If you got the Johnson & Johnson vaccine as your first COVID-19 shot, a booster dose of either the Moderna or Pfizer-BioNTech vaccine apparently could produce a stronger immune response than a second dose of J&J's vaccine. That's the finding of a highly anticipated study released Wednesday. And if you started out with either Pfizer or Moderna, it probably doesn't matter that much, the research suggests, as long as you get one of the two mRNA vaccines as a booster.