TORONTO -- As scientists scramble to learn as much as they can about COVID-19 and the new coronavirus that causes it, they’re still grappling with a number of major unanswered questions.

It’s still not clear why children do not seem to be affected by the disease as severely as adults are, how long it will take to develop a vaccine, or exactly how the disease was first transmitted to humans – but they are making headway on determining how contagious it is.

“I think we’re starting to understand a little bit more about how transmissible it is,” Jason Kindrachuk, an assistant professor of viral pathogenesis, said Monday on CTV News Channel.

Medical researchers measure the contagiousness of a virus by determining its basic reproduction number, or R0. This number measures how many people are likely to be infected by one sick person. It is calculated via factors including how it is transmitted and its infectious period.

An R0 of 1 means the average carrier can be expected to infect one other person. An R0 below 1 means the virus will die out on its own; a higher number raises the possibility of outbreaks, epidemics and pandemic.

The World Health Organization investigative team that visited China in late February, led by Canadian expert Dr. Bruce Aylward, reported last week that it places the R0 of COVID-19 at between 2 and 2.5 – a number it considers “relatively high.”

R0 numbers are not affected by the development of vaccines or other actions taken to stop viruses from transmitting; they only measure a disease’s contagiousness under conditions where nothing is done to halt it.

An R0 of 2.5 – the absolute maximum the WHO considers likely – would give COVID-19 an infection rate on par with the influenza pandemic of 1918 (2 to 3), at the high end of estimates for the 2014 Ebola outbreak (1.5 to 2.5) and at the low end of estimates for SARS (2 to 5).

Several common infectious diseases have much higher R0s, including measles (12 to 18), rubella (6 to 7) and mumps (4 to 7). Lower R0s were calculated for two recent outbreaks that caused pandemic fears – H1N1 influenza in 2009 (1.46 to 1.48) and MERS (0.3 to 0.8).


One study examining the COVID-19 outbreak on the Diamond Princess cruise ship estimated the disease’s maximum likely R0 at 2.28, within the WHO’s calculated range.

These two figures, based on the beginning of the outbreak in Wuhan, China and the early days of coronavirus spread aboard the Diamond Princess, represent the disease’s R0 without accounting for measures that may be taken to curb its spread.

Kindrachuk pointed to the similarities between these contagiousness measurements and the early estimates out of China as a positive sign that researchers are getting a handle on what they are working with.

“We are starting to get some of these data pieces together and see that they hold fairly strongly together as well,” he said.

The WHO team notes that the rate of new infections in China began to fall after the country’s government enacted lockdowns and other strict spread-halting measures. While this doesn’t affect COVID-19’s R0 – remember, that number is only based on its transmission rates when it is able to spread freely – it does provide experts with a sense of optimism.

“We have never seen before a respiratory pathogen that’s capable of community transmission but at the same time which can also be contained with the right measures,” WHO director-general Tedros Adhanom Ghebreyesus said Monday at a press conference.

“Containment of COVID-19 is feasible and must remain the top priority for all countries.”