Vital context to include in your measles coverage

Since a measles outbreak first began in west Texas in mid-January, there have been 1,001 confirmed cases across 30 U.S. states as of May 8. While not all of these cases are specifically linked to the Texas outbreak, 93% of them are linked to the dozen outbreaks recorded this year. There's reason to believe these numbers will continue to climb given the pockets of low vaccination rates spread unevenly across the country.

As these cases increase, however, so is the amount of misinformation coming from Health and Human Services Secretary Robert Kennedy, Jr., a long-time anti-vaccine advocate who already exacerbated a deadly measles outbreak in American Samoa in 2019. Hawaii Gov. John Green even alleged Kennedy was responsible for causing additional deaths during the outbreak.

It is therefore vital that journalists continue to include accurate information about measles and measles vaccination in their reporting on the cases and outbreaks while not repeating misinformation promoted by Kennedy or HHS press releases.

Facts about the measles vaccine

The only way to prevent measles is with the measles-mumps-rubella (MMR) vaccine, which is 93% with one dose and 97% with two doses against measles. The vaccine was tested in placebo-controlled trials and is very safe. The most common side effects are soreness at the injection site, temporary pain or stiffness in the joints and a mild fever.

About four in every 10,000 children who get the vaccine between 12-15 months old will experience a febrile seizure - a seizure caused by a fever - in the 7-10 days after getting the vaccine, but these seizures do not cause any long-term effects. The risk of a febrile seizure is actually higher (9.5 per 10,000) for children who get the vaccine between 16-24 months. A rarer risk is a temporary low platelet count, called immune thrombocytopenic purpura, which can occur in 1 out of 40,000 children receiving the vaccine. The rarest risk is anaphylaxis, estimated to occur in 1 to 2 per 1 million children.

More than a dozen studies have confirmed that there is no link between the MMR vaccine and autism. Only one death has ever been linked to the MMR vaccine in the U.S., a 21-month-old who developed measles inclusion-body encephalitis eight months after vaccination in 1999.

How contagious is measles?

Measles is the most contagious human disease known. It's airborne, and the droplets remain in the air up to two hours after an infectious person leaves an area. At least 90% of susceptible (non-immune) people exposed to the virus will develop measles.

The contagiousness (or transmissibility) of a disease is calculated as the basic reproduction number, expressed as the R-naught (R0). The R0 refers to the number of susceptible people a single individual with the disease will infect during the course of their illness. The number isn't static - it can shift based on multiple factors - but for measles, it's roughly 12-18.

What is the incubation period and infectious period for measles?

People infected with measles show the first symptoms, usually a rash, approximately 10-14 days after exposure to the virus. However, they are contagious starting about four days before the rash appears until about four days after the rash appears.

What is the herd immunity threshold of measles and how does it relate to current vaccination coverage?

The herd immunity threshold is the percentage of residents in a community who must be immune to a disease to prevent its spread through the population. Since measles is so contagious, the herd immunity threshold is high at 95%.

Based on the vaccination coverage of U.S. kindergarteners in the 2023-2024 school year, only 92.7% received both recommended doses of the measles-mumps-rubella (MMR) vaccine, so the average national vaccination rate is below the herd immunity threshold. It's important to remember, however, that the 92.7% represents the average, not the rate in individual states or communities. As seen in this table (or graphically here), rates vary considerably between different states.

In California, for example, which in 2016 eliminated non-medical exemptions for school vaccination requirements, 96.2% of kindergarteners were fully vaccinated against measles. The state with the highest rate of 98.3% is West Virginia, which has never had non-medical exemptions available. On the flip side, only 79.6% of kindergarteners are vaccinated against measles in Idaho, which recently restricted non-school vaccine mandates and previously halted the health department's ability to administer any Covid-19 vaccines.

Even within states, immunization coverage can vary widely, especially in states with looser laws on vaccination requirements. It's the pockets within a state - such as the highly unvaccinated region of Texas where the largest outbreak started - that indicates a community's susceptibility to an outbreak.

What are the complications and death rate of measles?

Common complications include ear infections, diarrhea and pneumonia. More serious complications include encephalitis (in approximately one of every 1,000 cases) or severe pneumonia (the most common cause of death in children with measles). In the U.S., an estimated one to three children per 1,000 with measles will die from complications of the disease during acute infection.

The most serious - and terrifying - complication of measles is subacute sclerosing panencephalitis (SSPE), a rare, progressive neurological disorder that develops when the measles virus invades the brain and nervous system and causes inflammation. The subsequent neurological impairment causes memory loss, irritability, language loss, mood changes, cognitive decline, vision loss, seizures, and eventually coma and death.

SSPE is always fatal with no cure, but symptoms do not present until seven to 10 years after the infection, and deterioration lasts an average one to three years, rarely beyond four years, though at least one case lasted 13 years. SSPE had previously been thought to occur in only five to 10 out of 1 million people who had measles, but the 1989-1992 measles outbreak resulted in a rate of four to 11 cases out of 100,000 cases. Another study in 2016 found a rate of one in 1,367 cases, with an even higher rate (one in 609) among children who had measles before 1 year old. The time to presentation was also longer, an average of 9.5 years.

How does measles harm the immune system?

One feature about measles that was only fairly recently discovered and thought to be unique relative to other viral diseases is that it can cause "immune amnesia." That is, a measles infection can cause the immune system to "forget" how to fight other diseases it's already learned to fight: it destroys many of the immune systems' existing antibodies to other diseases, so it no longer recognizes those pathogens. This phenomenon was first described in a 2015 study in Science, which found that measles can cause an increase in overall childhood infectious disease deaths even among those who initially survive the measles infection because the disease disables immune memory until reinfection or re-vaccination.

Is measles worse for kids or adults?

Measles can be serious at any age, but children under age 5 and adults over age 20 are at greater risk for complications. Others at higher risk include those who are pregnant or immune-compromised. Children under age 2 appear to be at higher risk for the compilation of SSPE.

Does vitamin A treat or prevent measles?

The only way to prevent measles is vaccination. The primary treatment of measles is supportive care, which means treating the symptoms and complications. There is no cure or direct treatment for measles. Vitamin A cannot prevent measles. In fact, cases of vitamin A toxicity have been reported in west Texas as Kennedy has continued inaccurately promoting vitamin A as a way to prevent the disease.

Measles does, however, deplete the body's stores of vitamin A, and the National Foundation for Infectious Diseases does recommend that physicians administer an age-appropriate dose of vitamin A to children hospitalized with measles, but it should not be administered by caregivers without a doctor's recommendation or supervision.

Who needs a booster for measles?

Children have been recommended to get two doses of the MMR vaccine since 1989. Most people born between 1957 and about 1983 likely only received one dose and would be eligible for a booster. People born before 1957 are presumed to have had a childhood infection and be immune. Those who got a vaccine between 1963 and 1967 may have received a less effective vaccine and might want to consider a booster.

Some physicians may advise people to check their titers (antibodies) to find out whether they are immune to measles before getting a booster. However, the MMR vaccine induces two types of immunity, only one of which is antibodies, so titers can be low even in people who are protected. Titers are therefore not necessarily clinically useful for determining whether someone needs a booster.

See this Q&A in Scientific American and this Yale Medicine article for more on who needs a booster.

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