The Ebola crisis in West Africa is one of the most striking public health emergencies in recent years. According to the Centers for Disease Control, this fast-spreading virus has killed over 9,951 people since the start of the outbreak in December of 2013. Currently, there are no FDA-approved vaccines or antiviral treatments for Ebola, and patients’ survival depends on their own immune response and the supportive care they receive. The mortality rate for this disease is currently estimated to be approximately 70 percent by the World Health Organization.

Now a new study published in the journal Science suggests that the Ebola crisis could leave countries vulnerable to epidemics of a more common virus, measles, due to its disruption of routine health care services in affected areas.

The authors of this study are affiliated with some of the most prominent public health institutions in the world, including Princeton University, Johns Hopkins’ Bloomberg School of Public Health, and the National Institutes of Health. They project that due to the loss of healthcare workers caused by the Ebola crisis, a cluster of children unvaccinated for measles will accumulate in Guinea, Liberia, and Sierra Leone. Because of the susceptibility of this population, the investigators expect a regional measles outbreak of 127,000 to 227,000 cases after 18 months, which will result in 2,000 to 16,000 measles-related deaths in the region.

Due to the efforts of the World Health Organization’s Expanded Program on Immunization, as well as supplemental immunization campaigns, the number of childhood deaths from preventable disease dropped by more than 50 percent from 2000 to 2010—a success story you don’t hear enough about. Vaccination against measles has been a key aspect of this public health victory, as the annual estimated measles mortality has decreased globally from 499,000 to 102,000 since 2004. The countries affected by Ebola have hugely benefitted from this decrease in measles mortality, but their susceptibility to measles has been growing in recent years. Prior to the Ebola outbreak, there had been a planned measles vaccination campaign in this area.

Historically, it is not uncommon for a measles epidemic to follow a humanitarian crisis. Measles is a highly transmissible infection, and its vaccination rates tend to be low. Once measles is introduced to a population, it can spread very quickly. This was recently seen in the US measles outbreak, which was likely instigated by a single infected traveler visiting a Disney amusement park in California. Due to the breakdown in health care systems that occurs during crisis, a post-crisis population is particularly vulnerable to measles. And historically, measles outbreaks have occurred after wars, natural disasters, and political crises.

To investigate the relationship between Ebola-related health care disruptions and increased risk from measles, the investigators of this study examined the geographical distribution of unvaccinated children and the measles susceptibility profile for the affected countries before and after the Ebola crisis. Data from the Demographic and Health Surveys in Guinea, Liberia, Sierra Leone, and surrounding nations were used to estimate vaccine coverage in the region. Investigators used a Bayesian analysis model, which allows statisticians to estimate the probability of future events based on knowledge of past events.

The investigators estimated the number of unvaccinated children at six months, 12 months, and 18 months after a 75 percent reduced rate of routine vaccination. Here, the authors modeled three scenarios based on three possible durations of disruption to usual health care service due to the Ebola Crisis—the crisis has not yet been resolved, so the duration of the potential disruption is not yet known. Their assumption of a 75 percent reduction in vaccination rates is based on some surveys of healthcare providers in the Ebola-affected region, though they note that no reliable information on the change in vaccination rates is available.

The authors estimated the full population susceptibility on the national level after eight months of disruptions by combining estimates of unvaccinated children with immunization histories and previous rates of natural infections. They also calculated the expected size of a regional measles outbreak using the previously observed relationship between population susceptibility to measles and the percentage of susceptible people who became infected in previous measles outbreaks. Finally, they were able to estimate the number of expected measles-related deaths after the Ebola crisis by using known information about the fatality of measles among infected individuals.

These calculations allowed the investigators to estimate that for every additional month of health care disruption due to the Ebola outbreak, the number of children not vaccinated against measles increases by an average of 19,514. After six months of health care disruption, they estimate that 964,346 children will be unvaccinated against measles. After 12 months, that number increases to 1,068,833 children, and after 18 months, it is estimated that 1,129,376 children in this region will not receive the vaccination.

If measles vaccination had continued at pre-Ebola rates during the Ebola outbreak, a generalized measles outbreak would have caused an estimated 126,868 cases. However, with 18 months of disruption to routine healthcare due to the Ebola crisis, a post-Ebola measles outbreak would result in almost twice as many cases (an estimated 227,484 cases) as well as an estimated increase of 5,209 deaths from measles.

Measles is just one of several childhood infections where vaccination rates have dropped due to the Ebola outbreak in West Africa; setbacks in vaccination programs could increase the occurrence of polio and tuberculosis in this region. Disruption to other public health services may also increase the effects of malaria and HIV.

The authors note that chronic diseases like HIV and non-age dependent infections like malaria require constant intervention and support, while the effects of childhood infections like measles can be countered with a single planned vaccination campaign. The authors recommend a heightened vaccination campaign in West Africa to prevent a second public health disaster on the heels of the Ebola crisis. Planning for this campaign should begin immediately, as the pool of susceptible children grows daily, and a second public health crisis would severely affect the population of an already devastated region.

Science, 2015. DOI: 10.1126/science.aaa3438 (About DOIs).