One of the key questions that we posed when this virus first emerged was is ‘silent’ transmission (that is transmission to others before symptoms become apparent) possible? And if it’s possible, is it the norm? A short letter published in yesterday’s New England Journal of Medicine answers the first part of that: silent transmission of 2019-nCoV is possible – but just how common is it?

The letter relates to a cluster of 5 patients with 2019-nCoV in Germany. The index patient was visiting Germany from their home in China on business, and managed to transmit the coronavirus to two patients, who subsequently transmitted the virus to two other colleagues (see the Figure). Whilst apparent simple transmission routes from epidemiological investigations can be undone by detailed molecular analysis (as was the case in this CPE outbreak), the key point here is that these transmission events all seemed to happen before symptoms arose.

Figure: Timeline of transmission between 5 patients in Germany; only two patients had contact with the index patient.

Furthermore, the concentration of the virus identified in the sputum of Patient 1 who had completed the course of his symptoms was high (10⁸ copies per mL of sputum). This raises the possibility of extended shedding of the virus in the post-symptomatic phase, which could be a risk for transmission – although as the authors say, the virus in the sputum was identified by PCR so there’s no guarantee that the virus is alive / viable.

There’s a remarkable statistic that H1N1 flu infected around a fifth of the global population. Could the 2019-nCoV follow the same course, with asymptomatic or low-grade diseases in a vast number of people? Time will tell, but this report is useful in demonstrating that transmission of the 2019-nCoV can happen before symptoms arise.

Finally, it’s pretty cool that the timeline of this study ends of 29 Jan…the day before the letter was published!