Measles Cases Surge in the Americas: Why Vaccination Matters More Than Ever

What Happened to Measles Elimination in the Americas?

The Region of the Americas achieved a historic milestone in September 2016 when PAHO declared it the first WHO region to have eliminated endemic measles transmission. This accomplishment represented decades of coordinated vaccination campaigns, surveillance infrastructure, and political commitment across 35 member states.

However, the foundation of this achievement began eroding well before the COVID-19 pandemic. Vaccination coverage with the first dose of measles-containing vaccine in the Americas dropped from 92% in 2019 to 85% in 2021, and while it partially recovered to 88% by 2024, it remained well below the 95% threshold needed for herd immunity against measles. The second dose coverage, equally critical, fell to just 81% across the region.

The consequences were predictable and devastating. From fewer than 100 confirmed cases in 2022, the Americas recorded over 4,300 confirmed measles cases by the end of 2025, a 43-fold increase. Brazil, Colombia, the United States, and Venezuela were among the hardest-hit countries, with clusters also reported in Argentina, Canada, and Mexico.

Why Is Measles Surging Now?

The COVID-19 pandemic created what UNICEF has called the "largest sustained backslide in childhood immunization in 30 years." Lockdowns, overwhelmed health systems, and supply chain disruptions meant that an estimated 25 million children worldwide missed routine vaccinations in 2021 alone. In the Americas, the disruption created a growing cohort of susceptible children, the so-called "immunity gap."

Compounding this, vaccine hesitancy has risen significantly in several countries. According to PAHO data, misinformation about vaccine safety, fueled by social media platforms, has contributed to parents delaying or refusing measles vaccination for their children. In the United States, non-medical exemptions for school vaccination requirements have increased in 40 states over the past five years.

International travel serves as the third critical factor. Measles is one of the most contagious diseases known, with a basic reproduction number (R0) of 12-18, meaning one infected person can transmit the virus to 12-18 susceptible individuals. The virus can remain airborne in an enclosed space for up to two hours after an infected person has left. Even small pockets of under-vaccination become vulnerable when the virus is reintroduced by travelers from endemic regions.

Who Is Most at Risk?

PAHO surveillance data reveals a clear pattern: the overwhelming majority of measles cases, 78%, occurred in individuals who had received no measles vaccination. An additional 14% had received only one dose of vaccine, providing approximately 93% protection but falling short of the 97% protection conferred by the full two-dose series. Only 3% of confirmed cases occurred in individuals who had received both recommended doses.

Children under 5 years of age account for the largest proportion of cases and the highest rate of complications. In this age group, measles can lead to pneumonia (the leading cause of measles-related death), encephalitis (brain inflammation), and severe dehydration from diarrhea. The case fatality rate for measles in children under 5 in low-resource settings can reach 3-6%.

Pregnant women who contract measles face increased risks of miscarriage, premature delivery, and low birth weight. Immunocompromised individuals, including those receiving chemotherapy, organ transplant recipients, and people living with untreated HIV, are unable to mount an adequate immune response even if previously vaccinated and may develop severe or atypical measles.

What Are the Symptoms of Measles?

Measles follows a characteristic clinical course. The incubation period is typically 10-14 days from exposure to the onset of symptoms. The illness progresses through several stages:

• Prodromal phase (2-4 days): High fever (often exceeding 40 degrees C / 104 degrees F), cough, coryza (runny nose), and conjunctivitis (red, watery eyes). Koplik spots, pathognomonic small white spots on the buccal mucosa, may appear 1-2 days before the rash.
• Exanthem phase (5-6 days): The characteristic maculopapular rash appears, typically starting on the face and behind the ears, then spreading to the trunk and extremities. Fever often peaks during rash onset, sometimes reaching 40.5 degrees C (105 degrees F).
• Recovery phase: The rash fades in the order it appeared, often leaving a brownish discoloration. Fever typically resolves 3-4 days after rash onset. Cough may persist for 1-2 weeks.

A person with measles is contagious from approximately 4 days before rash onset to 4 days after, making early transmission likely before the disease is recognized.

How Can You Protect Yourself and Your Family?

Vaccination remains the cornerstone of measles prevention. The measles, mumps, and rubella (MMR) vaccine has been used safely for over 50 years, with more than 4.2 billion doses administered worldwide. The vaccine's safety and efficacy have been confirmed by extensive research, including large-scale studies involving hundreds of thousands of participants.

The recommended vaccination schedule calls for two doses: the first at 12-15 months of age and the second at 4-6 years. This two-dose regimen provides approximately 97% lifelong protection against measles. For adults born after 1957 who lack evidence of immunity (documented vaccination, laboratory evidence of immunity, or healthcare-provider-diagnosed measles), at least one dose of MMR vaccine is recommended.

During outbreak situations, additional recommendations may apply. Infants as young as 6 months may receive an early dose of MMR if traveling to an area with active measles transmission, though this dose does not count toward the routine two-dose series. Healthcare workers, international travelers, and college students should ensure they have received two doses.