Background
Measles, also known as rubeola, is one of the most contagious infectious diseases, with at least a 90% secondary infection rate in susceptible domestic contacts. It can affect people of all ages, despite being considered primarily a childhood illness. Measles is marked by prodromal fever, cough, coryza, conjunctivitis, and pathognomonic enanthem (ie, Koplik spots), followed by an erythematous maculopapular rash on the third to seventh day. (See the image below.) Infection confers lifelong immunity.
A generalized immunosuppression that follows acute measles frequently predisposes patients to bacterial otitis media and bronchopneumonia. In approximately 0.1% of cases, measles causes acute encephalitis. Subacute sclerosing panencephalitis (SSPE) is a rare chronic degenerative disease that occurs several years after measles infection.
After an effective measles vaccine was introduced in 1963, the incidence of measles decreased significantly. Nevertheless, measles remains a common disease in certain regions and continues to account for nearly 50% of the 1.6 million deaths caused each year by vaccine-preventable childhood diseases. The incidence of measles in the United States and worldwide is increasing, with outbreaks being reported particularly in populations with low vaccination rates. [1]
Maternal antibodies play a significant role in protection against infection in infants younger than 1 year and may interfere with live-attenuated measles vaccination. A single dose of measles vaccine administered to a child older than 12 months induces protective immunity in 95% of recipients. Because measles virus is highly contagious, a 5% susceptible population is sufficient to sustain periodic outbreaks in otherwise highly vaccinated populations.
A second dose of vaccine, which is recommended for all school-aged children in the United States, [2] induces immunity in about 95% of the 5% who do not respond to the first dose. Slight genotypic variation in circulating strains has not affected the protective efficacy of live-attenuated measles vaccines.
Unsubstantiated claims that suggest an association between the measles vaccine and autism have resulted in reduced vaccine use and contributed to a recent resurgence of measles in countries where immunization rates have fallen to below the level needed to maintain herd immunity. [3, 4]
Considering that for industrialized countries, endemic transmission of measles can be reestablished if measles immunity falls to less than 93-95%, efforts to ensure high immunization rates among people in both developed and developing countries must be sustained.
Supportive care is normally all that is required for patients with measles. Vitamin A supplementation during acute measles significantly reduces risks of morbidity and mortality.
Pathophysiology
In temperate areas, the peak incidence of infection occurs during late winter and spring. Infection is transmitted via respiratory droplets, which can remain active and contagious, either airborne or on surfaces, for up to 2 hours. Initial infection and viral replication occur locally in tracheal and bronchial epithelial cells.
After 2-4 days, measles virus infects local lymphatic tissues, perhaps carried by pulmonary macrophages. Following the amplification of measles virus in regional lymph nodes, a predominantly cell-associated viremia disseminates the virus to various organs prior to the appearance of rash.
Measles virus infection causes a generalized immunosuppression marked by decreases in delayed-type hypersensitivity, interleukin (IL)-12 production, and antigen-specific lymphoproliferative responses that persist for weeks to months after the acute infection. Immunosuppression may predispose individuals to secondary opportunistic infections, [5] particularly bronchopneumonia, a major cause of measles-related mortality among younger children.
In patients with deficiencies in cellular immunity, measles virus can cause a progressive and often fatal giant cell pneumonia. [6]
In immunocompetent individuals, wild-type measles virus infection induces an effective immune response, which clears the virus and results in lifelong immunity. [7]
Etiology
The cause of measles is the measles virus, a single-stranded, negative-sense enveloped RNA virus of the genus Morbillivirus within the family Paramyxoviridae. Humans are the natural hosts of the virus; no animal reservoirs are known to exist. This highly contagious virus is spread by coughing and sneezing via close personal contact or direct contact with secretions.
Risk factors for measles virus infection include the following:
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Children with immunodeficiency due to HIV or AIDS, leukemia, alkylating agents, or corticosteroid therapy, regardless of immunization status
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Travel to areas where measles is endemic or contact with travelers to endemic areas
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Infants who lose passive antibody before the age of routine immunization
Risk factors for severe measles and its complications include the following:
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Malnutrition
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Underlying immunodeficiency
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Pregnancy
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Vitamin A deficiency
Epidemiology
United States statistics
Before the first measles vaccine was licensed in 1963, an estimated 3-4 million cases occurred each year in the United States. [8] The immunization program resulted in a decrease of more than 99% in reported incidence.
From 1989-1991, a major resurgence occurred, affecting primarily unvaccinated preschoolers. This measles resurgence resulted in 55,000 cases and 130 deaths [9] and prompted the recommendation that a second dose of measles vaccine be given to preschoolers in a mass vaccination campaign, which led to the effective elimination in the United States of endemic transmission of the measles virus by 2000. [10]
From 2001-2011, an average of 83 measles cases were reported to the US Centers for Disease Control and Prevention (CDC) each year. [11] However, the incidence started to rise after that period, with most cases linked either directly or indirectly to international travel. Incomplete vaccination rates facilitated the spread once the virus was imported into the United States.
In 2019, 1274 measles cases, the highest number of cases in the United States since 1992, were reported in 31 states. All cases were linked to traveler importations that reached at-risk US populations, the majority of whom were unvaccinated or undervaccinated. [12]
The COVID-19 pandemic led to setbacks in immunization efforts. The CDC estimates that measles-mumps-rubella (MMR) vaccine coverage among US kindergarteners has been below the 95% coverage goal since 2020, and the percentages are much lower in some communities. [13] The US-based outbreaks in 2025 have occurred in undervaccinated counties. [8, 14] Young children who are not appropriately vaccinated may experience a more than 60-fold increase in the risk of disease following exposure to measles cases.
International statistics
In developing countries, measles affects 30 million children a year and causes 1 million deaths. Measles causes 15,000-60,000 cases of blindness per year.
Worldwide, most reported cases of measles occur in Africa. [15] In 2019, the Samoan Ministry of Health declared a measles outbreak, the first Pacific island country to take action against the global resurgence of measles. [16]
Following a worldwide decline in measles vaccination coverage during the COVID-19 pandemic, measles-related deaths rose by 43% in 2022, compared with 2021. The number of total reported cases increased by 18% during the same period, accounting for about 9 million cases and 136,000 deaths globally, mostly among children. [17]
According to the World Health Organization (WHO), the total number of measles cases reported for the European region in 2024 was 127,350, which is double the number of cases in 2023 and the highest since 1997. [18]
Age-related demographics
Although measles is historically a disease of childhood, infection can occur in unvaccinated or partially vaccinated individuals of any age or in those with compromised immunity.
Unvaccinated young children are at the highest risk. Age-specific attack rates may be highest in susceptible infants younger than 12 months, school-aged children, or young adults, depending on local immunization practices and incidence of the disease. Complications such as otitis media, bronchopneumonia, laryngotracheobronchitis (ie, croup), and diarrhea are more common in young children.
Among the 285 US patients reported to have measles in 2024, most were aged younger than 20 years: 120 (42%) were younger than 5 years, 88 (31%) were aged 5-19 years, and 77 (27%) were aged 20 years or older. [12]
In heavily populated, underdeveloped countries, measles is most common in children younger than 2 years.
Sex- and race-related demographics
Unvaccinated males and females are equally susceptible to infection by the measles virus. Excess mortality following acute measles has been observed among females at all ages, but it is most marked in adolescents and young adults. Excessive non–measles-related mortality has also been observed among female recipients of high-titer measles vaccines in Senegal, Guinea Bissau, and Haiti. [19]
Measles affects people of all races.
Prognosis
The prognosis for measles is generally good, with infection only occasionally being fatal. The CDC reports that the childhood mortality rate from measles infection in the United States is 0.1-0.2%. However, many complications and sequelae may develop, and measles is a major cause of childhood blindness in developing countries.
Morbidity/mortality
Globally, measles remains one of the leading causes of death in young children. According to the CDC, measles caused an estimated 107,500 deaths worldwide in 2023, and most occurred in children. [20]
Case-fatality rates are higher among children younger than 5 years. The highest fatality rates are among infants aged 4-12 months and in children who are immunocompromised because of human immunodeficiency virus (HIV) infection or other causes.
Complications of measles are more likely to occur in persons younger than 5 years or older than 20 years, and morbidity and mortality are increased in persons with immune deficiency disorders, malnutrition, vitamin A deficiency, and inadequate vaccination.
Croup, encephalitis, and pneumonia are the most common causes of death associated with measles. Measles encephalitis, a rare but serious complication, has a 10% mortality.
Complications
Most complications of measles occur because the measles virus suppresses the host’s immune responses, resulting in a reactivation of latent infections or superinfection by a bacterial pathogen. Consequently, pneumonia, whether due to the measles virus itself, to tuberculosis, or to another bacterial etiology, is the most frequent complication. Pleural effusion, hilar lymphadenopathy, hepatosplenomegaly, hyperesthesia, and paresthesia may also be noted.
Complications of measles are more likely to occur in persons younger than 5 years or older than 20 years, and complication rates are increased in persons with immune deficiency disorders, malnutrition, vitamin A deficiency, and inadequate vaccination. Immunocompromised children and adults are at increased risk for severe infections and superinfections.
Common infectious complications include otitis media, interstitial pneumonitis, [21] bronchopneumonia, laryngotracheobronchitis (ie, croup), exacerbation of tuberculosis, transient loss of hypersensitivity reaction to tuberculin skin test, encephalomyelitis, diarrhea, sinusitis, stomatitis, subclinical hepatitis, lymphadenitis, and keratitis, which can lead to blindness. In fact, measles remains a common cause of blindness in many developing countries.
Rare complications include hemorrhagic measles, purpura fulminans, hepatitis, disseminated intravascular coagulation (DIC), subacute sclerosing panencephalitis (SSPE), thrombocytopenia, appendicitis, ileocolitis, pericarditis, myocarditis, acute pancreatitis, [22] and hypocalcemia. [23] Transient hepatitis may occur during an acute infection.
Approximately 1 of every 1000 patients develops acute encephalitis, which often results in permanent brain damage and is fatal in about 10% of patients. In children with lymphoid malignant diseases, delayed-acute measles encephalitis may develop 1-6 months after the acute infection and is generally fatal.
An even rarer complication is SSPE, a degenerative CNS disease that can result from a persistent measles infection. SSPE is characterized by the onset of behavioral and intellectual deterioration and seizures years after an acute infection (the mean incubation period for SSPE is approximately 10.8 years).
The complications of measles in the pregnant patient include pneumonitis, hepatitis, subacute sclerosing panencephalitis, premature labor, spontaneous abortion, and preterm birth of the fetus. Perinatal transmission rates are low.
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Koplik spots in measles. Photograph courtesy of World Health Organization.
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Enanthem of measles (Koplik spots).
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Measles conjunctivitis.
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Face of boy with measles.
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Morbilliform rash.
