Omar Bennett
Measles is a highly contagious illness that can easily cross borders. In 2023, an estimated 10.3 million people were infected with measles globally. Measles outbreaks can result in severe complications and deaths, especially among young, malnourished children. The risk of measles outbreaks is particularly high among refugees and unvaccinated individuals. In the United States, measles was officially eliminated in 2000, meaning there was no endemic measles transmission within the country. However, outbreaks can still occur due to unvaccinated international travelers. An outbreak is typically defined as three or more related cases, and in 2025, there have been several outbreaks reported across multiple states.
| Characteristics | Values |
|---|---|
| Number of cases that constitute an outbreak | 3 or more related cases |
| Number of cases that constitute an outbreak (WHO) | 5 or more reported suspected cases in one month per 100,000 population |
| Number of cases that constitute a confirmed outbreak | 3 or more confirmed cases, 2 of which should be lab-confirmed |
| Number of cases that constitute an outbreak (in absence of lab confirmation) | Sustained and progressive rise in clinically confirmed cases over a 3-week period |
| Number of cases that constitute an outbreak (in absence of lab confirmation) | Hospitalization of a proportion of suspected cases due to measles-related complications |
| Number of cases that constitute an outbreak (WHO) | 2 or more lab-confirmed cases that are temporally related (with dates of rash onset occurring 7-21 days apart) and epidemiologically or virologically linked |
| Number of cases in Texas outbreak (May 13, 2025) | 717 |
| Number of fatalities in Texas outbreak (May 13, 2025) | 2 |
| Number of hospitalizations in Texas outbreak (May 13, 2025) | 93 |
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What You'll Learn
A suspected outbreak is five or more suspected cases per 100,000 people in a month
The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) are currently working to control measles outbreaks in the United States and globally. Measles is a highly contagious airborne viral disease that can lead to severe complications and even death. It is important to note that the definition of a measles outbreak can vary depending on the context and phase of measles control.
In the context of a measles mortality reduction setting, a suspected outbreak is defined as the occurrence of five or more reported suspected cases of measles in a month per 100,000 population in a specific geographical area. This specific geographical area could refer to the catchment area of a health centre or a sector of a large city. This definition serves as a general guideline, and an assessment of the situation should be conducted on a case-by-case basis to determine if an outbreak is suspected or confirmed.
The presence of measles-specific IgM antibodies is the recommended method for diagnosing measles and confirming suspected outbreaks. A suspected outbreak can also be documented through a sustained and progressive rise in clinically confirmed cases over a short period, such as three weeks. Additionally, hospitalization of a proportion of suspected cases due to measles-related complications can help distinguish measles outbreaks from other infections with similar symptoms.
In the United States, measles was declared eliminated in 2000, but imported cases continue to be detected due to unvaccinated international travelers. In 2023, the vaccination coverage rate for two doses of the measles, mumps, and rubella (MMR) vaccine among kindergarten-aged children in the US was 92.7%. However, pockets of unvaccinated individuals can exist even in states with high vaccination coverage, and outbreaks can occur when measles reaches these communities.
To prevent and control measles outbreaks, it is crucial to maintain high vaccination coverage. The best way to protect oneself and others is to ensure immunization with two doses of the MMR vaccine, which is highly effective at preventing measles.
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Confirmed outbreak: three or more confirmed cases in a month
A measles outbreak is a serious public health concern. The World Health Organization (WHO) defines a confirmed measles outbreak as the occurrence of three or more confirmed measles cases in a health facility, district, or block (with a catchment population of approximately 100,000) within a month. At least two of these cases should be laboratory-confirmed, with the presence of measles-specific IgM antibodies, and the dates of rash onset occurring 7–21 days apart.
The highly contagious nature of measles means that it can spread rapidly within communities, especially those with low immunization rates or pockets of unvaccinated individuals. This is particularly true during times of high travel or when unvaccinated persons are in close quarters, such as summer camps or schools. As a result, localized outbreaks can occur within larger communities, and the disease can easily cross borders.
The recommended method for measles diagnosis is the detection of measles-specific IgM antibodies. A clinically confirmed case of measles is one where an individual meets the clinical case definition of suspected measles and cannot be discarded. Laboratory confirmation involves meeting the clinical case definition and having laboratory confirmation of measles virus infection. Epidemiologically confirmed cases are those that meet the clinical case definition and are linked epidemiologically to either a laboratory-confirmed case or another epidemiologically confirmed case.
During a measles outbreak, most cases are clinically confirmed. However, if laboratory confirmation is not possible, an outbreak may be documented through a sustained and progressive rise in clinically confirmed cases over a three-week period. Additionally, the hospitalization of a proportion of suspected cases due to measles-related complications, such as diarrhea or respiratory infections, can help distinguish measles outbreaks from other infections with fever and rash.
The ongoing measles outbreak in Texas, primarily in West Texas, as of May 13, 2025, has resulted in 717 confirmed cases, with 93 hospitalizations and two fatalities in unvaccinated school-aged children. This outbreak has been managed through the identification of designated outbreak counties, ongoing investigations, and the promotion of measles vaccination, such as the combination measles-mumps-rubella (MMR) vaccine.
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Two or more lab-confirmed cases are an outbreak
The World Health Organization (WHO) defines a measles outbreak as the occurrence of three or more confirmed measles cases, with at least two of them being laboratory-confirmed (IgM positive). This is in the context of a health facility, district, or block with an approximate catchment population of 100,000 within a month. The definition of an outbreak may vary depending on the phase of measles control and the geographical location. For instance, in measles-endemic regions, localized outbreaks can occur within larger communities, and each local community may experience outbreaks every 3 to 5 years.
The presence of measles-specific IgM antibodies is the recommended method for diagnosing a measles outbreak. A suspected case of measles that has been confirmed positive by testing in a proficient laboratory and where vaccine-associated illness has been ruled out is considered a laboratory-confirmed case. Epidemiologically confirmed cases are those that meet the clinical case definition and are linked epidemiologically to laboratory-confirmed cases or other epidemiologically confirmed cases.
When an epidemic occurs, most cases are clinically confirmed. A clinically confirmed case of measles is when an individual meets the clinical case definition of suspected measles and cannot be discarded. A suspected outbreak of measles is defined as the occurrence of five or more reported suspected cases of measles in one month per 100,000 people living in a specific geographical area.
If laboratory confirmation is not possible, an outbreak may be documented through a sustained and progressive rise in clinically confirmed cases over a three-week period. Hospitalization of a proportion of suspected cases due to measles-related complications, such as diarrhea or respiratory infections, can also help distinguish measles outbreaks from other infections with similar symptoms.
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Unvaccinated people can cause outbreaks
Measles is a highly contagious airborne viral disease that can lead to severe complications and death. It is one of the world's most contagious diseases, and it can easily cross borders. In 2023, an estimated 10.3 million people were infected with measles, and there were approximately 107,500 measles-related deaths, mostly among unvaccinated or under-vaccinated children under five years old.
Measles is a vaccine-preventable disease. However, when measles gets into communities of unvaccinated people, outbreaks can occur. This is because measles is highly contagious and can be transmitted through the air or via droplets from the nose, mouth, or throat of infected persons. The virus can remain active and contagious in the air or on infected surfaces for up to two hours after an infected person coughs or sneezes. In a completely susceptible population, one person with measles can infect an average of 12 to 18 other people, with some sources stating that this number could be as high as nine out of ten close contacts.
In the United States, measles was officially eliminated in 2000, meaning there was no measles spreading within the country, and new cases were only found when someone contracted measles abroad and returned. However, due to falling vaccination rates, misinformation, and the importation of measles cases from other countries, outbreaks have occurred in recent years. In 2019, the U.S. nearly lost its elimination status with almost 1,300 measles cases from a large outbreak in New York and cases in 30 additional states. In 2025, an ongoing outbreak of measles was reported in the United States, with 378 cases reported across 17 states as of March 20, including two deaths. The majority of cases were among children who were unvaccinated or had an unknown vaccination status.
Unvaccinated individuals can cause outbreaks of measles due to the highly contagious nature of the disease. When a community has a high proportion of unvaccinated individuals, the risk of measles outbreaks increases significantly. This is because there is a larger pool of susceptible individuals who can be infected and contribute to the spread of the disease. Therefore, it is essential to maintain high vaccination rates to prevent measles outbreaks and protect vulnerable members of the community who cannot be vaccinated.
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Outbreaks can be documented without lab confirmation
An outbreak of measles is defined as three or more related cases. Measles is a highly contagious airborne viral disease that can lead to severe complications and death. It is so contagious that it can easily cross borders. In 2023, an estimated 10.3 million people were infected with measles globally.
Outbreaks of measles can be documented without laboratory confirmation. When a measles outbreak is suspected, the following steps should be taken to reinforce routine vaccination: district staff, health facility staff, and partners should rapidly identify priority areas within the affected district, such as communities with low vaccination coverage and a high risk of morbidity and mortality. They should then jointly work on strengthening the available district immunization work plans, locating health centres that may need additional staff or vaccine supplies, and organizing corrective measures such as additional outreach services to communities with a high proportion of unreached children.
Response to a measles outbreak needs to be developed fast and aggressively. The type of outbreak response varies depending on several factors, including the level of susceptibility in the population, the risk of spread and complications, and the existing health service infrastructure. Districts should aggregate data on measles cases and deaths and send weekly reports to the next administrative level, such as the province. The data should include, at a minimum, information on age group, vaccination status, and the date of rash onset.
Once an outbreak has been laboratory-confirmed, block and district staff must immediately notify the next administrative level. The immediate notification report should include information on the number of cases and deaths by age group, vaccination status, and date of rash onset, the geographical location of the outbreak, and the activities planned to investigate and manage the outbreak. In addition, any supplies needed and technical support should be sought at this time.
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Frequently asked questions
According to the World Health Organization, an outbreak is defined as five or more reported suspected cases of measles in a month per 100,000 population in a specific geographical area. A confirmed outbreak is defined as three or more confirmed cases of measles, two of which should be laboratory-confirmed.
A suspected case of measles is defined as any person meeting the clinical case definition of suspected measles and cannot be discarded.
The clinical case definition of suspected measles is an illness in a patient with fever and generalized maculopapular (non-vesicular) rash.
If laboratory confirmation is not possible, an outbreak can be documented through a sustained and progressive rise in clinically confirmed cases over a three-week period. Hospitalization of a proportion of suspected cases due to measles-related complications can also be used to distinguish measles from outbreaks of other infections.
The best way to prevent getting sick during a measles outbreak is to be immunized with two doses of a measles vaccine, such as the combination measles-mumps-rubella (MMR) vaccine.
