Delta’s ‘R naught’ contagiousness rate doesn’t mean entire US will get COVID-19

A TikTok video with more than 535,000 likes claims that if the contagiousness of the COVID-19 delta variant is on the high end of what "they" estimate, avoiding infection will be virtually impossible, regardless of vaccination.

"They have found that the R naught of delta is anywhere between 5 and 9½. The R naught value of an infectious pathogen refers to its contagiousness," the user says in the three-minute video, which was posted July 31 and was still viral nearly a month later. 

"If the R naught is actually closer to 10, within six weeks the whole country will test positive for delta, regardless of vaccination or not. I mean, at that rate, it’s so contagious, all you have to do is just walk past someone with it. … If you’re in close quarters and you walk past someone that’s exhaling out air and you inhale their air — even you just barely brush by them, if you get in their space and they are delta-positive, you are going to get sick. And at this rate, I think that’s going to happen to you whether you are vaccinated or not."

The conclusion of the claim is incorrect because it doesn’t take into account measures such as vaccination and social distancing that reduce the chances of contracting COVID-19, said Dr. David Dowdy, an infectious-diseases specialist at Johns Hopkins University.

He added: "The whole country testing positive within six weeks is patently false." 

According to the federal Centers for Disease Control and Prevention, the delta variant is more than twice as contagious as previous variants of the coronavirus and caused nearly 99% of recent coronavirus cases in the U.S. Unvaccinated people are particularly vulnerable: in Los Angeles County as of July 25, unvaccinated people were five times more likely to be infected and 29 times more likely to be hospitalized.

The video alludes to one way health experts think about a virus’ infection rate: the basic reproduction number, or R0 (pronounced "R naught"). It indicates the average number of times one infected person will transmit the virus to another person. The higher the R0, theoretically, the faster the virus is spreading.

The CDC has estimated that delta’s R0 is in the range stated in the video.

But even if the R0 were 10, "that does not mean that everyone, regardless of vaccination status, will be infected," said Dr. Céline Gounder, a clinical assistant professor of medicine and infectious disease at New York University’s Grossman School of Medicine.

R0 is a theoretical figure that assumes everyone in a population is without immunity and susceptible to infection. 

"It is important to realize that that is the number in a completely unmitigated, ‘natural’ environment — no one vaccinated, no one social distancing, no one wearing masks, and no one immune from prior infection," said Dr. Perry Wilson, a Yale School of Medicine associate professor.

Wilson said the important indicator for predicting cases in the future is what researchers call the effective reproduction number, or Rt. It is the observed average number of people an infected person infects. Delta’s Rt likely between 1 and 2, he said.

Featured Fact-check

Instagram posts

stated on November 26, 2024 in an Instagram post

“The CDC says that little 6 month old babies must get SEVEN doses of the Covid vaccine in order to be considered fully ‘up to date’ on their vaccinations.”

"Due to vaccination, prior infection, and masking and distancing, we have mitigated the ability of delta to spread, so the Rt is quite a bit lower than the R0," Wilson said.

In some places, because of steps such as vaccination and masking, the Rt, while changing daily, is below 1, said Dowdy, indicating that the spread of the virus has been curbed.

The estimated Rt in the U.S. as of Aug. 28 is 1, according to a project led by researchers from the London School of Hygiene & Tropical Medicine.

A TikTok video claims that the contagiousness of the coronavirus delta variant is "valued between 5 and 9½," and if the value is closer to 10, "within six weeks, the whole country will test positive for delta, regardless of vaccination." 

One measure rates delta’s contagiousness in the range of what the video states, but it is a theoretical measure that doesn’t take into account steps such as vaccination and social distancing that reduce the risk of getting COVID-19. Accounting for those steps, the reproduction number is far lower.

There is no evidence to suggest that everyone, regardless of vaccination status, could become infected.

We rate the post Mostly False.