Photo Credit NIAID
Helen Branswell has the story this evening on a fresh study, just published in The Lancet Infectious Diseases, that looks at the likely extent of transmission of the MERS virus in the Middle East. The results – that for every case identified, there are likely 5 to 10 that go undetected – suggest that this virus may be transmitting more efficiently than previously estimated.
First, a link to Helen’s excellent review, then a link to the study, after which I’ll return with a bit more.
Helen Branswell / The Canadian Press
November 12, 2013 03:42 PM
A new analysis of MERS case data suggests a large number of infections are going undetected, with the researchers estimating that for each case that has been found, five to 10 may have been missed.
The scientific paper, from European researchers, further suggests that transmission of the MERS virus is occurring at a rate close to the threshold where it would be considered able to pass from person to person in a sustained manner.
In fact, the authors say based on the available evidence they cannot rule out the possibility that person-to-person spread is the main mode of transmission of the virus at this point. The other option, they say, is that the virus is spreading via a combination of animal-to-person and then person-to-person transfer.
Middle East respiratory syndrome coronavirus: quantification of the extent of the epidemic, surveillance biases, and transmissibility
Simon Cauchemez PhD a †, Prof Christophe Fraser PhD a †, Maria D Van Kerkhove PhD a, Prof Christl A Donnelly ScD a, Steven Riley PhD a, Prof Andrew Rambaut PhD b, Vincent Enouf PhD c, Prof Sylvie van der Werf PhD c, Prof Neil M Ferguson DPh
By showing that a slowly growing epidemic is underway either in human beings or in an animal reservoir, quantification of uncertainty in transmissibility estimates, and provision of the first estimates of the scale of the epidemic and extent of case detection biases, we provide valuable information for more informed risk assessment.
The epidemiological yardstick by which human transmission of an infectious disease is measured is called the R0 (pronounced R-nought) or Basic Reproductive Number.
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’.
Last July, in The Lancet: Transmissibility Of MERS-CoV, we looked at a preliminary analysis that calculated the R0 of the MERS virus to be between .60 and .69. Too low (at that time) to spark an epidemic.
But calculating the R0 is notoriously difficult, particularly since much hinges upon the existence and subtle differences between viral strains, the accuracy of surveillance and reporting, `seasonality’’ of the infection, and individual host responses to the virus (i.e. number of `super spreaders’).
Like the CFR (Case Fatality Ratio), the R0 can vary considerably over time or geography, often ends up being described as a `range’, and usually isn’t well established (or at least, generally agreed upon) until long after an outbreak has ended.
The authors in today’s study believe the R0 of the MERS virus is likely close to 1.0, or perhaps even higher, and write:
We conclude that a slowly growing epidemic is underway, but current epidemiological data do not allow us to determine whether transmission is self-sustaining in man. Our analysis demonstrates that the transmissibility of MERS-CoV in man is close to the critical threshold of R=1 required for self-sustaining transmission. If R is greater than 1, then the number of human cases we estimate to have occurred to date make it highly likely that self-sustaining transmission has already begun
Even assuming low levels of sustained transmission – with an R0 of greater 1.0 – the timely application of control procedures can sometimes contain, and even halt, an epidemic.
The R0 of SARS was estimated to be between 2 and 4, but since patients weren’t infectious prior to developing symptoms, aggressive quarantine efforts were able to quell that outbreak.
The question, of course, is whether the current surveillance and testing regimens in place are comprehensive enough to identify and the spreaders of this virus. And here, the Achilles heel may be asymptomatic or mild cases – which are less likely to be identified and isolated – but which may still be capable of spreading the virus.
SARS is believed to have had a low percentage of asymptomatic cases (see EID Journal Asymptomatic SARS Coronavirus Infection among Healthcare Workers, Singapore), and they did not appear to be aggressive spreaders of the virus.
Whether that will be the case with MERS remains to be seen.