Asymptomatic SARS-CoV-2 infection: the tip or the iceberg?

The pooled proportion of asymptomatic infection was 16% (95% CI 12% to 20%).2 Community RT-PCR testing of the entire village of Vo in Italy, at two time points, demonstrated an overall infection prevalence of 2.6% at the start of lockdown and 1.2% at the end of lockdown, with 43% of all infections being asymptomatic.3 Data from screening healthcare workers (HCWs), where unrecognised infection may have significant consequences for nosocomial transmission, suggest a low overall prevalence of asymptomatic infection. Asymptomatic HCW screening in a London hospital showed a prevalence of 7% in late march, reducing to 1% in late April.4 In Cambridge, UK, screening of 1032 HCW in April revealed 30 (3%) positive cases, of whom only 5 were considered truly asymptomatic.5 Perhaps more comparable to the contained environment of cruise ships are data from residential home outbreaks, revealing a high prevalence of infection. The cruise ship outbreak described by Ing et al raises another interesting question relating to the variability in risk of onward transmission from infected cases (the overdispersion parameter), where sometimes a high number of secondary cases arise from a presumed single source.11 Mathematical modelling has estimated that 80% of secondary transmissions may occur from as few as 10% of cases.12 If asymptomatic cases are demonstrated to contribute to these so-called ‘superspreading’ events, then any intervention in the assessment of an outbreak must be able to recognise and manage such cases. Mass indiscriminate RT-PCR testing of asymptomatic groups requires careful consideration of laboratory testing capacity and interpretation of test results in the context of non-infectious, prolonged RT-PCR positivity in convalescence.13 However, in outbreak scenarios, where a high prevalence of asymptomatic infection from point prevalence data has been repeatedly demonstrated, mass screening represents a valuable tool to identify asymptomatic (including paucisymptomatic, and presymptomatic) cases, that would otherwise go unrecognised.