Why lockdown? Simplified arithmetic tools for decision-makers, health professionals, journalists and the general public to explore containment options for the novel coronavirus

Gerry Killeen; Samson Kiware

On the other hand, the predictions presented here appear relatively optimistic when compared with recent reports suggesting viral reproduction rates are higher than generally thought 65,67 because previous analyses failed to consider the likelihood that large fractions of cases may go undetected [8] [9] [10] [11] [12] [13] 21 because they exhibit only mild, non-specific symptoms, if any 1-7,9,12, [14] [15] [16] [17] [18] [19] [20] [21] As underlined right at the outset of the global response, 79,120 the most important remaining question that needs to be answered to reduce the uncertainties of model predictions is the extent of asymptomatic carriage and infectiousness. Learning lessons from other diseases like endemic malaria, which is primarily a chronic illness transmitted by semi-immune adult carriers, 121 the term asymptomatic may well be a misnomer not only because some individuals become infectious before exhibiting symptoms, 1, 3, 5, 16, 19, 22, 23, 74 but also because it is often applied to those who shrug off mild symptoms to get on with their daily lives. 8, 122, 123 A particularly important caveat arising from current uncertainty about the role of cryptic carriers is that it also has a major influence on estimation of fatality rate for infections rather than clinical cases. The latest analyses allowing for this phenomenon suggest that fatality rates are may be 10 to 40 times lower per infection than per confirmed case, 63,65 consistent with our conclusion that the vast majority of cases are never confirmed. While fatality rates are difficult to estimate directly, 124, 125 these modelling analyses support the conclusions of the most controlled empirical epidemiological studies, indicating that the COVID-19 fatality rates may be comfortably below 1%. 21 The surge of severe cases and fatalities in an uncontained epidemic may therefore peak at a far lower level than those predicted in figure 2. However, much of the variation between fatality rate estimates appears related to geographic differences in health system capacity and burden, 125,126 so low income countries will be much more vulnerable. Even if the best worst-case scenario proves to be less catastrophic than previously projected, 63,65 it will nevertheless overwhelm critical care capacity several times over and should be avoided if at all possible.