The United States and Canada are experiencing one of the worst measles outbreaks in years, and the virus has spread to neighboring Mexico.

A total of 2,500 infected people and four deaths have been reported in the three countries. The culprit is people's unwillingness to get vaccinated. Misinformation and growing hesitancy have led to vaccination rates falling below the level needed to achieve herd immunity. In some US states and Canadian provinces, there are increasing numbers of exemptions from vaccination on non-medical grounds, such as personal or religious beliefs. This is leading to the emergence of large groups of unvaccinated people who are more vulnerable to epidemics when exposed to the virus.

Measles is a highly contagious viral disease that begins with symptoms such as high fever, cough, runny nose, red or watery eyes, and sensitivity to light. A few days after infection, a red, blotchy rash usually appears on the face and spreads to the body. Fatigue and loss of appetite are also common.

Most people recover within two to three weeks, but complications, especially in young children and people with weakened immune systems, can be serious. These include ear infections, diarrhea, pneumonia, and brain swelling (encephalitis), which can lead to permanent disability or death. In wealthier countries, measles causes about one death for every 5,000 cases, but in regions with lower incomes and weaker health systems, the death rate can be as high as one in 100.

Before the introduction of the measles vaccine in 1963, there were about 3 to 4 million cases of measles reported each year in the US, with hundreds of deaths and thousands of hospitalizations. By 2000, following widespread vaccination, measles was declared "eliminated" in the US, meaning that there had been no sustained transmission of the disease for more than 12 months. Countries that have eliminated a disease through vaccination are considered to have "herd immunity" as long as vaccines continue to be available to the entire population.

Herd immunity occurs when enough people in a community become immune to a disease, either through vaccination or by having had the infection.

The number of people who need to be immunized to achieve herd immunity for the entire community varies depending on the disease. For measles, which spreads very easily, 95% of the community needs to be immune. When most people are immunized, the disease spreads much more slowly, meaning that it is more likely to die out before it can be passed on to another carrier. This helps protect those who cannot be vaccinated for specific reasons, including newborn babies or people with certain health conditions.

Experts say that the safest way to maintain herd immunity is through vaccination, due to the potentially serious consequences of measles infection.

According to the World Health Organization (WHO), measles vaccination prevented an estimated 31.7 million deaths worldwide between 2000 and 2020.

The measles, mumps, and rubella (MMR) vaccine, widely used around the world, provides about 97% protection against measles. It is usually given to children in two stages: the first dose at 12 months and the second at around three years and four months (when the child is approaching preschool age), as part of routine immunization programs in countries such as the United Kingdom. Cases of infection in vaccinated people are rare and usually mild.

North America is experiencing a significant measles resurgence, with more than 2,500 confirmed cases in the US, Canada, and Mexico. Three deaths have been reported in the US and one in Mexico. The outbreak began in late 2024 in Ontario, Canada, before spreading to Texas and northern Mexico in early 2025, with Mennonite communities (Anabaptist Christian communities), which historically have low vaccination coverage, being the most severely affected.

A WHO report from April indicates that measles cases in North and South America in the first three months of this year were 11 times higher than in the same period in 2024. The region is now at "high" risk of measles epidemics, compared to "moderate" risk globally. | BGNES