They *could* be successful, at least one of the five or six they're already using. They have lab tests that people receiving the vaccine candidates produce a strong enough antibody response.
Problem is they're not waiting for testing and statistical confirmation. So, they can all be bad. Or, they can all be good.
They can also be good, but dangerous to a small population... we just don't know yet.
But they're certainly gambling on it.
"The West" has about four or five good candidates in Phase III. We have reasonable expectations for one or more to be successful. If we were China or Russia, we'd start implementing them. And that would seem like a good chance to have a successful vaccine. And also a possibility of causing harm. So, we don't take the gamble.