Interhuman transmissibility of Middle East respiratory syndrome coronavirus: estimation of pandemic risk

Abstract

Background: The new Middle East respiratory syndrome coronavirus (MERS-CoV) infection shares many clinical, epidemiological, and virological similarities with that of severe acute respiratory syndrome (SARS)-CoV. We aimed to estimate virus transmissibility and the epidemic potential of MERS-CoV, and to compare the results with similar findings obtained for prepandemic SARS.

Methods: We retrieved data for MERS-CoV clusters from the WHO summary and subsequent reports, and published descriptions of cases, and took into account 55 of the 64 laboratory-confirmed cases of MERS-CoV reported as of June 21, 2013, excluding cases notified in the previous 2 weeks. To assess the interhuman transmissibility of MERS-CoV, we used Bayesian analysis to estimate the basic reproduction number (R0) and compared it to that of prepandemic SARS. We considered two scenarios, depending on the interpretation of the MERS-CoV cluster-size data.

Results: With our most pessimistic scenario (scenario 2), we estimated MERS-CoV R0 to be 0·69 (95% CI 0·50-0·92); by contrast, the R0 for prepandemic SARS-CoV was 0·80 (0·54-1·13). Our optimistic scenario (scenario 1) yielded a MERS-CoV R0 of 0·60 (0·42-0·80). Because of recent implementation of effective contact tracing and isolation procedures, further MERS-CoV transmission data might no longer describe an entire cluster, but only secondary infections directly caused by the index patient. Hence, we calculated that, under scenario 2, eight or more secondary infections caused by the next index patient would translate into a 5% or higher chance that the revised MERS-CoV R0 would exceed 1--ie, that MERS-CoV might have pandemic potential.

Interpretation: Our analysis suggests that MERS-CoV does not yet have pandemic potential. We recommend enhanced surveillance, active contact tracing, and vigorous searches for the MERS-CoV animal hosts and transmission routes to human beings.

Funding: Agence Nationale de la Recherche (Labex Integrative Biology of Emerging Infectious Diseases), and the European Community's Seventh Framework Programme project PREDEMICS.

Figure 1

Map of Middle East respiratory syndrome coronavirus clusters included in the analysis Cluster sizes are shown in bold white text. We used a blue background if their location could be established within the country of origin and a red background otherwise. Each arrow corresponds to travel of one patient with Middle East respiratory syndrome coronavirus infection outside the Middle East, where they caused secondary cases. gov=governorate.

Figure 2

Mathematical modelling results (A) The average tree size versus R₀ as predicted by the theory of homogeneous branching processes. The dark blue and red dashed lines correspond to the values of R₀ that we calculated for scenarios 1 and 2, respectively. (B) Contour plot of the expected Middle East respiratory syndrome coronavirus (MERS-CoV) yearly incidence versus introduction rate into the human population and R₀. The very dark blue region corresponds to yearly incidence estimates below 10. The dark red region corresponds to yearly incidence estimates above 320. All other solid colour regions correspond to yearly incidence estimates bounded by the values shown on the contours. The black square and red circle show the parameter sets of scenarios 1 and 2, respectively; the error bars represent the corresponding 95% CIs. As a visual aid, we have shaded the region comprising the parameter sets compatible with scenarios 1, 2, and the in-between area compatible with intermediate scenarios. (C) The probability that R₀ exceeds 1 versus the size of the next MERS-CoV transmission tree; the horizontal dashed line corresponds to the 5% probability. (D) The probability that R₀ exceeds 1 versus the size of the next count of secondary cases of an index patient; the horizontal dashed line corresponds to the 5% probability. R₀=basic reproduction number (the number of secondary cases per index case in a fully susceptible population).