They are manufactured by Pfizer, Moderna and, most recently, Johnson & Johnson. Clinical trials found all three vaccines to be effective in preventing symptomatic COVID-19. The Pfizer and Moderna vaccines require two doses, with the second dose administered three or more weeks after the first. Each vaccine site is required to have on hand the medication, equipment and trained medical personnel needed to address potential serious allergic reactions. You can reduce potential discomfort at the injection site by exercising your arm prior to getting the vaccine, and by applying a cool, wet cloth to the injection site.