Journal Article > ResearchFull Text
Proc Biol Sci. 22 September 2010; Volume 277 (Issue 1695); DOI:10.1098/rspb.2010.0536
Ferrari MJ, Djibo A, Grais RF, Bharti N, Grenfell BT, et al.
Proc Biol Sci. 22 September 2010; Volume 277 (Issue 1695); DOI:10.1098/rspb.2010.0536
Seasonally driven cycles of incidence have been consistently observed for a range of directly transmitted pathogens. Though frequently observed, the mechanism of seasonality for directly transmitted human pathogens is rarely well understood. Despite significant annual variation in magnitude, measles outbreaks in Niger consistently begin in the dry season and decline at the onset of the seasonal rains. We estimate the seasonal fluctuation in measles transmission rates for the 38 districts and urban centres of Niger, from 11 years of weekly incidence reports. We show that transmission rates are consistently in anti-phase to the rainfall patterns across the country. The strength of the seasonal forcing of transmission is not correlated with the latitudinal rainfall gradient, as would be expected if transmission rates were determined purely by environmental conditions. Rather, seasonal forcing is correlated with the population size, with larger seasonal fluctuation in more populous, urban areas. This pattern is consistent with seasonal variation in human density and contact rates due to agricultural cycles. The stronger seasonality in large cities drives deep inter-epidemic troughs and results in frequent local extinction of measles, which contrasts starkly to the conventional observation that large cities, by virtue of their size, act as reservoirs of measles.
Journal Article > Short ReportFull Text
Science. 9 December 2011; Volume 334 (Issue 6061); DOI:10.1126/science.1210554
Bharti N, Tatem AJ, Ferrari MJ, Grais RF, Djibo A, et al.
Science. 9 December 2011; Volume 334 (Issue 6061); DOI:10.1126/science.1210554
Measles epidemics in West Africa cause a significant proportion of vaccine-preventable childhood mortality. Epidemics are strongly seasonal, but the drivers of these fluctuations are poorly understood, which limits the predictability of outbreaks and the dynamic response to immunization. We show that measles seasonality can be explained by spatiotemporal changes in population density, which we measure by quantifying anthropogenic light from satellite imagery. We find that measles transmission and population density are highly correlated for three cities in Niger. With dynamic epidemic models, we demonstrate that measures of population density are essential for predicting epidemic progression at the city level and improving intervention strategies. In addition to epidemiological applications, the ability to measure fine-scale changes in population density has implications for public health, crisis management, and economic development.
Journal Article > ResearchFull Text
Epidemiol Infect. 5 October 2011; Volume 140 (Issue 8); 1356-1365.; DOI:10.1017/S0950268811002032
Bharti N, Broutin H, Grais RF, Ferrari MJ, Djibo A, et al.
Epidemiol Infect. 5 October 2011; Volume 140 (Issue 8); 1356-1365.; DOI:10.1017/S0950268811002032
Throughout the African meningitis belt, meningococcal meningitis outbreaks occur only during the dry season. Measles in Niger exhibits similar seasonality, where increased population density during the dry season probably escalates measles transmission. Because meningococcal meningitis and measles are both directly transmitted, we propose that host aggregation also impacts the transmission of meningococcal meningitis. Although climate affects broad meningococcal meningitis seasonality, we focus on the less examined role of human density at a finer spatial scale. By analysing spatial patterns of suspected cases of meningococcal meningitis, we show fewer absences of suspected cases in districts along primary roads, similar to measles fadeouts in the same Nigerien metapopulation. We further show that, following periods during no suspected cases, districts with high reappearance rates of meningococcal meningitis also have high measles reintroduction rates. Despite many biological and epidemiological differences, similar seasonal and spatial patterns emerge from the dynamics of both diseases. This analysis enhances our understanding of spatial patterns and disease transmission and suggests hotspots for infection and potential target areas for meningococcal meningitis surveillance and intervention.
Journal Article > CommentaryFull Text
Sci Immunol. 19 May 2020 (Issue 47); DOI:10.1126/sciimmunol.abc6347.
Bryant JE, Azman AS, Ferrari MJ, Arnold BF, Boni MF, et al.
Sci Immunol. 19 May 2020 (Issue 47); DOI:10.1126/sciimmunol.abc6347.
Serological testing for SARS-CoV-2 has enormous potential to contribute to COVID-19 pandemic response efforts. However, the required performance characteristics of antibody tests will critically depend on the use case (individual-level vs. population-level).
Journal Article > ResearchFull Text
Trans R Soc Trop Med Hyg. 1 September 2006; Volume 100 (Issue 9); DOI:10.1016/j.trstmh.2005.10.014
Grais RF, Ferrari MJ, Dubray C, Bjørnstad ON, Grenfell BT, et al.
Trans R Soc Trop Med Hyg. 1 September 2006; Volume 100 (Issue 9); DOI:10.1016/j.trstmh.2005.10.014
The objective of this study is to estimate the effective reproductive ratio for the 2003-2004 measles epidemic in Niamey, Niger. Using the results of a retrospective and prospective study of reported cases within Niamey during the 2003-2004 epidemic, we estimate the basic reproductive ratio, effective reproductive ratio (RE) and minimal vaccination coverage necessary to avert future epidemics using a recent method allowing for estimation based on the epidemic case series. We provide these estimates for geographic areas within Niamey, thereby identifying neighbourhoods at high risk. The estimated citywide RE was 2.8, considerably lower than previous estimates, which may help explain the long duration of the epidemic. Transmission intensity varied during the course of the epidemic and within different neighbourhoods (RE range: 1.4-4.7). Our results indicate that vaccination coverage in currently susceptible children should be increased by at least 67% (vaccine efficacy 90%) to produce a citywide vaccine coverage of 90%. This research highlights the importance of local differences in vaccination coverage on the potential impact of epidemic control measures. The spatial-temporal spread of the epidemic from district to district in Niamey over 30 weeks suggests that targeted interventions within the city could have an impact.
Journal Article > ResearchFull Text
J R Soc Interface. 6 January 2008; Volume 5 (Issue 18); DOI:10.1098/rsif.2007.1038
Grais RF, Conlan AJK, Ferrari MJ, Djibo A, Le Menach A, et al.
J R Soc Interface. 6 January 2008; Volume 5 (Issue 18); DOI:10.1098/rsif.2007.1038
The current World Health Organization recommendations for response during measles epidemics focus on case management rather than outbreak response vaccination (ORV) campaigns, which may occur too late to impact morbidity and mortality and have a high cost per case prevented. Here, we explore the potential impact of an ORV campaign conducted during the 2003-2004 measles epidemic in Niamey, Niger. We measured the impact of this intervention and also the potential impact of alternative strategies. Using a unique geographical, epidemiologic and demographic dataset collected during the epidemic, we developed an individual-based simulation model. We estimate that a median of 7.6% [4.9-8.9] of cases were potentially averted as a result of the outbreak response, which vaccinated approximately 57% (84563 of an estimated 148600) of children in the target age range (6-59 months), 23 weeks after the epidemic started. We found that intervening early (up to 60 days after the start of the epidemic) and expanding the age range to all children aged 6 months to 15 years may lead to a much larger (up to 90%) reduction in the number of cases in a West African urban setting like Niamey. Our results suggest that intervening earlier even with lower target coverage (approx. 60%), but a wider age range, may be more effective than intervening later with high coverage (more than 90%) in similar settings. This has important implications for the implementation of reactive vaccination interventions as they can be highly effective if the response is fast with respect to the spread of the epidemic.
Journal Article > ResearchFull Text
Nature. 7 February 2008; Volume 451 (Issue 7179); DOI:10.1038/nature06509
Ferrari MJ, Grais RF, Bharti N, Conlan AJK, Bjørnstad ON, et al.
Nature. 7 February 2008; Volume 451 (Issue 7179); DOI:10.1038/nature06509
Although vaccination has almost eliminated measles in parts of the world, the disease remains a major killer in some high birth rate countries of the Sahel. On the basis of measles dynamics for industrialized countries, high birth rate regions should experience regular annual epidemics. Here, however, we show that measles epidemics in Niger are highly episodic, particularly in the capital Niamey. Models demonstrate that this variability arises from powerful seasonality in transmission-generating high amplitude epidemics-within the chaotic domain of deterministic dynamics. In practice, this leads to frequent stochastic fadeouts, interspersed with irregular, large epidemics. A metapopulation model illustrates how increased vaccine coverage, but still below the local elimination threshold, could lead to increasingly variable major outbreaks in highly seasonally forced contexts. Such erratic dynamics emphasize the importance both of control strategies that address build-up of susceptible individuals and efforts to mitigate the impact of large outbreaks when they occur.
Journal Article > ResearchAbstract
Epidemiol Infect. 1 June 2010; Volume 139 (Issue 2); DOI:10.1017/S0950268810001329
Metcalf CJ, Klepac P, Ferrari MJ, Grais RF, Djibo A, et al.
Epidemiol Infect. 1 June 2010; Volume 139 (Issue 2); DOI:10.1017/S0950268810001329
Measles vaccine efficacy is higher at 12 months than 9 months because of maternal immunity, but delaying vaccination exposes the children most vulnerable to measles mortality to infection. We explored how this trade-off changes as a function of regionally varying epidemiological drivers, e.g. demography, transmission seasonality, and vaccination coverage. High birth rates and low coverage both favour early vaccination, and initiating vaccination at 9-11 months, then switching to 12-14 months can reduce case numbers. Overall however, increasing the age-window of vaccination decreases case numbers relative to vaccinating within a narrow age-window (e.g. 9-11 months). The width of the age-window that minimizes mortality varies as a function of birth rate, vaccination coverage and patterns of access to care. Our results suggest that locally age-targeted strategies, at both national and sub-national scales, tuned to local variation in birth rate, seasonality, and access to care may substantially decrease case numbers and fatalities for routine vaccination.
Journal Article > ResearchFull Text
Am J Epidemiol. 15 January 2014; Volume 179 (Issue 2); 245-251.; DOI:10.1093/aje/kwt236
Minetti A, Hurtado N, Grais RF, Ferrari MJ
Am J Epidemiol. 15 January 2014; Volume 179 (Issue 2); 245-251.; DOI:10.1093/aje/kwt236
Current mass vaccination campaigns in measles outbreak response are nonselective with respect to the immune status of individuals. However, the heterogeneity in immunity, due to previous vaccination coverage or infection, may lead to potential bias of such campaigns toward those with previous high access to vaccination and may result in a lower-than-expected effective impact. During the 2010 measles outbreak in Malawi, only 3 of the 8 districts where vaccination occurred achieved a measureable effective campaign impact (i.e., a reduction in measles cases in the targeted age groups greater than that observed in nonvaccinated districts). Simulation models suggest that selective campaigns targeting hard-to-reach individuals are of greater benefit, particularly in highly vaccinated populations, even for low target coverage and with late implementation. However, the choice between targeted and nonselective campaigns should be context specific, achieving a reasonable balance of feasibility, cost, and expected impact. In addition, it is critical to develop operational strategies to identify and target hard-to-reach individuals.
Journal Article > ResearchFull Text
Int Health. 5 September 2014 (Issue 4)
Ferrari MJ, Fermon F, Nackers F, Llosa AE, Magone C, et al.
Int Health. 5 September 2014 (Issue 4)
In 2009, a large meningitis A epidemic affected a broad region of northern Nigeria and southern Niger, resulting in more than 75 000 cases and 4000 deaths. In collaboration with state and federal agencies, Médecins Sans Frontières (MSF) intervened with a large-scale vaccination campaign using polysaccharide vaccine. Here the authors analyze the impact (cases averted) of the vaccination response as a function of the timing and coverage achieved.