Some diseases seem to infect everyone with their contagious nature, while others fade out quietly. What’s the difference between how diseases spread? The answer can be explained with an estimate that researchers call R0.
Measuring how fast diseases spread with R0
Researchers measure disease spread with R0 (pronounced “arr naught”). R0 estimates how many unprotected people one sick person infects on average. Unprotected means no prevention measures: no vaccine, no social distancing and no masks. The higher the R0, the faster a disease can spread.
Let’s say the R0 of a pretend disease – we’ll call it Sneezies – has an estimated R0 of 3. The first patient, Fred, infects three friends. Fred’s three friends each infect three others, who then infect three more each. Before long, 40 people are infected with Sneezies. Each patient only infected three people, but Sneezies spreads easily through the unprotected population.
R0 estimates of diseases depend on a lot of things:
- Where you live. For example, R0 would be higher in a densely populated city than a rural farming town
- How the disease spreads. Different diseases spread in different ways. Ebola spreads through bodily fluids like blood or vomit. Measles and COVID-19 spread through the air through respiratory droplets
- The number of infectious particles needed to make someone sick. Some viruses need a lot of viral particles to transfer from one person to another. The amount of time you spend with a sick person affects how many viral particles you’re exposed to. For COVID-19, the number of viral particles isn’t known yet
- How long until someone becomes contagious. When do people start spreading (infectious) germs around? Each disease has a length of time between when you’re exposed to infectious particles and when you can infect others
- Human behavior. If people wear masks, socially distance and wash their hands, they can decrease the effective R0 of the disease in their area
If the R0 of a disease is greater than one, the number of infected people will grow over time.
If the R0 of a disease is less than one, the number of infected people will shrink over time. This is why public health measures like social distancing, mask-wearing and postponing gatherings work to slow the spread of COVID-19. These types of health practices avoided 62 million COVID-19 cases in six countries, according to a study.
Maximum estimates of R0 (infographic below) show how quickly a disease spreads without a vaccine. COVID-19 can spread quickly because we’re an unprotected population – no vaccine exists yet to protect us.
Since R0 is not precise and depends on so many factors, researchers often estimate a range of R0 values. The maximum R0 values above came from these sources: CDC, LiveScience, EID Journal, and medRxiv.
The R0 of COVID-19
When the pandemic began, researchers estimated the R0 of the coronavirus to be 2.5. So each person that got COVID-19 would infect approximately 2.5 other people.
Recent estimates put the R0 of COVID-19 at about 6:
If these estimates are true, one person with COVID-19 typically infects about six other people.
R0 is the number of unprotected people that one sick person infects – on average. There are “super-spreader” events that infect many more people. One person with COVID-19 infected 51 others at choir practice in Washington.
How to avoid spreading COVID-19
If you have symptoms of COVID-19, please stay home. Self-isolate for two weeks. If your symptoms worsen or you have shortness of breath, seek medical care.
Unfortunately, some people can also have COVID-19 without knowing it. Some people are asymptomatic (show no COVID-19 symptoms ever) or presymptomatic (in the early days of the infection). That’s why public health expert John-Martin Lowe, PhD, encourages everyone to wear a mask.
Wearing a mask is something everyone can do to protect people. Even if you don’t have COVID-19 symptoms, wear a mask to keep others safe.
How to stop COVID-19 (even without a vaccine)
According to experts, these actions stop the spread of infectious diseases:
- Increase the resistance of the hosts (get a vaccine)
- Reduce viral shedding (isolate sick individuals)
- Reduce the exposure of a susceptible host with PPE (wear masks)
Researchers are working on a safe, effective COVID-19 vaccine as fast as they can. In the meantime, that leaves us with the other two actions – isolating sick individuals and mask-wearing.
However, we are not helpless until a vaccine comes. New Zealand eliminated the coronavirus – and they don’t have access to a vaccine, either. See how we can end this pandemic, together.