A visual look at how the virus spreads.
Not long after the novel coronavirus — which causes the disease COVID-19 — was first discovered in China, scientists and doctors warned that the virus could spread around the world. It has now spread to at least 115 countries, with thousands of people in the United States now reported to have COVID-19. But how is the virus spreading so quickly? It all comes down to the basic reproductive number, or how many new people contract the virus from one infected person. This is how it's calculated — and how we can reduce it.
How contagious is coronavirus?
A key factor in studying the spread of disease is determining the average number of new people who will be infected by each person with the virus. This is called R0, or the basic reproductive number. There are many, many variables that can go into calculating R , but at 0 the most basic level it involves 3 numbers.
First, the number of people each infected person has contact with is estimated. In this hypothetical example, let’s say each infected person comes into contact with 3 people susceptible to the virus.
The number of contacts is then multiplied by the percent chance that 1 of those contacts has been infected. In this hypothetical, let’s say the chance was 33%.
That number is then multiplied by the length of time individuals are able to contract the disease from the infected person.
So if someone with this hypothetical virus had contact with 3 people per day, and there was a 1 in 3 chance of infection for each contact, and the infected person was contagious for three days, the R0 would be 3. About 3 new people would contract the illness from each person infected.
Again, there are a lot of other variables that epidemiologists can add into this equation — those are only the basics.
The R0 of coronavirus is usually estimated to be somewhere between 2 and 3. This means that if one person contracts the virus, the next wave of infection — assuming there was no intervention — would be 2 to 3 times as large. It’s why the number of people affected increases rapidly after the first case in an area.
Researchers don’t know exactly what the R0 is for certain because no one has complete data on who has been infected.
Most of the coronavirus charts and maps you’ve probably been seeing, including the charts above, show the number of reported COVID-19 cases. But many people who have COVID-19 — and no one knows how many — are not being counted by medical authorities. COVID-19 can have mild symptoms or even no symptoms at all, and some people are unaware they even have the disease.
This means that in order to calculate R0 and other variables, researchers need to make assumptions about the number of people infected and other missing data. As a result, statistical estimates from different studies can vary. This is why while we know that the basic reproductive number of coronavirus is probably between 1 and 3, the exact number is still uncertain.
Because we don’t know for sure how many people have the virus, an area with 10 reported cases could easily have 100 cases in reality. And because of the exponential growth of the number of people with the coronavirus, those 100 cases could become 1,000 within a week.