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Asymptomatic MERS Cases

Amesh A. Adalja, MD, FACP, FACEP, August 29, 2014

A new study published in the New England Journal of Medicine sheds light on whether asymptomatic or mild cases of Middle East Respiratory Syndrome (MERS) occur. Key to controlling the spread of any infectious disease is an understanding of its transmission dynamics. Are all infected individuals symptomatic? Does contagiousness completely coincide with symptoms? Are there mild or asymptomatic cases? The answers to such questions are very important because they have a direct impact on the control strategies implemented.

280 Contacts Studied

In this study by Drosten et al., 26 index patients with MERS in Saudi Arabia were identified and 280 of their household contacts studied. Contacts underwent throat swab PCR testing as well as serologic testing a median of 17.5 days after symptom onset in index patients.

2% of Contacts Positive by PCR

Of the 280 contacts, 2% (7 of 280) were positive for MERS on PCR assays. Viral loads varied from below 500 copies to 80,000 copies. Mild symptoms occurred in one contact who developed pharyngitis and the highest viral load. Importantly, 2 contacts had contact with camels, a species suspected to harbor the virus.

Separate serological studies were undertaken in a 2-step process. Serology revealed 5 additional unique possible cases of secondary transmission.

The Risk from Asymptomatic Individuals

The chief implication of this study is that a small proportion of individuals exposed to MERS cases may become silently infected with the virus. Such a phenomenon, even when it occurs on a low level, as demonstrated in this study, poses difficulties for control efforts if these individuals are able to transmit the virus to others.

When MERS cases are detected, one of the major interventions is to monitor contacts for symptoms of MERS. The existence of asymptomatic cases substantially diminishes the effectiveness of such steps, potentially allowing the virus a key opportunity for spread. Understanding the scope of asymptomatic infections and the role they play in the epidemiology of MERS will be important to optimizing control of this virus.


Drosten C, Meyer B, Muller MA, et al. Transmission of MERS-Coronavirus in household contacts. N Engl J Med 2014;371:828-835.