Estimating the contagiousness of COVID-19

There’s a possibility that COVID-19 is much more contagious than previously thought.

In a May 13, 2020 article in Harvard Magazine, experts discussed an indicator called R nought (R0) that shows how many other people one person with COVID-19 can infect. An R0 of 1 means that each infected person will infect one other person. The R0 of the coronavirus was originally thought to be in the 2 to 3 range, but experts now think it may be closer to 5 or 6.

The higher the R0 of a particular disease, the harder it becomes to control its spread with measures such as washing hands, wearing masks, social distancing, quarantines, and contact tracing, according to the article. An R0 in the 5-6 range also means that achieving herd immunity—when a large enough percentage of a population becomes immune so that transmission is greatly reduced—would require that more than 80% of the population be immune, as opposed to roughly 55% as had been previously estimated.

Harvard T.H. Chan School of Public Health epidemiologists quoted in the article included Michael Mina, Marc Lipsitch, and Caroline Buckee.

One indication that the R0 of the coronavirus is higher than 2 or 3 “is the difficulty that some places are having getting their cases down,” such as big cities or places that aren’t doing enough social distancing, said Mina, assistant professor of epidemiology.

Lipsitch, professor of epidemiology, said he’s uncertain about the possibility that the R0 of COVID-19 is as high as 5 or 6, because it’s been difficult to pin down the number of infected people, including those who are asymptomatic.

The R0 of the virus can vary depending on region—urban vs. rural, or nation vs. nation, noted Buckee, associate professor of epidemiology. When modeling the disease, “we use setting-specific contact rates to the extent that we can and they are … very much dependent on the population in question,” she said.

Mina agreed. “In many ways, it doesn’t make a lot of sense to say, ‘This is the R nought for the U.S.,’ because we are such a heterogenous country. I think it should be viewed in a much more localized fashion.” He added, “In cities like Boston and New York, in places where there is more potential for transmission, I think we will have to be more careful about how we open up. Continuing to wear masks, continuing to be diligent about washing your hands, about carrying hand sanitizer with you at all times—these things will help, but they will not prevent transmission.”

Read the Harvard Magazine article: COVID-19 May Be Much More Contagious Than We Thought