FEB 28, 2026
Epidemiology of Measles
Measles is the world’s most infectious disease. It has a basic reproductive number (R₀) of 12-18. That means that each infected person infects 12-18 other non immune people. As a result, 90% of susceptible people will get measles from exposure to 1 person with measles. The measles virus can linger in the air for 2 hours after a person with measles has left a room. Any susceptible person who enters that room during the 2 hour time period can be infected with measles. Here is a picture of a child with a typical measles rash.
Most US doctors have never seen a case of measles like this child. That, unfortunately is about to change.
For comparison of infectivity let’s look at the R₀ for other diseases we consider very contagious. R₀ for Ebola is 1.5-2.5, R₀ for COVID is 2.2-3.6 and R₀ for influenza is 1.2-1.4. As you can see none of these diseases comes even close to being as infectious as measles.
There are only two other diseases that are closer to being as contagious as measles. The R₀ for pertussis (whooping cough) is 5-17 (see my recent post on pertussis). The R₀ for varicella (chicken pox) is 10-12.
History of Measles in the US Prior to Measles Vaccine
Prior to the introduction of the measles vaccine in 1963, virtually all children in the US had measles by age 15. Having measles (and surviving it) provides lifetime immunity to measles, so adults born before 1954 have had measles and are already immune. People born before 1954 do not need to be vaccinated for measles.
Symptoms of Measles
Here is a description of measles symptoms from the Mayo Clinic
“Measles symptoms show up around 7 to 14 days after contact with the virus.
The first symptoms usually are:
- Fever, which may be as high as 105 degrees Fahrenheit (40.6 degrees Celsius).
- Dry cough.
- Runny nose.
- Red, watering eyes, called conjunctivitis.
About 2 to 3 days after the first symptoms, you also may see tiny white spots inside the mouth, called Koplik spots.
About 3 to 5 days after symptoms first start, or about 14 days after you come into contact with the virus, it’s common to see a rash. The rash is made up of large, flat spots and small raised bumps. The rash starts on the face or neck and spreads down the body from the chest to the arms and legs. A person with measles can spread the virus four days before the rash appears and four days after.”
Measles is obviously not a pleasant disease to have. Fever of 104-105 is pretty scary. All children with measles are miserable, but most children without complications recover from measles in about 10 days.
Complications of Measles
This information comes from the CDC.
Ear Infections
1 in 10 children with measles get bacterial ear infections that may need antibiotics
Hospitalization
About one in 5 children and adults with measles have to be hospitalized,
Pneumonia
One out of every 20 children with measles gets measles pneumonia. pneumonia is the most common cause of death from measles in children.
Encephalitis
One child out of every 1000 will develop encephalitis (inflammation of the brain). This condition can also cause death, but children who survive may have deafness and/or intellectual disability.
Death
Three children out of every 1000 children with measles will die from their disease, either from respiratory or neurological complications.
Subacute Sclerosing Pan Encephalitis (SSPE)
This is a uniformly fatal brain disease that can occur 7-10 years after a measles infection. Fortunately it is rare, but children who get measles before age 2 are at higher risk of SSPE.
Herd Immunity to Measles
The recommended regimen for MMR (measles, mumps and rubella) vaccination that gives the most protection is to be vaccinated at age 1 year and a second dose at age 4-5. This regimen gives lifelong 97% protection against contracting measles. This means that children under 1 year of age are protected only by everyone around them being immune to measles. Measles is a live vaccine (a weakened strain of the measles virus). People who have weakened immune systems because of chemotherapy or other causes of immune deficiency cannot receive live virus vaccines. Their only protection from measles is herd immunity. Herd immunity means that enough people are vaccinated or immune that a susceptible person who cannot be vaccinated is protected by the “herd” of people who are vaccinated.
Measles is so contagious that 95% vaccination rate is necessary to prevent measles entirely. Outbreaks start to occur if vaccination rates fall below 95%. Large outbreaks occur at vaccination rates below 90%.
In 2025 MMR vaccination rates ranged from a low of 75% in Idaho to 98% in Connecticut. Only 10 states had MMR vaccination rates above 95%.
MMR Side Effects
There are occasional side effects to MMR vaccine but the vast majority of those are mild. They include pain and swelling at the injection site, fever, a mild rash about 10 days after vaccination and some joint stiffness. When they happen these side effects disappear completely in a day or two. There are some very rare more serious reactions including febrile seizures (8 in 10,000 vaccinations), bruising or bleeding from low blood platelets (1 in 20,000 vaccinations) and severe allergic reactions (1 in 1 million vaccinations. Febrile seizure is scary for parents but does not recur and has no long term effects on the child. The other rare side effects are manageable. There are no known deaths from side effects of MMR vaccine. It is also important to point out that all of these side effects are much more common with measles infections.
MMR Vaccination Resistance
Worry about side effects of MMR vaccine has led to parents refusing MMR for their children. The main reason for vaccination resistance is worry that MMR vaccine causes autism. Very large well designed research studies have shown definitively that there is no connection between MMR vaccine and autism. The misinformation about MMR vaccine and autism unfortunately persists. This vaccination resistance has decreased the vaccination rate below the 90% threshold for serious outbreaks of measles in large parts of the US.
Current Outbreaks of Measles
I think the best way to show the resurgence of measles in the US is graphically. Below is a graph from the Johns Hopkins Bloomberg School of Public Health.
As you can see from this graph, measles cases started to rise after 2019 and went through the roof in 2025. 2026 is on track to have even more cases than 2025.
Deaths from Measles
Worldwide in 2025 95,000 children died from measles almost entirely from lack of measles vaccination. Three of those deaths were in the US. Given the resurgence of measles outbreaks across the US, we will undoubtedly see more deaths from measles.
Bottom Line
Measles is the most contagious disease in the world. Most children recover, but complications occur in 20% (one in five) children with measles. Complications include ear infections, hospitalizations, pneumonia, encephalitis and death. Vaccination rates must be at least 95% for complete herd immunity. Once vaccination rates fall below 90% large outbreaks of measles occur Vaccination rates are below 90% for large parts of the country and large outbreaks continue to occur in those communities with low vaccination rates. Measles is back with a vengeance!