If you are a passenger in an uninsured driver's vehicle does that mean that you are not covered either if he has an accident?

If you mean are your medical bills covered -- and assuming your state requires medical coverage on auto insurance policies -- then, no, you wouldn't be covered by any policy of the driver's since, of course, no policy exists.

However, if you have your own auto policy, that policy would then become primary. After that, your health insurance would be primary.

Unfortunately for the uninsured driver, if you do have to go through your own health insurance, there's a big possibility that they will go after the driver for any payments they make, particularly if a driver is legally required to carry auto insurance in your state (again, insurance that included medical coverage).