If we dont know the long term effects of the covid vaccine, how can we know its safe?

Predictions are very hard, especially about the future (Berra). Have no clue if longterm followup will reveal bad outcomes, but for the present, best to get the vaccine, as a severe case of COVID-19 could prevent surviving into the future.

Deal with reality. We have more data on these than many we developed in the past gathered in a short amount of time. In his memoir Ben Franklin wrote he regretted not getting his son the new cowpox vaccine shown to prevent smallpox. He lost his son to that disease. If you wait 20 years to collect all the data you will likely succumb to the disease or be killed in a car wreck before its available.

How can you tell that the Sun will come out tomorrow? From what we know, and the experts know, there is no indication that there will be long term adverse consequences, based on the constituents of the vaccine. Another cliche: Do not let perfect be the enemy of good. Get the vaccine, I did.