first is how long a person is contagious. Certainly, you want to quarantine someone during their infectious period, but, with COVID-19, that period is not known for certain.

For SARS, it was about 14 days, so that’s what they’re using for the related SARS-COV2 (the name for the virus that causes COVID-19). There are outliers, however, that range from 20-37 days. With a range that wide, how do they figure out when you’re no longer contagious?

If COVID-19 testing is available, they have determined three criteria for considering release from isolation:

•             You no longer have a fever without using fever-reducing drugs.

•             Symptoms like cough or shortness of breath have improved significantly.

•             you have received two negative tests in a row, 24 hours apart.

If testing is not available, the three criteria are:

•             You have had no fever for at least 72 hours without using fever-reducing drugs.

•             Symptoms like cough or shortness of breath have improved significantly.

•             At least 7 days have passed since your symptoms first appeared (I was surprised at that last one; perhaps 14 days is more prudent).

Aside: Recovering COVID-19 patients might be surprised when they feel better but are told that the X-ray still shows signs of pneumonia. This is because the x-ray appearance of pneumonia commonly seems to lag behind the patient’s clinical appearance.

The second parameter is how likely is it that contact with a susceptible person ends up in infection. That depends partially on the characteristics of the virus itself, but It might also depend on a person’s age, general health, lifestyle, or even bad habits.

Older folks may get it as often as younger folks, but seem to do worse across the board. In one study, if you were in your twenties and got COVID-19, your chances of dying was 0.2 percent. If you were in your eighties, it was closer to 22 percent.

What about bad habits? Consider smoking: Most COVID-19 victims are men. in China, 50% of men smoke there as opposed to about 5% of women. Therefore, you can probably conclude that women have healthier lungs, on average, than men.

Cultural differences might also play a role. In Iran and certain other countries, most men work or spend a good amount of time outside. From this, we can infer that they might be exposed more often than women, who probably spend more time at home.

The third parameter is the frequency of contact between the infected individual and the susceptible population. For example, there are people that are known as “super spreaders”. A super-spreader is an individual who is more likely, for one reason or another, to infect others. 20% of infected individuals are responsible for 80% of transmissions to others.

Although South Korea is held out as a model of success in the containment of COVID-19, that wasn’t always the case. In mid-February, confirmed cases of SARS-CoV-2 infection suddenly jumped in that country. The Korean CDC attributed the increase in cases to “Patient 31“, who had participated in a mass religious gathering in the city of Daegu.

In New York, a lawyer contracted the illness and then spread it to at least twenty other individuals in his community in New Rochelle. In the early going, he was thought to account at one point or another for more than half of coronavirus cases in the state

Super-spreaders aren’t confined to viral disease, 100 years ago, a woman named Mary Mallon worked as a cook in New York. She was an asymptomatic carrier of the bacteria Salmonella Typhi, and passed that disease to more than 50 other people, giving her the nickname “Typhoid Mary“.

Terminating Typhoid Mary’s employment and quarantining super-spreaders and their contacts helps, but only if it’s done rapidly. In South Korea, it can be said to be successful. In New York, well, not so much.

There’s more to R-noughts than those 3 parameters, like testing issues, the availability of personal protection equipment to a community, and much more. It’s interesting to think about what the R-Nought of the 1918 Spanish Flu would have been if it occurred today with commercial air travel so common.

More updates on issues relating to the pandemic in the near future.

Oh, and if you were wondering where we’ve been lately, we’ve been personally packing medical kits seven days a week as well as writing our latest book, Alton’s Pandemic Preparedness Guide: Emerging and Current Viral Threats. You can find it on and, soon, at

This is Joe Alton MD, that old Dr. Bones, wishing you the best of health in good times or bad. Thanks for watching.

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