The first three months of the COVID-19 epidemic: Epidemiological evidence for two separate strains of SARS-CoV-2 viruses spreading and implications for prevention strategies

Knut M. Wittkowski

In summary, there is a narrow window-of-opportunity for interventions to "flattening the curve" (reducing R0) to be successful in terms of public health: • Starting after the peak prevalence (of infections) has little effect (not shown). The curve goes down, but is not "flattened". • Starting at the peak prevalence gives the epidemic a "coup de grace", shortening its duration, albeit at the price of reducing the R/S ratio. The curve is narrower, but also not "flattened" (Fig 12) . • Starting at the peak incidence "flattens" (and broadens) the curve and may reduce the number of deaths prevented during the current epidemic, unless o they increase the proportion of high-risk (elderly) people in the susceptible population at risk of becoming infected (after the children are locked away) or o they cause behavioral adaptations that increase the contacts of high-risk people with infectious people (such as grandparents taking care of un-schooled children while the parents are working, e.g., in hospitals), but reduces herd immunity and, thus, the chance of another epidemic coming sooner ( Fig 13) . To prevent a rebound, however, the intervention needs to be extended for several months (Fig 14) . • Starting before the peak incidence "flattens the curve", but also broadens it and causes a rebound, unless the intervention is continued for many more months (Fig 15) .