The United States and Canada are experiencing one of their worst measles outbreaks in years, while the virus has also crossed to neighbouring Mexico.

A total of 2,500 cases and four deaths have been reported in the outbreaks across the three countries. Vaccine hesitancy has been blamed.

Here is what we know about the outbreaks so far and why measles cases are on the rise.

What is measles?

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

Most people recover within two to three weeks but complications, particularly in young children and those with weakened immune systems, can be serious.

These include ear infections, diarrhoea, pneumonia and brain swelling (encephalitis) which can lead to permanent disability or death. In wealthier countries, measles causes about one death in every 5,000 cases, but in lower-income regions with weaker healthcare systems, the fatality rate can be as high as one in 100.

Before the measles vaccine was introduced in 1963, about 3 to 4 million cases of measles were reported annually in the US, with hundreds of deaths and thousands of hospitalisations. By 2000, following widespread vaccination, measles was declared “eliminated” in the US – meaning there had been no continuous transmission of the disease for more than 12 months. Countries that have eliminated a disease through vaccines are considered to have “herd immunity” as long as vaccines continue to be offered to everyone.

What is herd immunity?

Herd immunity occurs when enough people in a community become immune to a disease, either through vaccination or by recovering from an infection.

The number of people who need to be immune in order to achieve herd immunity for the whole community varies from disease to disease. Measles, which spreads very easily, requires 95 percent of a community to have immunity.

When most people are immune, the disease has a much harder time spreading – meaning it is more likely to die before it can be transmitted to another host. This helps to protect those who cannot be vaccinated for specific reasons, including newborn babies or people with certain health conditions.

Experts say the safest way to maintain herd immunity is through vaccination due to the potentially severe effects of a measles infection.

How effective is the measles vaccine?

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

The measles, mumps and rubella (MMR) vaccine, widely used around the world, provides about 97 percent protection against the measles virus, according to public health agencies.

The vaccine is normally given to children in two stages: The first dose at 12 months and the second at about three years and four months of age (when the child is nearing pre-school age), as part of routine immunisation programmes in countries like the United Kingdom.

Breakthrough cases in vaccinated people are rare and usually mild.

North America is experiencing a significant measles resurgence, with more than 2,500 confirmed cases across the US, Canada and Mexico. Three deaths have occurred 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 congregations), a historically under-vaccinated population, being the hardest hit.

A WHO report in April stated that measles cases in the Americas during the first three months of this year were 11 times higher than during the same period in 2024. The region now faces a “high” risk of measles outbreaks, compared with a “moderate” risk globally, the WHO said.

United States

The US has so far reported 935 confirmed measles cases across 30 states, with Texas being the epicentre of the outbreak, accounting for 683 of those cases. The outbreak has now also spread to New Mexico (67 cases), Oklahoma (16) and Kansas (37).

Three people died from measles between February and April, including two unvaccinated children in Texas and one unvaccinated adult in New Mexico. These mark the country’s first measles deaths since 2015.

In 2019, the US experienced its most significant measles outbreak since 1992, with 1,274 confirmed cases reported across 31 states, but no deaths.

Canada

Meanwhile, Canada is experiencing its worst measles outbreak in more than 25 years. Since mid-October 2024, Ontario alone has recorded more than 1,000 cases.

Canadian officials confirmed that the outbreak had originated from a large gathering in New Brunswick province that involved Mennonite communities, which strive for a simple life and tend to avoid modern technology and medicine.

Mexico

In Mexico, the number of measles cases has risen sharply this year in the northern state of Chihuahua, where 786 cases have been reported so far. Just seven cases were reported in the whole country last year. A 31-year-old, unvaccinated man from Ascension, Chihuahua, died from the disease in early April.

Why is measles spreading?

According to the WHO and the US Centers for Disease Control and Prevention (CDC), the main reason measles is spreading is that fewer people are taking up vaccines. Misinformation and growing hesitancy have seen vaccination rates dropping below the level needed for herd immunity.

In some US states and Canadian provinces, exemptions – giving non-medical reasons for declining vaccines, such as for personal or religious reasons – are becoming more common. This has led to the existence of clusters of unvaccinated people who are more vulnerable to outbreaks when exposed to the virus.

The US CDC reported that during the 2023-2024 school year, exemptions from one or more vaccines were claimed by parents or carers of 3.3 percent of preschool children, compared with 3 percent the previous year. Exemptions were higher in 40 states and the District of Columbia, with 14 states reporting exemption rates exceeding 5 percent.

Health officials point to how closely connected the affected communities in the US, Canada and Mexico are, with people frequently travelling between them. Mennonite groups, in particular, have strong cross-border ties and lower vaccination rates, making it much easier for the virus to spread from one region to another.

Who are the ‘anti-vaxxers’ and what are their concerns?

Anti-vaccine advocates, often referred to as “anti-vaxxers”, oppose vaccinations for a number of reasons, including fears about side effects, distrust of pharmaceutical companies or government agencies, and religious or personal beliefs.

Mennonite groups in Ontario and Chihuahua have also historically been hesitant about taking up vaccines, and health authorities say a significant share of recent measles cases in these areas are concentrated within this population.

One of the most persistent myths driving vaccine hesitancy is the claim that vaccines cause autism. This belief stems from a 1998 study that has since been discredited and retracted.

Will Trump’s pick of RFK Jr as health secretary push vaccination levels down further?

Robert F Kennedy Jr’s appointment as US Secretary of Health and Human Services prompted concern among health experts, particularly as measles cases are on the rise across North America.

Kennedy, a longstanding vaccine sceptic, has repeated false claims that vaccines are linked to autism and that the MMR vaccine contains “aborted fetus debris”.

Health experts have debunked this claim. The rubella part of the vaccine was made using lab-grown cells which were copied from a single foetal sample taken in the 1960s. These cells have been multiplied in labs for decades and are used to grow the virus needed for the vaccine. But the cells themselves are removed during the process, so there is no fetal tissue in the actual shot.

Kennedy has been criticised for downplaying the severity of the current measles outbreak, and for promoting unproven treatments for measles. He has suggested alternatives including vitamin A supplements and cod liver oil, which experts say do not replace vaccination and are not backed by strong scientific evidence.

Kennedy has also made major changes to the Department of Health and Human Services, firing thousands of employees and cutting budgets for key agencies like the CDC, which experts say could weaken the country’s ability to respond to health emergencies.

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