How many people tried them out:
Moderna: trials included more than 30,000 volunteers across the US.
Pfizer: trials included more than 43,000 volunteers across six countries: the US, Argentina, Brazil, Turkey, Germany, and South Africa.
Johnson & Johnson: trials included more than 43,000 volunteers across eight countries: the US, Argentina, Brazil, Chile, Colombia, Mexico, Peru, and South Africa.
About half of the volunteers in each company's study got real shots, and half got fake (placebo) jabs, to test out how well the vaccines work, compared to nothing.
How they work:
Moderna: mRNA.
Pfizer: mRNA.
Johnson & Johnson: adenovrius.
How you take them:
Moderna: 2 shots in the arm, given 28 days apart.
Pfizer: 2 shots in the arm, given 21 days apart.
Johnson & Johnson: 1 shot in the arm. 
© Hugh Hastings/Getty Images A nurse at the Royal Cornwall Hospital prepares to administer a COVID-19 vaccine in Truro, United Kingdom. Hugh Hastings/Getty Images
How long it takes for protection to kick in:
Moderna: some protection from infection begins 10-14 days after first shot, with full protection achieved two weeks after the second shot.
Pfizer: some protection from infection begins 10-14 days after first shot, with full protection achieved one week after the second shot.
Johnson & Johnson: some protection from infection begins as early as 14 days after the shot, with full protection measured 28 days after the jab. That protection may get even better as time goes on, though, as zero COVID-19 cases were reported in patients who'd been vaccinated for 49 days or more.
How long hospitals and clinics can store it in the fridge
Moderna: 30 days.
Pfizer: 5 days.
Johnson & Johnson: 3 months.