Infectious disease experts are weighing the need for booster shots of the Pfizer/BioNTech or Moderna mRNA-based vaccines for Americans who received Johnson & Johnson's one-dose vaccine due to the increasing prevalence of the more contagious Delta coronavirus variant.

A few say they have already done so themselves, even without published data on whether combining two different vaccines is safe and effective or backing from US health regulators.

Canada and some European countries are already allowing people to get two different Covid-19 shots.

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