How infectious is Covid-19?

Patrick Bardill, Ph.D.:

When compared to more familiar diseases, this is much less contagious than measles, but more contagious than seasonal flu or norovirus.

This comparison is based on R-nought, or basic reproduction number. R0 is what R is at the start of an outbreak, before immunity or interventions. R0 more closely reflects the power of the virus itself, but it still changes from place to place. For example, R0 is higher in dense cities than sparse rural areas. R0 for seasonal flu is about 1.28; estimates of R0 for SARS-CoV-2 range from 2.2 to 5.7. Measles has an R0 of 18.

 R0 and R are different but have been conflated in most reporting. This has caused much confusion about what these numbers mean. 

 This resource has a very good explanation of R nought and R: https://ncase.me/covid-19/

Jack Lipton, Ph.D.:

It’s pretty infectious, from what we understand. At first, people were worried about it on surfaces. It doesn’t appear that touching a common surface is a particularly significant way for the virus to transmit. Some viruses are more hearty than others; their packaging are more conducive to living on surfaces.

For example, cold viruses are mild but are very infectious because they can last on door handles and other common touch areas. There is evidence that touch is important with coronaviruses, but more of the risks are associated with sharing air in the same space for an extended period of time with someone who is infected. That is because of microscopic droplets, which is why we’re all supposed to wear masks.

Could you explain what is meant by “R value,” and why it’s important to our understanding of Covid-19?

Patrick Bardill, Ph.D.:

R stands for current reproduction number, and is a measure of how contagious a disease is.

R is the average number of people a person with the disease will infect before they recover or die.

For example, an R value of 5 means that, on average, each person with a disease infects five other people. If the R value is below 1, the number of cases of the disease should drop. If the R value is above 1 the number of cases of a disease will increase. 

Note that both biological properties of an infectious agent as well as the behavior of people in an affected area influence the R value. Thus, it is possible for the exact same virus to have different R values in different areas depending on how people interact.

R can change over the course of an outbreak as people gain immunity and interventions change, as well. This is important with COVID-19, as practices such as scrupulous hygiene and social distancing can and have lowered the R value. 

Like any disease, the R value of SARS-CoV-2 has varied. Before public health interventions in a variety of places, the R value was as high as 3 or 4. With interventions such as self-isolation of the sick and social distancing, the R value has been brought below 1 in some places.