How contagious is the Delta variant compared to other infectious diseases?

This article was published on
August 13, 2021

This explainer is more than 90 days old. Some of the information might be out of date or no longer relevant. Browse our homepage for up to date content or request information about a specific topic from our team of scientists.

The R-naught for the COVID-19 Delta variant likely falls between 5 and 9 with some estimates putting it between 6 and 7. The R-naught for chickenpox is estimated to fall between 9 and 10.

The R-naught for the COVID-19 Delta variant likely falls between 5 and 9 with some estimates putting it between 6 and 7. The R-naught for chickenpox is estimated to fall between 9 and 10.


What our experts say

Since first being identified in India in December of 2020 the Delta variant has spread rapidly around the world. The reasons for this are numerous, but they include:

  • The Delta variant may replicate more quickly than the original strain of COVID-19
  • The Delta variant may generate more viral particles than other variants
  • The Delta variant appears to be at least twice as contagious as the original strain of SARS-CoV-2
  • The Delta variant appears to be 60% more contagious than the alpha variant (the formerly dominant strain in many countries)

In order to measure how many people might become infected by a virus when no one in a population is immune, scientists use something called the R0. The basic reproductive number—the R-naught or R0—is the number of people that one person with COVID-19 (or any virus) will go on to infect. For example, the R-naught for HIV is four, which means we can estimate that the average person infected with HIV will infect four other people, on average.

R-naught estimates are complex and not as straightforward as many assume. The number is constantly changing and is not an exact science. The variables that scientists use to calculate the R-naught can range across populations. The R-naught value is a representation—not an exact number— of how infectious a disease may be. The more time that passes, and the more data that is collected, the more accurate the number will be.

The R-naught for influenza is two, which means an average person sick with the flu will infect two others. Some people will infect more than two others, and some will infect fewer. However over a long enough period of time, we can estimate that the average is two.

The R-naught for the original strain of COVID-19 was recently estimated to be three. That is greater than the R-naught for influenza and lower than the R-naught for HIV. Because the Delta variant is more contagious than the original strain of COVID-19, its R-naught is higher than original strains of the virus.

The current R-naught for the Delta variant falls between five and nine, according to recent documents from the United States Centers for Disease Control and Prevention (U.S. CDC). Recently the U.S. CDC updated their estimate to reflect the Delta variant having an R-naught of 8.5. Many other estimates predict the R-naught of the Delta variant as falling between six and eight with several scientists predicting an average of seven. This represents a substantial increase in the virus' ability to spread amongst and infect people.

A more specific estimate by evolutionary biologist and biostatistician Professor Tom Wenseleers at the University of Leuven in Belgium, and biologist Karthik Gangavarapu (a computational biologist at Scripps Research Institute) is that the Delta's R-naught falls between six or seven. Wenseleers was one of the first scientists to use a mathematical computer model to estimate how fast the Delta was spreading in India. He applied the data from the variant to create “Wenseleer curves” which estimate how the variant may impact a population.

As more time passes and data about the R-naught is collected, we will have a more precise idea of the value for the Delta variant. Chickenpox (varicella-zoster virus) has an R-naught between 9 and 12 (most estimates start at 10) making it likely to be significantly more contagious than COVID-19 and the variants we have currently detected.

Recent comparisons between chicken pox and the Delta variant of COVID-19 have been made to express the idea that the Delta variant spreads much more easily and has the ability to infect many more people than the original strain of COVID-19 and earlier variants.

To calculate the R-naught, three main factors are considered:

  • Infectious period of the disease (how long someone with the virus is contagious so the longer this period, the higher the R-naught)

  • Mode of transmission (how the virus is transmitted between people; this is higher with airborne infections than those that require an exchange of bodily fluids or an insect bite, for example)

  • Contact rate (how many people the average infected person will come into contact with; location, age, public health prevention measures, and more influence this number so it is heavily modifiable)

Several challenges exist in determining the R-naught value for a disease, including:

  • How healthy or young a population is as younger, active people may be contagious for a shorter amount of time than older adults with underlying conditions.

  • When, exactly, an infected person becomes contagious and how long that period last may be different with variants like Delta.

  • Quantifying how many infections are caused by aerosolized particles, airborne droplets, or other methods can be difficult to gauge as well.

It should also be noted that generating an R-naught value for a large, even global population must take into account many variables ranging from town to town, country to country, continent to continent.

For instance, a 64-year old Vietnamese grandmother who is highly active and social, has never smoked, and routinely visits her physician may have a very different rate of contact or susceptibility to certain viruses than a 24-year old cancer survivor and computer programmer from Norway who sees few people each day and has his groceries delivered.

Applying a number to calculate a disease's ability to spread from person to person is complex and becomes more accurate as time passes, but emerging variants of concern will likely impact these numbers by increasing or decreasing the transmissibility of the virus. How many people have obtained some sort of immune protection through vaccination or prior infection will also factor in greatly.

Despite the fact that the Delta variant's R-naught range is likely not as high as chickenpox, the presentation of the virus is still highly contagious, easily transmissible, and even after vaccinating some populations, can cause infected people to infect others at a higher rate than they would have with the original virus.

Context and background

Recent reports from a United States Centers for Disease Control and Prevention document likened the R-naught of COVID-19's Delta variant to that of chickenpox, a highly contagious disease most often found in children. While this number was an early estimate and some of the data was presumed to be from a small sample size with more homogeneity than most populations hold, it is still a good indicator that this variant is highly transmissible and strong public health precautions should be taken to prevent its further spread.


  1. Shedding of Infectious SARS-CoV-2 Despite Vaccination when the Delta Variant is Prevalent - Wisconsin, July 2021 (medRxiv)
  2. Demystifying R Naught: Understanding What Does it Hide? (Indian Journal of Community Medicine)
  3. Why the proportion of transmission during early-stage HIV infection does not predict the long-term impact of treatment on HIV incidence (PNAS)
  4. Why is Delta more infectious and deadly? New research holds answers (National Geographic)
  5. How We’ll Know It’s Finally Time to Stop Masking (Slate)
  6. The Delta Variant Isn't As Contagious As Chickenpox. But It's Still Highly Contagious (NPR)
  7. ‘Goldilocks virus’: Delta vanquishes all variant rivals as scientists race to understand its tricks (The Washington Post)
  8. Improving communications around vaccine breakthrough and vaccine effectiveness (The Washington Post)
  9. Measuring Disease Dynamics in Populations: Characterizing the Likelihood of Control (Johns Hopkins)
  10. R0: How Scientists Quantify the Intensity of an Outbreak Like Coronavirus and Its Pandemic Potential (University of Michigan)
  11. How the pandemic ends (The Atlantic)
  12. Understanding Predictions: What is R-Naught? (Harvard Global Health Institute)

Media briefing

Media Release

Expert Comments: 

No items found.


No items found.