so,

1) while existing immunity from the 2003 sars can potentially defeat weak cases of covid-19,

2) the new virus is different enough that the immunity is very partial and really reliant on good luck. it would have to be very marginal transmission. but, that's the bulk of it, right?

3) however, it remains to be seen if existing immunity may lead to quick synergy with newly produced antibodies; that is, while existing immunity to the old sars won't beat the new virus, it very well might help dramatically. and that might explain what we're seeing in these small, isolated southeast asian countries.