R0, pronounced “R naught” or “R zero,” is a mathematical term that indicates how contagious an infectious disease is. It’s also referred to as the reproduction number. R0 of 1 means an infection in one person will spread to one other person on average.

R0 is known as the basic reproduction number or reproductive ratio.

It tells you the average number of people who will contract a contagious disease from one person with that disease.

It specifically applies to a population of people who were previously free of infection and haven’t been vaccinated. It also relates to how contagious the disease is before herd immunity has developed.

Three possibilities exist for the potential transmission or decline of a disease, depending on its R0 value:

  • If R0 is less than 1, each existing infection causes less than one new infection. In this case, the disease will decline and eventually die out.
  • If R0 equals 1, each existing infection causes one new infection. The disease will stay alive and stable, but there won’t be an outbreak or an epidemic.
  • If R0 is more than 1, each existing infection causes more than one new infection. The disease will be transmitted to more people, and there may be an outbreak or epidemic.

Importantly, a disease’s R0 value assumes everyone in a population is completely vulnerable to the disease.

This means:

  • no one has been vaccinated
  • no one has had the disease before
  • there’s no vaccine or immunization effort available

For example, if a disease has an R0 of 2, a person who has the disease will transmit it to an average of 2 other people. That cycle will continue if no one has been vaccinated against the disease and no one has immunity to the disease already.

However, thanks to medical advances, it’s rare that an entire population is completely vulnerable to a disease. Many diseases that were deadly in the past can now be contained and sometimes cured.

For example, in 1918, a worldwide outbreak of swine flu (H1N1) killed 50 million people. The R0 value of the 1918 pandemic was estimated to be between 1.4 and 2.8. However, when it returned in 2009, its R0 value was between 1.4 and 1.6, according to researchers in the journal.

The existence of vaccines and antiviral drugs made the 2009 outbreak much less deadly than the previous one.

The R0 for COVID-19 is a median of 5.7, according to a study published online in Emerging Infectious Diseases. However, R0 can change as new data comes out. For example, an earlier R0 estimate was around half this, at 2.2 to 2.7.

An R0 of 5.7 means that one person with COVID-19 can potentially transmit the coronavirus to 5 to 6 people, rather than the 2 to 3 researchers originally thought.

Researchers calculated the new number based on data from the original outbreak in Wuhan, China. They used parameters such as the virus incubation period (4.2 days) — how much time elapsed from when people were exposed to the virus and when they started to show symptoms.

The researchers estimated a doubling time of 2 to 3 days, which is much faster than earlier estimates of 6 to 7 days. The doubling time is how long it takes for the number of coronavirus cases to double. The shorter the time, the faster the disease is spreading.

With an R0 of 5.7, at least 82% of the population needs to be immune to COVID-19 to stop its transmission. Herd immunity is achieved through vaccinations and natural immunity.

The following factors are taken into account to calculate the R0 of a disease:

Infectious period

Some diseases are contagious for longer periods than others.

For example, according to the Centers for Disease Control and Prevention (CDC), adults with the flu are typically contagious for up to 8 days. Children can be contagious for longer than that.

The longer the infectious period of a disease, the more likely a person who has it can transmit the disease to other people. A long period of infectiousness will contribute to a higher R0 value.

Contact rate

If a person who has a contagious disease comes into contact with many people who have not had the infection or have not been vaccinated, the disease will be transmitted more quickly.

If that person remains at home, in a hospital, or otherwise quarantined while they’re contagious, the disease will be transmitted more slowly. A high contact rate will contribute to a higher R0 value.

Mode of transmission

The diseases that are transmitted the fastest and easiest are the ones that can travel through the air, such as the flu or measles.

Physical contact with a person who has such a disease isn’t needed to transmit it. You can contract the flu from breathing near someone who has the flu, even if you never touch them.

In contrast, diseases that are transmitted through bodily fluids, such as Ebola or HIV, aren’t as easy to contract or transmit. This is because you need to come into contact with infected blood, saliva, or other bodily fluids to contract them.

Airborne illnesses tend to have a higher R0 value than those spread through direct contact.

R0 can be used to measure any contagious disease that may spread in a susceptible population. Some of the most highly contagious conditions are measles and the common flu. More serious conditions, such as Ebola and HIV, spread less easily between people.

This illustration shows some commonly known diseases and their estimated R0 values.

R0 of diseases

However, not all infectious diseases can be measured with R0. For example, diseases that don’t transmit via humans, such as tetanus, are spread through the environment.

R0 is a useful calculation for predicting and controlling the transmission of disease. Medical science continues to advance. Researchers are discovering new cures for different conditions, but contagious diseases aren’t going to disappear anytime soon.

Take these steps to help prevent the transmission of contagious diseases:

  • Learn how different contagious diseases are transmitted.
  • Ask your doctor about steps you can take to stop the transmission of infection. For example, wash your hands regularly with soap and water, especially before you prepare or eat food.
  • Stay up to date on routine vaccinations.
  • Ask your doctor what diseases you should be vaccinated against.

What’s the difference between R0 and Re?

Re is the effective reproduction number, also known as Rt.

It’s the number of people who are able to get an infection from an individual at a specific point in time. This number decreases as the population becomes immune or people die. Re can also be affected by people’s behavior, such as social distancing.

The effective reproduction number gives information on how the disease is spreading in real time. Scientists use both R0 and Re to monitor a disease.

Is R0 the same as doubling time?

No, R0 is not the same as doubling time. Doubling time is how long it takes for the number of infections in a population to double.

R0 is how many people get an infection as a result of one person being infected.