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Fast initial Covid-19 response means greater caution may be needed later

As the Covid-19 pandemic unfolds it is becoming increasingly clear that the strength of the first wave of the epidemic varies significantly between countries. In this study a simple numerical model is used to illustrate the impact the timing of initial measures against Covid-19 has on the first wave of infection and possible implications this may have for the measures taken as the first wave is ebbing. The results highlight that delaying measures by 10 days is sufficient to largely account for the differences seen between countries such as the UK and Germany for the first wave of infections. A pronounced first wave means that a larger fraction of the total population will have been infected and is therefore likely to display immunity. Even if this fraction is far below the level needed for "herd immunity" the effective reproduction factor Re is decreased compared to a population that had no prior exposure to the virus. Even a small reduction in Re can have major influence on the evolution of the epidemic after the first wave of infections. A large first wave means the resulting value for Re will be lower than if the first wave was mild. Without either vaccine or effective treatment countries who experienced a small first wave should therefore relax measures at a slightly lower pace than countries where the first wave was strong.