In the first 5 months of 2024, there were 151 cases of measles reported from a total of 22 U.S. states.¹ By comparison, 58 cases were reported in all of 2023, and 121 cases were reported in 2022.¹ From the 2019–2020 to the 2022–2023 school year, national measles, mumps, and rubella vaccine coverage among children in kindergarten declined slightly from 95% to 93%.² Groups of children with low vaccination rates are more vulnerable to measles outbreaks.³ Hopefully what happened last year in Europe, where there were more than 30,000 cases reported from a total of 40 countries—a 30-fold rise from 2022—will not occur in the United States.⁴

Measles is caused by a Morbillivirus, a double-stranded RNA virus. Transmitted by infectious droplets and aerosolization, it is highly infectious. An enclosed space is considered a potential source of infection for 2 hours after someone with measles has left.

Measles is not a benign illness. Common complications include otitis media, bronchopneumonia, laryngotracheo-bronchitis, and diarrhea. Encephalitis occurs in 1 out of 1,000 people, and 1 out of 500 people with measles will die.⁵ Those at highest risk of serious complications include young children and patients who are immunocompromised or pregnant.⁵ Before the introduction of the measles vaccine in 1963, there were an estimated 3 million to 4 million cases of measles each year, leading to 48,000 hospitalizations, 1,000 cases of encephalitis, and 400 to 500 deaths.⁵

The incubation period for measles is typically 10 to 12 days but can be as long as 21 days. Illness starts with a high fever and the 3 Cs: cough, coryza, and conjunctivitis. After about 4 days, an erythematous, maculopapular rash appears on the face and spreads downward to cover the body. This rash lasts about 6 days. Koplik spots (tiny white spots inside the mouth) may also appear 2 or 3 days after the first symptoms (Figure 1⁶). A person with measles is infectious 4 days before through 4 days after the appearance of the rash.

Most physicians in practice today have never seen a patient with measles. This can lead to missing the diagnosis or confusing it with other illnesses that cause fever and a rash, such as rubella, Rocky Mountain spotted fever, fifth disease, and scarlet fever. The clinical characteristics of measles are shown on the Centers for Disease Control and Prevention’s website.

In addition to delayed diagnosis, medical errors can lead to the spread of the infection in the community. Medical settings are a common source of measles exposures during an outbreak. If a family physician suspects a patient has measles, the following steps should be taken immediately:

• Place a mask on the patient and keep them isolated in an examination room, preferably one with negative pressure. A patient with a rash and fever should never wait in a common waiting area.

• Collect samples for confirmation, including blood for measles-specific immunoglobulin M and a nasopharyngeal swab for reverse transcriptase–polymerase chain reaction testing.⁷

• Contact the local health department to report the suspected case and to obtain information on how to process the samples.

• Inform patients with suspected measles that they need to self-isolate while awaiting confirmation. They will likely be contacted by the local health department for investigating potential contacts.

To avoid spreading measles in the clinical setting, all personnel should be fully vaccinated, and infection control practices should be followed. Protocols should be in place to separate patients with a rash illness from others, physically and temporally. The measles vaccine is safe and effective, and family physicians should encourage all children older than 12 months and unvaccinated adolescents and adults to receive two doses of the measles, mumps, and rubella vaccine, as recommended by the Centers for Disease Control and Prevention.⁸ The American Academy of Family Physicians does not support nonmedical exemptions from vaccination requirements for school entry.⁹

Measles is preventable with widespread use of the safe and effective vaccine. Unfortunately, a summary of the clinical and public health aspects of measles I wrote in American Family Physician in 1990 is still pertinent today.¹⁰