New Delhi: People who got Johnson & Johnson Inc’s COVID-19 vaccine as a first shot had a stronger immune response when boosted with vaccines from Pfizer-BioNTech or Moderna, a study run by the National Institutes of Health showed.

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. Moderna’s and Pfizer’s vaccines are based on messenger RNA while J&J’s uses viral vector technology.

These three groups were each divided into three new groups to receive one of the available vaccines as a booster. The nine groups consisted of about 50 people each. Researchers then analyzed antibody levels 15 days after the booster shot.

For people originally inoculated with J&J, antibody levels were four times higher after a J&J booster, 35 times higher after a Pfizer booster and 76 times higher after a Moderna booster.

And antibody levels for those who had originally received Moderna shots were higher “irrespective of the booster vaccine administered,” when compared with those who had initially received Pfizer or J&J, the study said.

Additionally, “no safety concerns were identified” after booster doses were administered, it found.

The study, which has not yet been peer-reviewed, has several limitations, however.

The number of participants was small, and the immune response could evolve over time, beyond the 15 days observed during the study.

“Important not to get too carried away with the findings,” tweeted Peter Hotez, a professor at Baylor College of Medicine.

Results from trials on a second J&J booster shot conducted by the company itself were “impressive,” he said.

The NIH study should fuel discussions by a US Food and Drug Administration (FDA) expert committee, which is scheduled to consider applications for a booster dose from Moderna and J&J on Thursday and Friday, respectively.

(With inputs from AFP)