West Africa's Ebola outbreak could be stanched by mid-year – but in the epidemic's wake, another public health crisis looms. Disruption of the region's already feeble health care systems has derailed health campaigns targeting childhood diseases, leaving the door wide open for measles and other preventable illnesses.

Above: A mother brings her sick child for treatment at Redemption Hospital, formerly an Ebola holding center, on February 2, 2015 in Monrovia, Liberia, where the Ebola epidemic has claimed at least 3,700 lives. But as Ebola wanes, a new threat is brewing: Unvaccinated children, vulnerable to highly contagious measles infections, could spark a devastating secondary epidemic | Photo by John Moore/Getty Images

In this week's issue of Science, researchers led by Johns Hopkins epidemiologist Justin Lessler report that stress placed by Ebola on West Africa's public health infrastructure has given rise to large, interconnected clusters of unvaccinated children throughout Guinea, Liberia, and Sierra Leone, the three countries hit hardest by the Ebola epidemic.

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Using high-resolution population data and reports from local health officials, the researchers modeled the geographic distribution of unvaccinated children and the measles susceptibility for each country, before and after the outbreak. From these models, Lessler and his colleagues estimate West Africa's "growing pool of susceptibility" could increase the size of a regional measles outbreak from 127,000 to 227,000 cases, resulting in anywhere from 2,000 to 16,000 deaths.

To be clear, that's 2,000 to 16,ooo additional deaths, on top of whatever death toll Ebola winds up causing directly. The latest figures from the World Health Organization, released Wednesday, indicate Ebola has now claimed 9,961 lives in Guinea, Liberia, and Sierra Leone. In other words, says Lessler, "the secondary effects of Ebola – both in childhood infections and other health outcomes – are potentially as devastating in terms of loss of life as the disease itself."

Modeling a Measles Outbreak

West Africa's current Ebola outbreak began 15 months ago, in December 2013. In their investigation, Lessler and his colleagues model several post-Ebola measles-outbreak scenarios, but assume in their main analysis that the Ebola outbreak will disrupt local health care systems for a total of 18 months.

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The researchers also forecast a 75% drop in measles vaccination rates relative to pre-Ebola vaccination levels. A decline of this size would leave an estimated 1,129,376 children 9 months and 5 years of age vulnerable to measles infection, compared with 778,000 prior to the Ebola outbreak. The animation on the left depicts the accumulation of additional children susceptible to measles after each month of Ebola-related disruptions to vaccination efforts, under these assumptions. [Credit: Justin Lessler and Saki Takahashi]

The prediction of a 75% drop in routine vaccination rates is based on surveys of local health officials, though Lessler and his co-authors note in their paper that the ever-changing nature of the Ebola epidemic limits the availability of reliable vaccination information. In a press conference held Wednesday morning, Lessler reiterated this point, telling reporters that recent information suggests his team's primary scenario of a 75% decline "may have been a touch too pessimistic."

"But even our least pessimistic scenario of a 25 percent reduction would be expected to result in tens of thousands of additional cases," added Lessler, whose team also modeled scenarios involving 25, 50, and 100% drops in vaccination rate, over periods as long as 24 months. A 25 percent reduction following 18 months of disruption, says Lessler, would translate to between 500 to 4,000 additional deaths.

The impact of Ebola related healthcare disruptions on measles risk | Image Credit: Justin Lessler and Saki Takahashi]

Whatever the projected death toll, the immediate course of action is clear: "[The] path to avoiding outbreaks of childhood vaccine-preventable diseases once the threat of Ebola begins to recede," write the researchers, is "an aggressive regional vaccination campaign aimed at age groups left unprotected because of health care disruptions."

Measles epidemiologist James Goodson – a senior scientist in the Centers for Disease Control's Global Immunization Division, who was unaffiliated with the study – tells io9 that he agrees.

"The main takeaway from [Lessler's team's study] is that there's an urgent need to address a very high risk for measles outbreaks in the Ebola affected countries," says Goodson, who, through the CDC, belongs to the Measles & Rubella Initiative, a global partnership dedicated to the control and elimination of measles worldwide. He adds that the CDC has conducted similar risk assessments in Liberia, Guinea, and Sierra Leone. Their conclusions, he says, corroborate those of Lessler and his colleagues.

The risk of a large measles outbreak in all three countries is very high, due to curtailed child immunizations, he says. "The need to vaccinate is urgent."

Not If But When

Public health officials specializing in the disease tell io9 a large measles outbreak in West Africa is imminent. "It's just a matter of time," says Goodson.

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Small outbreaks have already been reported. In a field update issued March 3, Médecins Sans Frontières reported outbreaks of measles in the Liberian counties of Lofa, Margib, and Montserrado counties. In fact, Goodson says the CDC has heard reports of measles cases from all three countries.

The fact that measles is already circulating, says Goodson, is cause for grave concern. "Every day that passes, the number of unvaccinated children grows," he says. "At some point the levee breaks, and the outbreak explodes."

"It's really not a matter of if," adds Goodson, "but when."

What's less clear is how deadly the measles outbreak – or outbreaks – will be. Predictions by Lessler's team range from 500 to 16,000 deaths, depending on how long Ebola lingers in West Africa and the actual vaccination status of the region's children. That said, the researchers report measles mortality could well fall at the high end of their predictions; left reeling from Ebola, the region's health care services will be ill-equipped to confront a measles outbreak full-bore.

Why Target Measles?

Measles is not the only threat brewing in Ebola's wake. The epidemic has stemmed the delivery of polio, tuberculosis, influenza, pertussis, tetanus, hepatitis B, and diptheria vaccines, as well. And the outbreak has had a devastating impact on maternal health. Inpatient beds are scarce, to say nothing of actual healthcare workers, over 400 of whom have perished in the course of the epidemic. The provision of health care to people with malaria, HIV and tuberculosis has also been interrupted.

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So why target measles, in particular, which Lessler himself told reporters "may not even be the most dangerous" threat?

Because, said Lessler, "it's one we can do something about."

Measles is the most infectious virus known to humankind, a dangerous and dogged opportunist. When a chink appears in a population's immunological armor, the disease tends to exploit it, and quickly. The same could be said of many vaccine-preventable diseases, but "measles in particular is known to show up during or after humanitarian crises," Lessler told reporters Wednesday morning, "because it is so infectious."

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"Few infections can stage a comeback as quickly or dramatically as measles," Dr. Paul Offit, Professor of Pediatrics in the Division of Infectious Diseases at the Children's Hospital of Philadelphia, tells io9. Take pertussis (whooping cough), for example, or influenza. The vaccines for these diseases are not as effective as the vaccine for measles, so it is common for them to persist at low levels, even in health communities.

The measles vaccine, on the other hand, is highly effective. "When it's administered, people are protected, and they're protected well," says Offit. But when a population's vaccination status wanes, the highly infectious nature of measles allows it to rebound with frightening speed. In the United States, we are well-equipped to respond with commensurate urgency. In Liberia, Guinea, and Sierra Leone, a frayed public health infrastructure has left room for measles to rebound with a vengeance.

The key now is to respond, and quickly. Liberia, whose last Ebola patient was released March 5, is poised to lead the charge by launching a measles vaccination campaign as early as May.