A recent Pan American Health Organization (PAHO) epidemiological update (March 18) focused on what may be a resurgence of measles in the Americas. Measles was declared eliminated in the Americas in 2016, the first region in the world to do so, but imported cases and outbreaks still occur throughout the region. WHO officials also recently expressed concern over a fourfold increase in measles cases in Europe from 2016 to 2017, owing largely to low vaccination rates in many countries across the continent. Despite the successful implementation of vaccination programs around the world, measles remains a global health threat, and growing anti-vaccine sentiments worldwide threaten a re-emergence of the disease in areas previously thought to be a relatively low risk.

What is Measles?

As we have discussed in previous Outbreak Thursday posts, measles is a highly contagious respiratory virus. The US CDC estimates that infectious cases can transmit measles to 90% of close contacts who are not already immune, and the virus can remain viable on surfaces or in the air for as long as 2 hours. Symptoms include high fever, cough, runny nose, and red/watery eyes and typically present 7-14 days after exposure. Complications range from ear infections in approximately 10% of children that can lead to permanent hearing loss to potentially life-threatening pneumonia in 5% of children and encephalitis in 1 out of every 1,000 children that can lead to deafness and intellectual disabilities. Approximately 1-2 out of every 1,000 children with measles dies.

The first measles vaccine was authorized for use in the US in 1963, and modern vaccines are estimated to be 93% effective (97% effective with 2 doses). The WHO estimates that measles caused 2.6 million deaths annually before the vaccine, which has fallen to 89,780 deaths in 2016, the first year that the global total was below 100,000. Even with this high efficacy, extremely high vaccination coverage is needed to achieve herd immunity. The US Department of Health and Human Services’ Healthy People 2020 objective is to maintain greater than 90% coverage for the first dose of the vaccine by age 35 months, and PAHO/WHO’s measles elimination strategy calls for maintaining 95% coverage for the second dose by age 4.

A Re-emerging Threat

The PAHO report notes that 4 countries in the Americas reported confirmed cases of measles in all of 2017, but 9 countries reported cases in just the first 3 months of 2018. The ongoing outbreak in Venezuela is by far the most serious in the region (727 cases in 2016 and 159 as of the March 16, 2018 report), and cases of measles have been confirmed in Venezuelan nationals in several other South American countries. Peru was the only other country in the region with measles cases not linked to either international travel (including Europe) or Venezuela.

The US has also experienced several notable measles-related health events over the past several years. Recently reported measles cases in San Francisco and New Orleans have been linked to individuals returning or arriving from Europe. In addition, several infectious travelers passing through major airports in Detroit, Michigan and New York recently caused concern that transmission at major travel hubs could result in outbreaks across the country. Several major outbreaks have also occurred recently in the United States—in 2014 (Amish communities in Ohio) and 2015 (liked to Disneyland in California).

During 2017, measles outbreaks exceeding 100 cases were reported in more than 25% of European countries, with Romania (5,562 total cases), Italy (5,006), and Ukraine (4,767) topping the list. Measles cases in Europe have waxed and waned over the past several years, but generally trended downward, so after a record low of 5,273 cases across the continent in 2016, the jump in 2017 probably was not unexpected. An increase of more than 300% in a single year, however, is worrisome, especially considering the recent importation of measles cases to the Americas by individuals originating from European countries.

The WHO maintains that low immunization coverage is leading to these sharp spikes in the number of measles cases being seen across Europe, and the reason for sub-optimal vaccination against measles in Europe may be multi-faceted. In Romania, which has been battling one of the worst measles outbreaks the country has seen in decades, it has been reported that vaccine shortages and limited healthcare infrastructure may be limiting measles vaccination efforts. In Italy, low vaccination coverage may be a result of anti-vaccination campaigns, which became a key issue in recent national elections following the passage of a law that made 12 vaccinations—including measles—compulsory for school children.


As Outbreak Observatory has addressed in previous posts, increasing global travel inherently increases the risk of the global spread of infectious disease. Dr. Peter Hotez, a world renowned infectious diseases expert, has posted several tweets lately warning the US and other countries to expect measles outbreaks as a result of rising incidence in Europe and the Americas, particularly for areas with low vaccination coverage. The measles vaccine has substantially decreased the global burden, but lapses in attention and active efforts on the part of anti-vaccine campaigns threaten this progress. And as we have seen with imported cases to the Americas, entire regions can effectively eliminate the disease, but the threat remains as long as measles remains endemic or causes outbreaks elsewhere in the world.


Photo: A young girl who suffered a severe corneal ulceration as a complication of measles.

Photo courtesy of CDC/ Paul Chenoweth

Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly to allow others to learn from these experiences in order to improve global outbreak response capabilities.