  The death toll for Covid-19 may be reduced by dividing the population into
two classes, the vulnerable and the fit, with different lockdown regimes.
Instead of one reproduction number there now are four parameters. These make it
possible to quantify the effect of the social distancing measures. There is a
simple stochastic model for epidemics in a two type population. Apart from the
size of the population of the vulnerable and the fit, and the initial number of
infected in the two classes, only the four reproduction parameters are needed
to run the two type Reed-Frost model. The program is simple and fast. On a pc
it takes less than five minutes to do a hundred thousand simulations of the
epidemic for a population of the size of the US. Epidemics are non-linear
processes. Results may be counterintuitive. The average number of vulnerable
persons infected by an infectious fit person is a crucial parameter of the
epidemic in the two type population. Intuitively this parameter should be
small. However simulations show that even if this parameter is small the death
toll may be higher than without shielding. Under certain conditions increasing
the value of the parameter may reduce the death toll. The article addresses
these blind spots in our intuition.
