[deanhills]

It is a good thing to have health checkup before taking the vaccine. But still, my point is that sometimes it is difficult to judge who is absolutely unsuitable for taking vaccine.

Because there are always some people who are in marginal conditions. Say they may appear to be healthy enough but actually they feel sick inside.

I totally agree with this. Even with a checkup before, it will always be hard to predict how the immune system of any given person will respond to the vaccine. But collectively however, it must be working on enough people to be able to slow down the aggressive spread of the virus for the whole of the population, so that health systems can survive the pandemic. I guess those who aren't good candidates for the vaccine are probably regarded as collateral damage. It's just not loudly said.