Studies suggest the Johnson & Johnson COVID-19 vaccine is very effective at protecting against hospitalization and death. But because just 14 million U.S. residents have received the J&J shot — compared to more than 160 million for the mRNA vaccines from Pfizer and Moderna — scientists don’t have as much information on the strength of immunity after vaccination or whether a booster shot is safe or effective. Here’s what they do know, so far.
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