The R0, pronounced R-naught, is a number epidemiologists use to determine how contagious a disease is and a community’s susceptibility to it. It represents the basic reproduction number, which is the number of susceptible individuals a single infected person will transmit the disease to during the course of their illness. For example, the estimated R0 of measles in 12-18, which means a single person with measles will infect 12-18 other susceptible people.
Deeper Dive
However, there are some important caveats to understand with R0. First, the key word is “susceptible.” Most people vaccinated against a particular disease are not susceptible to an infection from that pathogen, so a person with measles in a highly vaccinated community may not transmit the disease to anyone. In the case of measles specifically, where an estimated 97% of fully vaccinated people are immune, about 3% of vaccinated people remain susceptible to the disease. If a person with measles who is still contagious visits a church or airport with thousands of people then, even in a highly vaccinated community, it’s likely some of them will contract the disease.
Out of 1,000 fully vaccinated people, for example, 30 of them will be susceptible. About 90% of susceptible people exposed to the measles virus will develop an infection, so if those 1,000 vaccinated people visit the same area as an infected person within two hours of the infected person’s visit (since measles droplets remain airborne for up to two hours), then 27 of them (90% of 30) will likely develop measles, demonstrating how incredibly contagious it is.
A real-life example of a similar situation was described in NEJM in 2006. An unvaccinated person who was incubating measles—before she knew she was infected — visited a church gathering while contagious. Among about 500 people, 50 did not have evidence of immunity, and a third of them got measles at the gathering. Within six weeks, a total of 34 cases of measles were confirmed, most of whom (94%) were unvaccinated.
Another caveat to the R0, as demonstrated by its variance depending on the vaccination rate of a community, is that it’s a dynamic number, dependent on context. Some communities may have greater susceptibility than others for reasons beyond vaccination, especially for vaccines that do not provide a high level of effectiveness or sterilizing immunity, the elimination of a pathogen before it replicates in the host. One estimate of the R0 for COVID-19, for example, is 1.4 to 2.4, but that number would be much higher in a crowded room with poor ventilation and much lower for someone outside with nearby people spread out.Despite these limitations, the R0 can provide a sense of how comparatively more or less contagious certain diseases are. Measles is the most contagious human disease and therefore has the highest R0. Other extremely contagious human diseases include pertussis (an R0 of 15-17) and chickenpox and polio (R0 of 10-12). The common cold has an R0 of about 2-3, HIV is 2-5, and flu ranges from 1-3.