Americans have had two extremely similar authorized COVID-19 vaccines since December: one mRNA vaccine from Pfizer, and another mRNA vaccine from Moderna.But now, there is another coronavirus shot authorized for use across the US: Johnson & Johnson's adenovirus jab, which got a green light from the US Food and Drug Administration on Saturday after an expert committee unanimously recommended on Friday evening that it should receive emergency-use authorization.In trials around the world, J&J's shot was shown to be 66% effective at preventing coronavirus infections altogether, and 85% effective at preventing severe COVID-19 cases, when given four weeks to take effect.Taken...