How many doses of a COVID-19 vaccine will I need? What is the time period between the initial vaccine and the 2nd vaccine shot/booster?

Both vaccines will require two primary doses.  An initial vaccination and then a second dose either three or four weeks later. The Pfizer vaccine requires a booster 21 days later and the Moderna vaccine requires a second dose 28 days later.  The different vaccine products are not to be interchangeable. The second dose must be completed with the same vaccine brand as the first dose.  Both doses are important to ensure full protection.