Relative Infectuousness of Asymptomatic and Symptomatic COVID-19 Infectives: Review and Quantitative Synthesis

The relative infectuousness of asymptomatic and symptomatic COVID-19 infectives is surrounded by contradictory clinical findings and confusion. This paper provides a critical review of the available clinical and virological literature, and traces the source of the present confusion. We provide a quantitative synthesis of the empirical results based on a time profile of effective infectuousness, separately for symptomatics and asymptomatics. For mild to moderate symptomatic cases we calculate that this period is 14 days on average, while for asymptomatic cases it is 9 days. Most of the replication-competent virus material is emitted during the first 4 days of this period, with few differences between symptomatics and asymptomatics. Because they shed virus over a longer interval, symptomatic infectives are likely to constitute the largest source of secondary infections. In early phases of the epidemic when medical isolation for symptomatics is not yet very effective, the relative infectuousness of asymptomatics is 0.80, versus 1.20 for symptomatic infectives. In the later phases of the epidemic, the infectuousness of asymptomatics increases in importance for overall transmission of infections: the relative infectuousness of asymptomatics and symptomatics becomes, respectively, 0.93 and 1.07. This shift in relative infectuousness is caused by two factors: the counter-intuitive fact that asymptomatics emit most virus material in a short period, while medical isolation of symptomatics often halves the much longer period during which the latter could otherwise have transmitted the virus.