How do we know the vaccine is safe and effective?

The Covid-19 vaccines available in the United States have been tested in clinical studies of tens of thousands of people and in real-world observational studies of millions of people. We know from these studies that the available vaccines are almost 100% effective in preventing hospitalization and death from Covid, and that they are also highly effective in preventing most Covid disease and new infections.  

The safety and effectiveness of the vaccines is constantly being evaluated.  But it’s already clear, after 100s of millions of doses, that it’s far safer to be vaccinated than to risk getting Covid, which can be deadly even to young people.

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1. I already had Covid, so why would I need the vaccine?
2. Will the vaccine protect me against variant strains of the coronavirus?
3. Are there important differences between vaccines?
4. How do we know the vaccine is safe and effective?
5. I read the vaccine is 95% effective. Does that mean there is still a 5% chance I could get Covid?
6. I am fully vaccinated. Do I need a booster shot?
7. What do we know about side effects?
8. I have doubts about the Covid vaccines because they were developed so quickly.