Fact Check: The measles virus has an R number of 12 to 18, indicating that one infected person can spread it to up to 18 others in an unvaccinated population.

Fact Check: "The measles virus has an R number of 12 to 18, indicating that one infected person can spread it to up to 18 others in an unvaccinated population."

What We Know

The basic reproduction number, often denoted as R₀ (R nought), is a critical metric in epidemiology that indicates the average number of secondary infections produced by one infected individual in a completely susceptible population. For the measles virus, R₀ is widely reported to be between 12 and 18. This means that each person infected with measles can potentially infect 12 to 18 other individuals if they are unvaccinated and susceptible to the virus (Guerra et al., 2017).

Measles is recognized as one of the most contagious infectious diseases, primarily due to its mode of transmission through respiratory droplets and its ability to remain airborne for extended periods (WHO). In 2022, measles was responsible for approximately 136,000 deaths globally, predominantly affecting unvaccinated populations (Tranter et al., 2024).

Analysis

The claim that the measles virus has an R number of 12 to 18 is supported by multiple credible sources. The World Health Organization (WHO) and various epidemiological studies consistently cite this range when discussing measles' transmissibility (WHO, Guerra et al., 2017). The high R₀ value reflects the virus's efficiency in spreading, particularly in populations with low vaccination rates.

While some studies have indicated that R₀ can vary based on specific population dynamics and local conditions, the consensus remains that the 12 to 18 range is a reliable estimate for measles in a fully susceptible population. This consensus is further supported by systematic reviews and meta-analyses that aggregate data from various outbreaks and studies (Tranter et al., 2024).

However, it is essential to consider the context of these numbers. The R₀ value is an average; actual transmission can be influenced by factors such as population density, social behavior, and public health interventions. For example, during outbreaks, the presence of unvaccinated individuals significantly increases the potential for transmission (CDC).

Conclusion

The claim that the measles virus has an R number of 12 to 18, indicating that one infected person can spread it to up to 18 others in an unvaccinated population, is True. This figure is well-supported by scientific literature and reflects the highly contagious nature of the virus, particularly in populations lacking adequate vaccination coverage.

Sources

1. Onward Virus Transmission after Measles Secondary ...
2. The basic reproduction number (R 0 ) of measles
3. Measles Cases and Outbreaks
4. Measles Update — United States, January 1–April 17, 2025
5. What to Know About Measles and Vaccines | Johns Hopkins
6. Measles - World Health Organization (WHO)
7. Measles - World Health Organization (WHO)
8. Measles and rubella - World Health Organization (WHO)