How to Flatten the post-lockdown epidemic trajectory

Populations are locked down during an epidemic to slow down the rate of infection so that epidemic trajectory is "flattened". This helps to keep cases at a manageable level. Given the enormous economic damage and misery caused by a lockdown, it is imperative to keep the lockdown period limited. A lockdown is useful only if it can be ensured that after the lockdown is lifted, the epidemic trajectory does not rise sharply again. We present here the results from a mathematical model of the epidemic which examines how the timing, strength and duration of the lockdown affects the post-lockdown epidemic trajectory. Our results show the following:

• A early lockdown (imposed when less than 1% of the population has been infected), of any reasonable duration, cannot prevent the return of the epidemic when the lockdown is lifted. The curve starts climbing soon after lifting the lockdown and reaches a peak of the same height as the no-lockdown curve

• The post-lockdown trajectory can be flattened only if the lockdown is imposed after about 10% of the population has recovered after infection.

• The slope of the post-lockdown epidemic curve depends on the level of immunity built up in the population before and during the lockdown period. Application of lockdown around the inflexion point of the epidemic curve (the point of maximum slope of the curve) ensures that the post-lockdown curve is also flattened.