How Contagious Are Chickenpox, Measles As CDC Document Reveals Delta Variant's R0

Among the well-known diseases, measles and chickenpox are two of the most infectious. But how does their contagiousness compare to the Delta variant of SARS-CoV-2 that is now causing the majority of new COVID-19 cases in the U.S.?

According to an internal Centers for Disease Control and Prevention (CDC) document that was obtained by The Washington Post, the Delta variant appears to have an R0, or basic reproduction rate, of between around 5 and 9.5.

The R0 value of an infectious pathogen refers to its contagiousness and transmissibility. The number gives an idea for how quickly a particular pathogen will spread through a given, susceptible population in which there is no immunity.

Specifically, the number refers to the average number of people that will be infected by one sick person. So, an R0 of 7, for example, means that, on average, one sick person will infect seven other individuals.

The reproduction rate of an infectious pathogen can be affected by various factors such as preventative public health measures and the presence of immunity, acquired either naturally or through vaccination.

As immunity to a particular pathogen builds up in a given population, a measure known as Re, or effective reproduction number, becomes more appropriate to use when estimating the number of people who can be infected by an individual at any specific time.

In the CDC document, the agency says that the Delta variant is more transmissible than the related coronaviruses that cause MERS and SARS, Ebola, the common cold, seasonal flu, the 1918 Spanish flu and smallpox.

In addition, the agency says on one page of the document that the variant is as transmissible as chickenpox. On this page, a graph is featured that was originally published by The New York Times in 2020 showing the transmissibility of various pathogens, including the ancestral SARS-CoV-2 virus.

The CDC has updated the graph to include information on the new Delta variant. According to the graph, the variant appears to be as transmissible as chickenpox, which is shown to have an R0 value of around 8.5.

But other common estimates for the R0 value of chickenpox indicate a range of 10-12, which would mean that infection caused by the varicella-zoster virus is slightly more transmissible than the Delta variant of SARS-CoV-2.

Nevertheless, the Delta variant appears to be less transmissible than measles, which is one of the most infectious pathogens that we know about. The R0 of measles is often cited to be 12-18, meaning that on average, one infected person would transmit the disease to 12-18 others in a totally susceptible population.

In fact, measles is so contagious that up to 90 percent of people close to an infected person who are not immune will also become infected, according to the CDC.

The internal CDC document suggests that fully vaccinated people infected with Delta may be capable of spreading the variant at the same rate as unvaccinated people. The document also says that the variant has been associated with a slightly increased risk of more severe illness.

CDC Director Dr. Rochelle Walensky confirmed the authenticity of the document, telling CNN: "I think people need to understand that we're not crying wolf here. This is serious. It's one of the most transmissible viruses we know about. Measles, chickenpox, this—they're all up there."

"When you think about diseases that have an R0 of eight or nine—there aren't that many."

The document notes that while vaccine breakthrough cases are expected and will increase as a proportion of total cases as vaccine coverage increases, the COVID-19 shots available in the U.S. still provide high levels of protection against severe disease caused by the Delta variant.

"Vaccines prevent more than 90 percent of severe disease, but may be less effective at preventing infection or transmission," the document reads. "Therefore, more breakthrough and more community spread despite vaccination."

Coronavirus particles
Stock image showing an artist's illustration of SARS-CoV-2. The Delta variant of SARS-CoV-2 is more transmissible than the ancestral virus. iStock