Just How Contagious Is Measles? More So Than Ebola, COVID-19, Or Even Smallpox

Is measles more contagious than other viruses?

In the league table of infectious diseases, measles sits right near the top. It’s said that nine out of 10 unvaccinated people exposed to the virus will catch the disease. That might sound unsurprising, but it’s not always the case – with polio, for example, only about 1 percent of infected people get the serious paralytic symptoms we all associate with the disease. 

Measles can have a reproduction number (R₀) of somewhere between 12 and 18. You might remember people talking about R₀ early in the COVID pandemic – it’s the average number of new infections caused by one patient in a population where no one is protected. For measles, that means that in a vulnerable population, each infectious case could be expected to lead to 12-18 more.

To put that in perspective, we can look to an article by professor of epidemiology Joseph Eisenberg in The Conversation, in which he outlined the R₀ numbers of several human viral infections.  

Against measles’ 12-18, smallpox – one of the most feared and deadliest diseases humanity faced, until it was eradicated – had an R₀ of 5-7. Polio is about the same, while the Ebola virus that caused the 2014 West Africa outbreak had an R₀ of 1.51-2.53.

Remember: this is only a measure of how infectious a disease is. It doesn’t say anything about how severe the illness is once you catch it. But it does illustrate why measles outbreaks can grow so quickly and are so hard for health authorities to contain. 

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How is measles transmitted?

Another factor that makes measles such a threat to public health is how it spreads. It is airborne, meaning that you don’t need to be in particularly close contact with an infected person to be at risk. 

Droplets containing infectious particles are exhaled, coughed, or sneezed into the surrounding air and onto nearby surfaces, where they can remain for up to two hours. 

This is why during outbreaks, we see alerts about potential exposures in busy public places like airports and churches. Hypothetically, you could unknowingly enter a room where someone with measles has been and contract the infection, even if you never meet them face-to-face.

Once you are infected with measles, you can pass the infection on from up to four days before the characteristic rash appears until four days afterwards – that’s quite a big window, and before the rash comes up, it can be easy to misidentify the early symptoms of measles as something like a cold or the flu. That means people could infect others before they’re even aware they have measles.

Herd immunity – the role of vaccines

It’s not hyperbole to say that the development of a vaccine was a game changer when it comes to measles control. The MMR and MMRV vaccines used today, as well as being very safe, are supremely effective – the two recommended doses typically provide around 97 percent protection against infection. 

This protection persists at a good level for decades, as was confirmed in a 2024 study from the London School of Hygiene & Tropical Medicine. 

But in order to keep measles at bay, it’s vital that this level of protection is very widespread within the community. Even small gaps in coverage give the virus an opportunity to spread.

There are people of all ages who are immunocompromised or have medical conditions that prevent them from receiving vaccinations; there are also babies who are too young for their first vaccines. All of these people remain vulnerable to infection, so it’s up to everyone else to get vaccinated to protect them.

According to the European Centre for Disease Prevention and Control, at least 95 percent of eligible people should be vaccinated against measles. Data from recent outbreaks has shown that overwhelmingly, those falling sick have been the unvaccinated. 

Health authorities around the world have a clear message: it’s never too late to check on your immunization status and to get up to date, or to catch up on missed vaccines for your child. 

The content of this article is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of qualified health providers with questions you may have regarding medical conditions.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.