Introduction
Measles is not just another childhood illness—it’s one of the most contagious diseases known to humans. Its ability to spread rapidly through communities has led to explosive outbreaks in areas with low vaccination coverage. The secret to its high transmissibility lies in a concept called R₀ (basic reproduction number). Understanding R₀ helps explain why controlling measles requires extremely high levels of immunity in a population.
What is R₀ (Basic Reproduction Number)?
- Definition:
R₀ represents the average number of people that one infected individual will transmit the disease to in a completely susceptible population (meaning no one is immune). - Example:
If a disease has an R₀ of 2, each sick person will infect 2 others, on average.
For measles, the R₀ is estimated between 12 and 18, making it one of the highest among all infectious diseases.
This means that one person with measles can infect up to 18 other people if no one is immune.
Why is the R₀ for Measles So High?
- Airborne Transmission
- Measles spreads through tiny droplets released when an infected person coughs, sneezes, or even breathes.
- These droplets can remain airborne for up to 2 hours, allowing the virus to infect people long after the infected person has left.
- High Viral Load
- The measles virus multiplies rapidly in the respiratory tract and is shed in large quantities, making it more likely for others to inhale an infectious dose.
- Infectious Before Symptoms Peak
- People with measles can spread the virus 4 days before and 4 days after the rash appears, often before they even realize they’re sick.
- High Susceptibility in Unvaccinated Populations
- Nearly everyone who is not immune will get infected if exposed.
- Immunity only comes from vaccination or prior infection.
- Crowded Settings Increase Spread
- Schools, daycare centers, refugee camps, and densely populated urban areas are prime environments for measles outbreaks due to close contact.
What Does R₀ Mean for Herd Immunity?
- Herd immunity threshold formula:
Herd Immunity Threshold=1−1R0\text{Herd Immunity Threshold} = 1 – \frac{1}{R₀}Herd Immunity Threshold=1−R01
- For measles with an R₀ of 12–18:
Threshold=1−112 to 1−118\text{Threshold} = 1 – \frac{1}{12} \ \text{to} \ 1 – \frac{1}{18}Threshold=1−121 to 1−181
This equals 92–95% immunity required in the community.
- This extremely high threshold is why two doses of the MMR vaccine are recommended for lasting protection.
Conclusion
Measles is so contagious because of its airborne nature, high viral load, long-lasting environmental survival, and ability to spread before obvious symptoms appear. Its R₀ of 12–18 means that controlling measles requires nearly universal vaccination coverage. When immunity drops below the herd immunity threshold, outbreaks can spread like wildfire—even from a single case.
FAQs:
What is the R₀ value of measles?
It ranges between 12 and 18, meaning one person can infect up to 18 others in a fully susceptible population.
How does measles spread so easily?
It spreads through airborne droplets that can linger in the air for up to 2 hours.
Can measles spread before symptoms appear?
Yes. An infected person can spread the virus 4 days before and 4 days after the rash develops.
What herd immunity level is needed to stop measles?
About 92–95% of the population must be immune through vaccination or past infection.
Does measles spread more easily than COVID-19?
Yes. Measles is one of the most contagious diseases known, with a much higher R₀ than most respiratory infections.
