If antibodies are low or zero after taking 2 doses of covid19 sinopharm vaccine, when is it safe for my dad, 78, to take another more effective vaccine such as astrazeneca?

There is no data on this. I would check with your local medical school and sino pharm to see if they are doing such a study. Take part and we will all be better off. Also notify Sino anc ask their advice.

If the antibody levels are low, your Dad is not adequately protected against the disease. Since there is no large scale data, please discuss with your physician and the companies.

Low as defined in what test ? If your dad was tested by the simple Covid antibody test he would not be expected to show them. The vaccines are a separate antigen/antibody system and unless he was tested for the antibodies to the specific vaccine the reported result would be negative. The vaccines do not introduce the covid virus, just a synthetic material that mimics the spike protein.