What is the R-naught (R0) value?

R-naught (R0) is the Reproduction number of a disease.  It’s a measure of to how many people an infected person will spread the disease into a population that has little or no immunity.

  • If R0 is less than 1, each existing infection causes less than one new infection: The disease will decline and eventually die out.
  • If R0 equals 1, each existing infection causes one new infection: The disease will continue in the population but there won’t be an epidemic.
  • If R0 is more than 1, each existing infection causes more than one new infection:  The disease will spread and there is a high probability of an epidemic.

What was the R0 of the 1918 Influenza Pandemic?

In 1918, the Swine Flu (H1N1 virus)  had an estimated R0 value in the U.S. of  between 1.34 to  3.21.  Among sailors it was as high as 4.97.

When the swine flu, or H1N1 virus, returned in 2009, the  R0 value dropped to around 1.4 to 1.6, still pretty contagious but not as bad as 1918 because many people had some measure of immunity.   The 1918 pandemic killed an estimated 50 million people worldwide. One contributing factor was apathy by public officials in the beginning and the continued transportation of solders back and forth from Europe.

SARS-CoV-2 virus causes the disease COVID-19

This virus is apparently new to humans although it is known to exist in animals and similar strains have been isolated and stored in containment laboratories around the world, such as those in Galveston Texas, and Wuhan, Hubei, China. These laboratories are classified as Biosafety Level 4 laboratories where the most virulent and dangerous pathogens are safely stored for research purposes.

Genetic research on the SARS-CoV-2 virus recovered from patients suggests that the variant causing the pandemic is  similar to that recovered from horseshoe bats but as of May 2020 its origin and direct ancestral viruses have not been identified.

What is the R0 of  SARS-CoV-2?

The only totally reliable way to know the R0 of  SARS-CoV-2 is to wait until the end of the pandemic and then survey various populations around the world. It will vary from one population to another.

Yet wouldn’t it would be useful to know approximately what R0 is now?

Judging from the reaction of health authorities around the world, it is perceived to be around 2–3, suggesting  that 50–60% of the population will eventually be infected because nobody appears to be immune.  As more people are infected and recover, the pool of carriers diminishes and the pandemic slowly dies out. By then vaccines will be developed and susceptible populations can be vaccinated. It is not yet known if a vaccine will impart long-term immunity, or if it will be like the seasonal flu.

Ç’est certainement la fin de la bise

Best Estimates of R0 for SARS-CoV-2

Medical News Today reports a study buy  Joacim Rocklöv, who is a professor of sustainable health at Umeå University in Sweden. They analyzed 12 studies that they regarded to be of high quality. Their conclusion was that SARS-CoV-2 likely has an R0  ranging from 1.4 to 6.49!  The World Health Organization estimates between 1.4 and 2.5. Differences can vary depending on how calculations are made. Some use statistical methods; other simple mathematical calculations.

Calculating the R0 of a disease involves knowing how long the infectious period lasts, how long infected people are in contact with susceptible people, and how the disease is physically spread.  COVID-19 produces huge amounts of virus in the upper respiratory tract before a person even feels sick. Sneezing and coughing at this early stage spreads the virus far and wide.

This is why wearing a mask does not prevent you from contracting the disease. The sick person does not know she is sick so she sneezes and you pick it up all over – your face, your hands, and the pen you are writing with. That is why washing your hands frequently with soap and warm water is important.  Wash your face and neck with soap and water as frequently as possible.

Nobody really knows at this point how long the sickness lasts or how long a person can pass the virus on. The duration of the illness seems to be about 3 days without symptoms, and then a week of mild to severe symptoms, and then another week of recovery.   The length of infectivity is not know with any certainty until more laboratory testing in carried out on infected patients. It most certainly will vary from one individual to the next because everyone’s immune system is different.

Early quarantining of people who have even been exposed is wise.  It may be the most effective way of slowing the spread.  They call it flattening the curve.

The mode of transmission is by droplets or mucous from the upper respiratory tract that land on you or on other surfaces where you pick it up.   These secretions are invisible on doorknobs, counter tops, pens and pencils, and anything handed from one person to another.  The question is still valid as it was in 1918 when mailmen spread the flu around rural communities that had quarantined themselves.  Can I catch it from the mail?  Answer: Unlikely, unless you are very friendly with the mailman.

You can also catch a cold, or flu, or COVID-19 the same way.

Fortunately, the same measures that help prevent the spread of flu and the common cold, also reduce the spread of COVID-19:  frequent and thorough hand washing; not touching your face;  coughing and sneezing into a large tissue or towel, or better yet cough outside far from people; staying out of crowds or maintaining larger than customary distances from people. The Swiss health authorities recommend about 6 feet.

Doing all of these things will reduce the R0 of COVID-19, and will allow hospitals to keep up with the inevitable severely sick people.

Because of the uncertainty of how well people will comply with recommendations of our healthcare leaders, the R0 of SARS-CoV-2 will probably be a rather wide range displaying geographical differences. Seasonal factors may also come into play as North America enters the summer months.  Anything from preventive measures to weather that slows the velocity of the virus’s spread will lower the eventual R0.


Creative Commons License CC BY-SA