[tiwil]

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.

Define what "feel sick inside" is. I don't know how the checkup procedure is (since I haven't take any vaccine yet) so I cannot comment way too far. But AFAIK there is some prerequisites to take the vaccine as I said before. Some of the prerequisites are:
1. More than 18 years old (since our vaccine "test" was for 18 years old and up)
2. Blood pressure under 180/110 mmHg
3. Haven't confirmed positive for COVID-19 in the last 3 months (I'll explain about my opinion about this prerequisite below)
4. Isn't pregnant
5. Doesn't have allergy to vaccine such as hard to breath or fever
6. Doesn't have asthma, heart disease, etc.
7. Doesn't have immune deficiency, blood clotting disorders, etc.

It's a very strict prerequisites and if one doesn't pass the prerequisites, they cannot take the vaccine. We have tested the vaccine before in phase 3 mid-2020 and got this prerequisites. Therefore for people that have any other disease still can take the vaccine as long as it's not listed there.

About the number 3: I believe we shouldn't vaccine someone that have caught COVID-19 and recovered. Their body already know how to fight the vaccine therefore it's actually not needed. But our government allows to vaccine them but only if more than 3 months just to give the body time to recall the memory they have before.