What is this Ro and why does it matter?

Updated: Jul 16, 2021

You may have heard the term Ro associated with COVID-19. It is pronounced “R Naught”, not “R Zero”. Why this pronunciation? I don’t have a clue!


Ro is the basic reproduction number of infectious microorganisms. This number is based on the how infectious the microorganism is in a social environment. With the coronavirus, if one person is infected and gives the virus to another person, the Ro would equal 1. If ten people were infected by the virus, the Ro would be 10. The higher the Ro, the greater the chance of being infected.


The lockdowns have decreased our social interaction. We have not been going to arenas or rock concerts! (Though hopefully we have been going to Mass Immunization Centres for our COVID-19 vaccine!) This decreased socialization has resulted in a lower Ro. As the Government of Ontario releases the “Emergency Brake”, i.e. lifts the state of emergency, the Ro will naturally increase. How much will this change the infection rate? No one knows until it already has happened. The increase in contagion is not the result of a more infectious variant but simply more social interaction. The BBC has a great one minute video explaining this concept.


COVID-19 variants have different infectious rates and thus different Ro. Here is a chart.

Vaccines help herd immunity. I have previously written a blog about herd immunity. Herd immunity helps protect unvaccinated people from the virus. The vaccine efficacy has dropped because of the new, Delta variant. A research study has placed this efficacy value at 88%, whereas previously it was 95% for the Alpha variant.


This is the formula to calculate herd immunity

I would like to thank Dr. Philippe Lagacé-Wiens for providing the equation.


Now, if we assume the Ro=5, herd immunity is achieved when 91% of the population is fully vaccinated. If the Ro=8, herd immunity is achieved at 99% of the population being fully vaccinated. Relying on others to get the vaccine is a poor strategy to preventing infection, given how many people need to be vaccinated for herd immunity.


Dr. Kieran Moore has issued a “Call for Arms” to be vaccinated. Now is the time to talk to your loved ones and friends about vaccinations. I have found that asking the question, “Do you know of anyone who needs a COVID-19 vaccine?”, is extremely helpful. They may tell you that they are not interested in the vaccine. If you can, try to understand their point of view before giving information. This can be challenging when two parties have different views about vaccination. Listening and respect are most important. Defensiveness destroys any chance of a productive discussion.


The Mass Immunization Clinics are in full swing. Here is the link to book your appointment. You may receive Pfizer or Moderna. It does not matter which one you are getting. They are both equally as effective. We are all in this together. Thanks for reading!















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