People who got Johnson & Johnson COVID-19 vaccine as a first shot had a stronger immune response when they boosted it with the Pfizer or Moderna vaccine, a study by the National Institutes of Health showed Wednesday. The study, which is preliminary and hasn’t been peer reviewed, is the latest challenge to J&J’s efforts to use its COVID-19 vaccine as a booster in the United States. The study, which included more than 450 adults who received initial shots from Pfizer, Moderna, or Johnson & Johnson, showed that “mixing and matching” booster shots of different types is safe in adults. The Moderna and Pfizer vaccines are based on messenger RNA, while J&J’s uses viral vector technology. The finding comes as an advisory group to the U.S. Food and Drug Administration prepares to meet later this week to discuss the merits of a booster shot for Moderna and J&J vaccines. FDA officials on Wednesday said J&J’s regulatory submission for its booster raised red flags such as small sample sizes and data based on tests that had not been validated. U.S. health officials have been under pressure to offer advice on booster doses of the J&J and Moderna COVID-19 vaccines since the White House announced in August that it planned to roll out boosters, beginning last month, for most adults. The NIH study contrasted the safety and immune responses of volunteers who were boosted with the same shot used in their initial vaccination with those of volunteers who received a different type of shot as a booster. Mixing and matching doses for a booster produced side effects like those seen in primary inoculations and raised no significant safety concerns, the study said. The study of the three COVID-19 vaccines authorized in the United States showed that using different types of shots as boosters generally appeared to produce a comparable or higher antibody response than using the same type. The trial took place in 10 U.S. cities and used a total of nine combinations of initial shots and boosters. Mixing booster doses “may offer immunological advantages to optimize the breadth and longevity of protection achieved with currently available vaccines,” researchers wrote in the study.