Can I be immune after I got the the 1st dose of covid - 19 vaccine? ?

You would have to be fully vaccinated to gain any immunity.

If it's 50 percent effective then you still have some risk of getting covid 19. But your symptoms may not be as sever as if you didn't get the vaccine. 

You should continue to wear masks around people that haven't been fully vaccinated. 

You could be "immune" prior to getting the vaccine.

It's estimated that about 50% of American's are naturally immune. 

Apparently, you haven't been watching the news.  There are many people in the USA right now who have had their vaccinations but who have contracted Covid anyway.  

We aren't invincible after the vaccines.

You like to live dangerously, right?  Taking the Chinese vaccine?  You could end up with some serious disability.  Chinese vaccines are so notorious that even Chinese subjects, in spite of their controlled government Media, resist taking them.

You may still get the virus even after getting vaccinated. Continue wearing masks even after getting both doses of the vaccine

HAve nont heard of Sinovac.  Must be a new one.  If it is a two-dose cavvine and if it is only 50% effective, then of course you can still get the virus after one dose.