Date Published: May 12, 2018
Publisher: The American Society of Tropical Medicine and Hygiene
Author(s): Matthew Graham, Jonathan E. Suk, Saki Takahashi, C. Jessica Metcalf, A. Paez Jimenez, Vladimir Prikazsky, Matthew J. Ferrari, Justin Lessler.
We report on and evaluate the process and findings of a real-time modeling exercise in response to an outbreak of measles in Lola prefecture, Guinea, in early 2015 in the wake of the Ebola crisis. Multiple statistical methods for the estimation of the size of the susceptible (i.e., unvaccinated) population were applied to weekly reported measles case data on seven subprefectures throughout Lola. Stochastic compartmental models were used to project future measles incidence in each subprefecture in both an initial and a follow-up iteration of forecasting. Measles susceptibility among 1- to 5-year-olds was estimated to be between 24% and 43% at the beginning of the outbreak. Based on this high baseline susceptibility, initial projections forecasted a large outbreak occurring over approximately 10 weeks and infecting 40 children per 1,000. Subsequent forecasts based on updated data mitigated this initial projection, but still predicted a significant outbreak. A catch-up vaccination campaign took place at the same time as this second forecast and measles cases quickly receded. Of note, case reports used to fit models changed significantly between forecast rounds. Model-based projections of both current population risk and future incidence can help in setting priorities and planning during an outbreak response. A swiftly changing situation on the ground, coupled with data uncertainties and the need to adjust standard analytical approaches to deal with sparse data, presents significant challenges. Appropriate presentation of results as planning scenarios, as well as presentations of uncertainty and two-way communication, is essential to the effective use of modeling studies in outbreak response.
Between January 23 and April 4, 2015 (weeks 4–13 of the year), 284 cases of measles were identified in Lola, a prefecture of approximately 180,000 people in southeast Guinea within the Nzérékoré region (Figure 1). Given healthcare system disruptions caused by the Ebola outbreak, there was concern that reductions in measles vaccination may have increased susceptibility in the younger population.1 A supplementary immunization activity, aimed at decreasing measles susceptibility, was planned for Guinea in 2014. However, this campaign was interrupted by the Ebola outbreak and never reached Lola prefecture. In addition, within the Nzérékoré region of Guinea, measles vaccination coverage has been relatively low (reaching only 61% of children aged 9–59 months in 20122), suggesting that a large proportion of the population aged less than 5 years was susceptible to a measles outbreak. These factors raised concerns that these 284 reported cases heralded a large and potentially deadly measles outbreak (e.g., the estimated case–fatality ratio of measles cases in Africa was 3.7%).3
The aims of this analysis were 2-fold: to estimate the level of measles susceptibility in the population and to forecast the number of expected future cases in Lola prefecture. Initial analyses focused on N’Zoo subprefecture as, along with a high level of observation of the epidemic, it had the longest history of case reports, likely improving the accuracy of analysis relative to other locations.
The Week 13 report gave observed measles cases in several subprefectures of Lola up to week 13 of 2015. N’Zoo subprefecture was the first to report cases of measles, beginning in the 4th week of the year. In the 9th week of the year, two cases were observed in Kokota subprefecture. The following week, cases were observed in N’Zoo and Kokota, along with a third subprefecture, Foumbadou; and by week 12, cases had been observed in eight subprefectures in total. Tounkarata subprefecture had four reported cases in week 15 and no other reported cases, and so is excluded from analysis entirely.
Here we describe an analysis of a measles outbreak in Lola prefecture of Guinea beginning in the 4th week of 2015, including estimates of population susceptibility and projection of future cases in the Nzérékoré region and Lola prefecture using several methods of different complexity. Projections made from the Week 13 report were supplied to the WHO on April 12, 2015. A measles vaccination campaign deployed to compensate for the interrupted Guinea-wide campaign originally planned for 2014 began in Lola prefecture on April 18 (week 16). This campaign was conducted over 7 days (the entirety of week 17), and it is estimated that ultimately 92% of children from 6 months to 10 years old were vaccinated.4 Despite this vaccination effort, the area has remained vulnerable to measles, as demonstrated by the subsequent measles outbreak in Guinea in 2017. More than 2,100 children were infected with measles from January 2017 to March 12, 2017, with 675 of these cases reported in Nzérékoré prefecture20 (located within the Nzérékoré region and bordering Lola prefecture), a greater number of cases than were reported in Lola in 2015. A vaccination campaign was initiated on March 12, 2017, in Nzérékoré prefecture, aimed at protecting 140,000 children between the ages of 6 months and 10 years.