Covid-19: four fifths of cases are asymptomatic, China figures indicate

Dear Editor
“Four fifths of cases” of coronavirus disease 2019 (Covid-19) are not asymptomatic. The fraction of people testing positive for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is not the same as the fraction with Covid-19. Also, the numbers cited do not show that “the large majority of coronavirus infections do not result in symptoms.” It is not possible to extrapolate a proportion of people without symptoms at a single time point, to a general statement about the natural history of an infection. Michael Day’s News item (https://www.bmj.com/content/bmj/369/bmj.m1375.full.pdf) is inaccurate and misleading.
The data cited are not “new evidence”. They are routine reports of “166 new infections… identified in the 24 hours to the afternoon of Wednesday 1 April” in China. The people tested are a mixture of people with symptoms that they report, people with mild or non-specific symptoms that they don’t report, people who are pre-symptomatic but will develop symptoms later, and people who are and will remain asymptomatic. If the aim of testing is case-finding, it makes sense to test people who might not yet know they are infected and to isolate the cases to break transmission chains. This is the point of contact tracing.
An asymptomatic SARS-CoV-2 infection can only be established by follow-up of people who test positive. Those who continue without symptoms until at least the end of the incubation period, generally accepted as 14 days from the date of infection,1 have had an asymptomatic infection. This group is thought to comprise a small proportion of all those who have been infected with SARS-CoV-2.1 It is not known how transmissible a truly asymptomatic infection is. SARS-CoV-2-infected people who are asymptomatic when tested, but who go on to develop symptoms of coronavirus disease 2019 (Covid-19) are pre-symptomatic.2 These people might be infectious 1 to 3 days before they develop symptoms.2
The difference between asymptomatic and pre-symptomatic is not just semantic. Stating that there is a large fraction of “asymptomatic cases” is misinterpreted as an indication that “large swathes of the population” have been infected, infected others and become immune. This hyperbole implies a level of herd protection which is at odds with modelling suggesting that, as of 28 March 2.7% (95% credibility estimate 1.2%-5.4%) of the United Kingdom population had been infected.3
We need better empirical evidence about the proportions of people who remain asymptomatic during the course of SARS-CoV-2 infection and of those who are pre-symptomatic. Through a living systematic review (https://osf.io/9ewys/), we will publish and update the evidence as it emerges. Accurate communication about the findings of this review should improve public understanding about the definitions, frequency and implications of asymptomatic and pre-symptomatic SARS-CoV-2 infection.

1. World Health Organization. Report of the WHO-China Joint Mission on Coronavirus Disease 2019 (COVID-19). Available at https://www.who.int/docs/default-source/coronaviruse/who-china-joint-mis... (Accessed 03.04.2020)
2. We WE, Li Z, Chiew CJ, et al. Presymptomatic Transmission of SARS-CoV-2 — Singapore, January 23–March 16, 2020. MMWR Morb Mortal Wkly Rep. ePub: 1 April Estimating the number of infections and the impact of non-pharmaceutical interventions on COVID-19 in 11 European countries2020. DOI: http://dx.doi.org/10.15585/mmwr.mm6914e1 (Accessed 03.04.2020)
3. Flaxman S, Mishra S, Gandy A, et al. Report 13 – Estimating the number of infections and the impact of non-pharmaceutical interventions on COVID-19 in 11 European countries. Available at https://www.imperial.ac.uk/media/imperial-college/medicine/mrc-gida/2020... (Accessed 03.04.2020)