Thursday, July 09, 2015

Eurosurveillance: Estimating The Odds Of Secondary/Tertiary Cases From An Imported MERS Case


Credit ECDC MERS Risk Assessment June 30th


# 10,311


Over the past three years there have been 23 documented importations of the MERS virus to nations outside of the Middle East (not including Turkey), with the number of secondary cases ranging from 0 (in 19 of those events) to 185 (in Korea).  


In between there have been three other countries with imported cases (France, Tunisia & The UK) that produced between 1 and 3 additional cases. 


While one might be tempted to assume that Korea’s bad luck with MERS was simply that  - bad luck – and unlikely to be repeated, an analysis that appears in today’s Eurosurveillance calculates the odds of seeing multiple generations of MERS transmission as the result of a single importation of the virus.


Along the way, and using math I won’t even pretend to understand, they calculated the estimated R0 (pronounced R-nought) or Basic Reproductive Number of the virus.  Essentially, the number of new cases in a susceptible population likely to arise from a single infection.


With an R0 below 1.0, a virus (as an outbreak) begins to sputter and dies out. Above 1.0, and an outbreak can have `legs’.

As we’ve seen with other analyses in the past (see The Elusive R0 of MERS), their calculations put the R0 of MERS well below the critical 1.0 mark (.75 to .80).


Their model suggests the odds of seeing at least one secondary case derived from an imported case is 22.7% , while the odds of seeing at least one tertiary case is 10.5%.  


Based on their calculations, the odds of seeing at least 8 cases as the result of a single importation is estimated at non-trivial 10.9%.

Follow the link below to read the study in its entirety.


Assessing the risk of observing multiple generations of Middle East respiratory syndrome (MERS) cases given an imported case

H Nishiura ()1,2, Y Miyamatsu1,2, G Chowell3,4, M Saitoh1,2,5


Date of submission: 22 June 2015

To guide risk assessment, expected numbers of cases and generations were estimated, assuming a case importation of Middle East respiratory syndrome (MERS). Our analysis of 36 importation events yielded the risk of observing secondary transmission events at 22.7% (95% confidence interval: 19.3–25.1). The risks of observing generations 2, 3 and 4 were estimated at 10.5%, 6.1% and 3.9%, respectively. Countries at risk should be ready for highly variable outcomes following an importation of MERS.

Middle East respiratory syndrome (MERS) has continued to spread globally [1]. A large cluster of MERS cases has been observed in the Republic of Korea since May 2015 [2]. Until 1 July 2015, so-called quaternary cases (i.e. generation 3 counting from the index case as generation 0) have been reported in South Korea [2]. Although the inter-human transmission potential of MERS is considered to be too low to cause large-scale epidemics [3-5], high variability and heterogeneity in the transmission potential have been underscored [6]. As MERS continues to spread globally, it is vital that risk assessment involves an evaluation of the potential outcomes following MERS importation events [7]. Among a total of 23 importation events in countries outside the Middle East region, there have been four MERS case importations that have given rise to at least one secondary transmission event [2].

While the basic reproduction number R0, i.e. the average number of secondary cases produced by a primary case in a fully susceptible population, is less than 1 for MERS and a major epidemic may therefore not occur immediately, it is critical to quantitatively assess several risks of MERS transmission following an importation event, e.g. the expected numbers of cases and generations. The present study aims to analyse the observed importation events of MERS and estimate the expected size of MERS clusters and the number of generations using a stochastic epidemic model.

(Continue . . . )


Those with mathochistic tendencies will no doubt want to read the entire analysis, and examine the accompanying materials.  For our purposes we’ll skip to a couple of paragraphs from the Discussion section.




An important outcome of the present study is that the risk of observing multiple generations of MERS cases was estimated at 22.7% and that of a tertiary case at 10.5% in our model. Since the distribution of secondary cases per single primary case is highly over-dispersed, superspreading events can occur, and thus, the expected total number of cases is highly variable. The finding echoes a recent re-analysis of clusters of MERS cases reported up to August 2013 [17]. Of course, we can expect that secondary transmission events could be prevented by a combination of contact tracing, monitoring suspected cases, early diagnosis and isolation of infectious individuals. Besides, the present study suggests that countries at risk of importation should keep in mind that a large cluster of cases with multiple generations may well occur, even though R0 is clearly below the epidemic threshold at 1.0 [18].


Despite these limitations, the present study successfully characterised the risk of observing a certain number of cases and generations given a case importation of MERS. The risk of at least one secondary case in our model was 22.7%, and that of a tertiary case was 10.5%. Once an untraced case is imported, countries at risk should assume that the secondary transmission risk as well as the risk of observing multiple generations of cases exists and should be ready to respond effectively by following their preparedness plans to manage emerging infectious diseases.


While outbreaks the size of the Korean cluster are obviously uncommon, I suspect a lot of people will be surprised to find the odds of seeing clusters of 8 or more cases result from a single imported case at nearly 11%.  


And that is without having seen any importations into some of the highly populated and problematic regions like Mumbai, Lagos, or Jakarta.

Reason enough to take the calls for better surveillance, diagnosis, and infection control for this virus (and other emerging infectious diseases) seriously.

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