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NEW YORK -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 U.S. health regulators. Canada and some European countries are already allowing people to get two different COVID-19 shots.

The debate centers on concerns over how protective the J&J shot is against the Delta variant first detected in India and now circulating widely in many countries.

Delta, which has also been associated with more severe disease, could quickly become the dominant version of the virus in the United States, Centers for Disease Control and Prevention (CDC) Director Rochelle Walensky has warned.

There is no substantial data showing how protective the J&J vaccine is against the new variant. However, UK studies show that two doses of either the Pfizer/BioNTech or AstraZeneca vaccines are significantly more protective against the variant than one.