Fig. 1.

Hypothetical representation of phases (A-G) of surveillance and control of COVID-19 at human population level. The dashed red curve represents the true prevalence (TP) of infected people that could be estimated by active random surveillance to inform about the “true” population infection status in real time. A) Surveillance surveys are addressed to initial detection of disease, and if no cases are found, the aimed confidence in freedom PFree could be reached (confidence to be below design prevalence Pu). B) Disease is known to be circulating in the population and random surveys are addressed to estimate the TP and avoid reaching the threshold prevalence (ThreTP, X1), above which the health care system could go under pressure due to high number of severely ill persons. C) Critical situation starts at the hospitals. Red vertical bar connecting ThreTP = X1 and hospitalization limit Y1 of severely ill people (red horizontal bar). D) Critical situation due to TP > ThreTP and application of “draconian” measures. E) Critical situation reduces at the hospitals due to TP = ThreTP (orange bar Y2-X2). F) Disease is still known to be circulating in the population and random surveys are addressed to estimate the TP to monitor the situation before relaxing restrictions. G) Surveillance surveys are addressed at early detection of eventual relapses of disease, and if no cases are found, the aimed confidence in freedom PFree is confirmed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)