People who have had it may be susceptible too.
We don't know if having had it means you are immune. If you are immune is that for all time - immunity might only last a few months.
This is where "the science" surrounding Herd Immunity and CV19 falls down. In a real Herd Immunity project a vaccine exists, is given to the weaker "animals", the rest are allowed to succumb and it's chance whether they survive or not - farmer/govt saves money on not having vaccinate the whole stock. Not knowing if immunity is certain or if a vaccine will ever come along whilst pursuing Herd Immunity is just russian roulette.