Measles is caused by the measles virus (MeV), which belongs to the family Paramyxoviridae. MeV is a single-stranded RNA virus with a spherical shape coated with an envelope composed of structural proteins. It contains eight proteins, six structural, including hemagglutinin, fusion, nucleocapsid, matrix, phosphoprotein, and large protein. The hemagglutinin protein binds the virus to the host cell receptor (CD46). The fusion protein facilitates the viral envelope’s attachment to the host cell membrane and their neighboring cell, forming a multinucleated cell (3). Initially, the virus inhaled from airborne droplets or fine aerosols first targets dendritic cells, lymphocytes, and alveolar macrophages in the respiratory tract of a susceptible host. The virus initially reproduces in the epithelial cells of the respiratory tract, followed by dissemination to the local and other lymphatic sites through the bloodstream (4).

The incubation period for measles ranges from 7 to 21 days, with an average of 13 days (5). Measles infection occurs in two phases: prodromal and exanthematous. The prodromal phase begins 10 to 12 days after exposure, presenting with fever above 101°F, malaise, anorexia, coryza, conjunctivitis, cough, and Koplik’s spots (small white lesions on the buccal mucosa). Koplik’s spots appear in 60–70% of patients and typically last 12–72 h. The exanthematous phase follows, characterized by a red, bumpy rash that gradually coalesces and then turns coppery brown before scaling. The rash appears in a craniocaudal pattern, progressing and disappearing similarly (4, 6).

Differentiating measles from other conditions like rubella, roseola, chickenpox, erythema infectiosum (parvovirus B19) and scarlet fever is crucial. Diagnosis is confirmed by detecting IgM antibodies in serum or plasma specific to the measles virus. These antibodies are a susceptible and specific test for detecting measles and typically peak four days after the onset of the rash (7). Antigen-specific IgG antibody synthesis occurs swiftly after IgM production and typically lasts for an extended time, often throughout the individual’s life. As a result, serological detection of IgM or IgM plus IgG suggests current contact with the pathogen. Still, just IgG indicates infection at an uncertain time in the past. Because most persons in the United States have been immunized and are immune to measles, IgM is typically found in isolated cases. Without contact with a known case, detecting IgM alone is likely to be false positive reactivity rather than actual measles infection (8). RT-PCR (reverse transcriptase polymerase chain reaction) is also helpful for detecting the virus in blood, urine, nasopharyngeal, throat, or nasal samples before the appearance of IgM antibodies in serum. Moreover, it enables the genotyping of the measles virus, which is valuable for monitoring the importation and transmission of the virus (4, 7).

There is no specific antiviral treatment for measles; however, the MMR (measles, mumps, and rubella) vaccine remains the most effective preventive measure. The recommended age for children for vaccination is 12 months to 12 years. Post-exposure prophylaxis (PEP) with the MMR vaccine is strongly recommended for individuals with unknown vaccination status or those known to be unvaccinated. The window period for PEP is 72 h (9).

Inadequate vaccination is a primary cause of measles outbreaks in the United States. To combat this, it is crucial to ensure over 95% vaccination coverage with both doses of the measles vaccine, including catch-up vaccinations for those who missed earlier doses. Travelers entering and leaving the U.S. should be up-to-date with MMR vaccinations. Not vaccinated travelers should reconsider travel to regions with endemic measles or implement extra preventive measures when undertaking such travel. High-quality surveillance is necessary to quickly identify, report, and isolate potential cases in environmentally isolated rooms until the infectious period ends. Healthcare workers treating suspected measles cases are at an increased risk of infection. Therefore, using N-95 respirators, gloves, and pre-exposure prophylaxis is recommended.

The following recommendations are advised for patients exposed to measles without immunity based on clinical circumstances: In pediatric patients, immune globulin should be administered to infants aged 0–5 months within six days of exposure to measles. For infants aged 6–11 months, either immune globulin within six days or the measles vaccine within 72 h is recommended. Unvaccinated children over 12 months should receive the measles vaccine within 72 h of exposure, with the vaccine being preferred over immune globulin (10).

Pregnant women without immunity should receive immune globulin since the MMR vaccine is contraindicated during pregnancy. Immunocompromised patients should be administered immune globulin regardless of their immune or vaccination status. Healthcare workers who are unvaccinated and without laboratory evidence of immunity or confirmed disease should receive two doses of the measles vaccine. In cases of exposure, healthcare workers without immunity should be isolated from the workplace from day 5 to day 21 post-exposure to prevent the spread of the disease (11).

Public awareness should be raised about the potential complications of measles and the benefits of vaccination, particularly to counteract misinformation that incorrectly links the MMR vaccine to autism (12).

Measles cases have dramatically increased recently, with over 500,000 cases reported to the World Health Organization (WHO) in 2019 (13). Beginning in late 2018, two closely related outbreaks were reported within Orthodox Jewish communities in New York City (14). These outbreaks highlighted the ongoing risk posed by under-vaccinated communities and raised concerns about the United States’ measles elimination status.

From 2021 to 2022, 170 measles cases were reported, with 133 cases (78%) linked to distinct outbreaks. Of these, 47 cases (96%) in 2021 occurred among Afghan evacuees temporarily housed at U.S. military bases during Operation Allies Welcome. In 2022, 86 cases (71%) were associated with an outbreak in central Ohio (2). However, only a single outbreak, reported in Chicago in 2024, has been directly linked to a migrant facility at the U.S. southern border (15).

These events emphasize the potential for isolated importations contributing to localized outbreaks in vulnerable settings.

From January 1, 2020, to March 28, 2024, 338 measles cases were reported in the U.S., with 97 cases during the most recent outbreak in the first quarter of 2024 (Figure 1). Nearly all these cases involved individuals who were either unvaccinated or had unknown vaccination statuses. Despite these outbreaks, as of the end of 2023, the U.S. maintained its measles elimination status. The global rise in measles incidence and decreasing vaccination coverage have heightened the risk of importation into U.S. communities, as observed in the first quarter of 2024 (16).

As of November 21, 2024, a total of 280 measles cases had been reported in 32 states: Arizona, California, District of Columbia, Florida, Georgia, Idaho, Illinois, Indiana, Louisiana, Maryland, Massachusetts, Michigan, Minnesota, Missouri, New Hampshire, New Jersey, New Mexico, New York City, New York State, North Carolina, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, South Dakota (17).

In 2024, 16 measles outbreaks are reported, accounting for 70% (197 of 280) of all cases this year. This represents a huge increase from 2023, when just 4 outbreaks were reported, with 49% (29 out of 59) of outbreaks-related cases (Figure 2). Similar to previous years, most cases in 2024 occurred among unvaccinated individuals under 20 years old, shelter residents, or those linked to importations (2).

Managing measles outbreaks in the U.S. faces several key challenges. Vaccine hesitancy, often fueled by misinformation about vaccine safety, has made specific communities particularly vulnerable to outbreaks. The COVID-19 pandemic caused significant disruptions in global immunization and disease surveillance efforts. With immunization services being suspended and vaccination rates dropping worldwide, millions of children were left vulnerable to preventable diseases, such as measles. These setbacks have increased the risk of outbreaks and undermined progress in controlling vaccine-preventable illnesses. Additionally, international travel continues introducing measles cases into the U.S., particularly from regions where the disease remains endemic. This global resurgence and the strain on public health resources needed to contain outbreaks complicate efforts to manage the disease. Legal and policy barriers, such as non-medical exemptions for vaccinations in some states, hinder efforts to maintain high vaccination rates (18).

In addressing these challenges, it is essential to prioritize sensitive and robust disease surveillance systems capable of detecting and responding to cases promptly. Effective communication with public health authorities is critical to facilitate timely case investigations, identify potential transmission chains, and implement appropriate interventions. Additionally, it requires a comprehensive approach, including public education, stricter vaccination policies, and improved healthcare access to prevent and quickly contain outbreaks (19).

Measles remains a global health problem, especially in Europe and developing countries. Nearly 95,000 measles cases were reported worldwide in 2024, with projections indicating the total number could match or exceed the previous year’s figures. On January 19, 2024, the UK Health and Security Agency declared a national incident in response to the escalating measles cases nationwide, cautioning that the disease could further spread. Despite the UK achieving measles elimination status in 2021, since October 2023, there have been 216 confirmed cases and 103 probable cases, primarily affecting children under 10 in the West Midlands. Between January and October 2023, over 30,000 cases were reported in the WHO European region. Outbreaks are also ongoing in Yemen (25,216 cases), India (14,927 cases), Kazakhstan (12,985 cases), and Ethiopia (11,227 cases) with provisional data from early December 2023 indicating thousands of cases in each of these countries (20, 21).