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Johnson & Johnson Covid-19 vaccine recipients are better off getting a booster shot from Pfizer or Moderna, a highly anticipated U.S. study suggested Wednesday.

The National Institutes of Health study on "mixing and matching" Covid vaccines included more than 450 adults who have received one of the three regimens currently available in the United States: J&J's, Moderna's or Pfizer's. The study, which looks at whether there are any advantages or drawbacks to using different boosters, hasn't yet been peer-reviewed, CNBC reports.

Volunteers were divided into groups and received an extra shot of their original vaccine or a booster from a different company. Antibody levels were measured two weeks and four weeks after the boosters were given.

All the combinations boosted antibody levels higher, though Pfizer's and Moderna's boosters appeared to work best. People who received a booster dose of either the Moderna or Pfizer vaccines had a higher increase in their antibody responses more often than those who received an extra dose of J&J, according to the study.

The study showed recipients of Moderna or Pfizer's original vaccines could easily swap third doses; the results were about the same. Volunteers who originally received the J&J vaccine appear to have gotten a better immune response if they got a booster made by Pfizer or Moderna.

There were no serious side effects tied to the additional shots and no new symptoms emerged after participants received the booster shots, researchers said. Two participants vomited after their boosters; one had received Moderna and the other J&J. Two other people who got a J&J booster reported fatigue or insomnia.

"These data suggest that if a vaccine is approved or authorized as a booster, an immune response will be generated regardless of the primary Covid-19 vaccination regimen," researchers wrote in the study. "Heterologous prime boost strategies may offer immunological advantages to optimize the breadth and longevity of protection achieved with currently available vaccines."