MERS: 2015 Outbreak in Riyadh, Saudi Arabia

There’s been a pretty serious outbreak of MERS going on in the Riyadh region of Saudi Arabia for the last 3-4 weeks. Since August 1 alone, there have been 82 cases reported – a statistic that is especially alarming given the upcoming Hajj season.

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Granted, this is not the first time that Riyadh has proven to be a hotspot for MERS transmission. Since 2012, ~40% of Saudi MERS cases have been reported out of Riyadh.

monthly_epicurve_KSA_8.22.15

What’s particularly curious about this outbreak is its timing. To date, we’ve seen upticks in Saudi MERS cases in the spring. This was most notable during the Spring 2014 outbreak, when over 500 MERS cases were reported out of Saudi Arabia between March and May.

monthly_epicurve_comb_8.22.15

…So, what’s the deal with the ongoing outbreak in Riyadh? One possible hypothesis is bimodal seasonality – namely, a surge of cases in the fall followed by another in the spring. After aggregating Saudi MERS data from 2012 to 2015, this trend becomes a little bit more apparent.

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However, with only three complete years of data, it’s tough to defend this theory statistically. If bimodal seasonality among human cases does indeed exist, a possible mechanistic explanation may be seasonal variability in viral load among dromedary camels – currently considered to be the primary animal reservoir for MERS-Coronavirus. Simply put, if there are times of year when dromedary camels are more likely to be carrying the virus, spillover into humans would be more likely during those time periods as well. These spillover events may then result in human-to-human transmission during care-seeking, thus resulting in nosocomial outbreaks – such as the one we witnessed in Spring 2014 or the one in Riyadh that is currently ongoing.

Understanding the emerging seasonality associated with human MERS infections in Saudi Arabia will be critical to outbreak prevention in the future. Though MERS started off in 2012 as a sporadic illness, there have been nearly 1500 cases – in over 25 countries – reported since.

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Needless to say, this is a disease that’s worth keeping and eye on – especially due to its propensity for super-spreading and frequency of importation events.

Note: Click through figures for high-resolution images. Feel free to use, but please give credit where credit is due!

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