How Contagious Is Monkeypox? R Number vs. COVID

There were 237 suspected and confirmed cases of monkeypox reported outside of Africa on Tuesday as the virus spread to the United Arab Emirates and the Czech Republic for the first time in this outbreak. The cases of the contagious disease spark concern given that the world is still navigating its way through the COVID-19 pandemic.

As cases continue to climb governments are warning their citizens to be vigilant, with some recommending that those infected and their contacts self-isolate to prevent further spread.

Addressing this, U.S. Centers for Disease Control and Prevention (CDC) official Jennifer McQuiston told reporters on Monday: "This is not COVID. It's not a situation where if you're passing someone in the grocery store, they're going to be at risk for monkeypox."

Virus on hands
Monkeypox is not thought to be as transmissible as COVID, according to health experts. This stock image depicts a magnifying glass revealing virus particles on someone's hands. vchal/Getty

The way that scientists label how well a virus spreads between people is by working out its R number, also called R0. The R number of a virus refers to how many people one infected person will go on to infect, so if a virus has an R number of 3 then a person who catches it will, on average, go on to infect three other people.

The R number for the Omicron BA.2 variant of COVID, for example, was calculated to be about 12 by professor Adrian Esterman, chair of biostatistics at the University of South Australia, in March.

The R number for monkeypox is thought to be considerably lower. According to a study published in the Bulletin of the World Health Organization journal in 2020, monkeypox was calculated to have an R number of about 2.13 based on data collected in the Democratic Republic of the Congo between 1966 and 1984.

Other estimates vary. Andrea McCollum, the poxvirus epidemiology team lead in the CDC's division of high consequence pathogens and pathology, told medical news outlet Stat last week: "We don't really have any estimates of R-naught for the West African clade. Most of our estimates come from Congo Basin. And most of those estimates are less than 1."

She added that even a virus with an R number of less than 1 can still be transmitted between people.

In short, the R number of monkeypox is debated but is thought to be far less than that of COVID, and thus less easy to spread.

"It's not as highly transmissible as something like smallpox, or measles, or certainly not COVID," Anne Rimoin, an infectious disease epidemiologist at the University of California Los Angeles, told Vox.