The Delta variant is not as contagious as chickenpox. But it’s still very contagious

In a leaked report, the Centers for Disease Control and Prevention made a startling claim about the delta variant of the coronavirus: The Washington Post July 26.

Chickenpox is one of the most contagious viruses known. Each individual can transmit the virus to up to “90% of people close to that person,” reports the CDC.

Is the delta variant also contagious?

The short answer is no, says evolutionary biologist and biostatistician Tom Wenseleers of the University of Louvain in Belgium.

“Yes, I didn’t find the CDC’s statement to be entirely accurate,” says Wenseleers, who was one of the first scientists to formally calculate the inheritance advantage of the alpha and delta variants over the original versions of the SARS-CoV-2.

Nonetheless, the delta is still highly transmissible, he adds. It is one of the most contagious respiratory viruses that we know of, he says.

Here’s why.

When scientists measure the transmissibility of a virus, they often use something called R0, or “R zero”. This is the number of people that a sick person will infect when the general population is vulnerable to the virus.

“So this is the potential for the virus to spread, under ideal conditions for the virus, when no one has immunity,” says computer biologist Karthik Gangavarapu of the Scripps Research Institute.

For example, the flu has an R0 of about two. Each person infected with the flu transmits the virus to an average of two people. Some people will infect more than two people and others will infect less. But over time, the average will be around two.

Chickenpox, on the other hand, is much more contagious, says Gangavarapu. Chickenpox has an R0 of about nine or 10. So each person with chickenpox infects about 10 other people on average. Epidemics are explosive.

For SARS-CoV-2, the R0 actually increased during the pandemic as the virus evolved. When the coronavirus first appeared in 2019, SARS-CoV-2 was slightly more contagious than the flu, Gangavarapu says. “The initial COVID-19 strain had an R0 between two and three. “

Then about a year later, the virus began to mutate rapidly. The alpha variant emerged, probably in the UK, and was more transmissible than the original strain. A few months later, the delta variant emerged, most likely in India. It was even more transmissible than alpha.

“For the delta variant, the R0 is now calculated between six and seven,” explains Wenseleers. It is therefore two to three times more contagious than the original version of SARS-CoV-2 (R0 = 2 to 3) but less contagious than chickenpox (R0 = 9 to 10).

So why did the CDC say that the delta variant was “just as transmissible as” chickenpox?

On the one hand, the disclosed document underestimated the R0 for chickenpox and overestimated the R0 for the delta variant. “The R0 values ​​for delta were preliminary and calculated from data from a rather small sample,” a federal official told NPR. The value for chickenpox (and other R0s in the slideshow) came from a chart of The New York Times, which was not entirely correct.

“Ultimately, this delta variant is much more transmissible than the alpha variant,” the official added. “That’s the message people need to get from this.” The official requested anonymity as they were not authorized to speak to the media on this matter.

The difference between an R0 of three and six is ​​huge. For example, with the original strain of SARS-CoV-2, one person would infect about three people, and each of those people would infect three more. Thus, after only two transmission cycles, the cases would increase by nine (3 x 3 = 9). After three rounds, the cases would increase by 27 (3 x 3 x 3 = 27). But with the delta variant, the first person would infect six more, who would each infect six more people. Thus, after two cycles of transmission, the cases would already increase by 36 (6 x 6 = 36). After three rounds, the cases would increase by 216 (6 x 6 x 6 = 216).

With an R0 of six, delta will be extremely difficult to slow down unless populations reach high levels of immunization, Wenseleers says. And even then, increases in cases will always occur, as is currently happening in Iceland and parts of the United States. And people who are vaccinated can still spread the virus. Additionally, people who are not vaccinated have a very high risk of infection, Wenseleers says. “Anyone who chooses not to be vaccinated will in all likelihood be infected with the delta variant within the next few months.”

For example, in San Francisco, daily case levels are rising rapidly towards those seen last winter despite more than 70% of the population being vaccinated, according to reports from the San Francisco Department of Public Health.

While delta cases are inevitable, hospitalizations are not, Wenseleers points out. “As long as people get vaccinated, we won’t have huge waves of hospitalizations.” For example, the city of San Francisco has had 3,041 people hospitalized with COVID-19 since March 18, 2020. Only 16 of them have been fully vaccinated.

Copyright NPR 2021.