A measles milestone is giving public health officials throughout the world renewed hope that one of the most infectious diseases in the world can be conquered. North, Central and South America together comprise the first region in the world to eliminate the disease, according to the Pan American Health Organization (PAHO). The triumph has been more than two decades in the making as public health workers aimed for widespread immunization against the disease.
Elimination means that a disease is no longer endemic, or circulating on its own, throughout a region. Any outbreaks would therefore result from imported cases of the disease from a different region of the world. Elimination is different from eradication, in which a disease has been completely wiped off every continent, such as smallpox was.
“This is a very big deal,” said Saad Omer, PhD, a professor of global health, epidemiology and pediatrics at Emory University School of Public Health and Emory Vaccine Center. “It’s significant because measles is a big killer that impacts real life and predisposes babies to other diseases.”
Research last year, for example, revealed that vaccination against measles has reduced death from other diseases as well because measles itself causes changes in the immune system—a type of immune system “amnesia”—that make people more susceptible to other infections after they have had measles.
PAHO announced the public health victory Sept. 27, during the 55th Directing Council of PAHO/WHO. The goal to eliminate measles by the turn of the millennium was first set in 1994, according to PAHO/WHO Director Carissa F. Etienne—just a decade and a half after mass vaccination began in 1980.
Until then, an estimated 2.6 million people died from the disease every year, including more than 100,000 deaths in the Americans from 1971 to 1979. Those who do not die from the disease can develop pneumonia or encephalitis (brain swelling) and suffer severe long-term disabilities such as deafness and brain damage. The most serious complication of measles besides death during the disease is subacute sclerosing panencephalitis (SSPE), an always-fatal progressive brain disease that can begin years after recovery from a measles infection.
The only two other diseases eliminated from the Americas have been polio in 1994 and smallpox in 1971, but it’s typical for the Americas to lead the world in setting such precedents.
“The PAHO region specifically outside North America has been a trendsetter in disease elimination,” Omer said. “It is rare that a major disease control program is launched globally before its feasibility is shown in the PAHO region.”
Measles elimination from the Americas follows that of rubella and congenital rubella from both continents just last year, an accomplishment resulting from widespread immunization with the MMR (measles-mumps-rubella) vaccine and successful management of outbreaks, said Omer.
“This particular achievement is a testament to the resilience of the region in addressing and controlling outbreaks,” he said, though he acknowledges it’s hard to say how lasting or fragile the achievement might be in light of continuing introduced outbreaks.
“It does give me hope, but it also says that even in regions that have been successful, there’s a need for ongoing vigilance so that vaccination coverage remains high and an effective outbreak response occurs in situations of reintroduction,” Omer said.
Public health researchers estimate that measles vaccination will have prevented an estimated 3.2 million measles cases and 16,000 deaths from measles in the Americas between years 2000 and 2020. But outbreaks such as last year’s Disneyland outbreak have periodically threatened to derail progress, especially since low immunization rates contributed to such outbreaks. In fact, public health officials thought they had nearly achieved elimination early this century until it was interrupted in 2002, when the last reported endemic case of the disease in the Americas occurred in Venezuela.
Today, cases continue to be imported to the Americas through travel, sometimes leading to outbreaks that officials eventually control, but data from every country in the region analyzed in 2015 and 2016 established that endemic transmission had truly ended from the North Pole to Patagonia.
“It required a lot of hard work, using data to guide disease and outbreak control, basic good epidemiology and program management both at the country level and the regional level,” Omer said.
His call for continued vigilance, however, was echoed by Etienne in PAHO’s official statement about the elimination.
“I would like to emphasize that our work on this front is not yet done,” Etienne said in the statement. “Measles still circulates widely in other parts of the world, and so we must be prepared to respond to imported cases. It is critical that we continue to maintain high vaccination coverage rates, and it is crucial that any suspected measles cases be immediately reported to the authorities for rapid follow-up.”
According to PAHO, 5,077 measles cases were imported into the Americas between 2003 and 2014.
“Some of our recent outbreaks have come from unexpected areas, such as Europe, so continued global success in saving lives by preventing measles would require having high vaccination coverage in all regions of the world,” Omer said. He pointed out that the interruption of elimination progress in 2002 was a bit of a psychological setback for other world regions.
“Many regions want to eliminate measles from their region, but measles is an unforgiving disease because it’s one of the most infectious common diseases,” Omer said. “It’s so infectious that you need really, really high vaccination rates to maintain elimination status at the country and regional level,” Omer said.
Epidemiologists measure how contagious a disease is by its basic reproduction number, known as R0. This number represents how many people a single person will infect with a disease during the full course of their infection if they are in a population that is not otherwise immune, either from vaccination or from a previous infection from the disease. The flu, for example, has an R0 of 2 to 3. That means if a college student becomes ill from the flu within a student population that is not immune, he will most likely infect two or three other non-immune people exposed to him during the period he is contagious.
Polio has an R0 of 5 to 7, which means a single person with polio is likely to infect somewhere between five and seven other non-immune people as long as the original person’s infection remains contagious. Diphtheria has one of the higher R0s, about 6 to 7. Measles, however, has an R0 of 12 to 18, believed to be the highest among all existing serious diseases that can infect humans.
Still, success in the Americas is inspiring to officials in the world’s other regions.
“Many regions have now set their own goals for their elimination of measles,” Omer said. “This success goes a long way in reassuring many people who want to see this disease gone from their regions that they have the potential to save millions of lives by preventing measles.”
