Mixing Covid vaccines produces as much or more antibodies as using the same shot as a booster, according to preliminary results of a widely awaited US government-sponsored trial.

The trial is the first major US study to compare the effects of using different vaccines as boosters from the initial shot or shots. The complicated, 9-arm trial involved over 450 people and measured the effects from giving a booster shot of the Moderna Inc, Pfizer Inc.-BioNTech SE or Johnson & Johnson vaccines to those who had originally gotten a different vaccine.

Overall, the results found that mixing-and-matching resulted in comparable or higher levels of neutralizing antibodies compared to same-vaccine boosting, the researchers said in the preprint posted on medRxiv.org. Rates of adverse events were similar across all the different booster groups, the study found.

"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," the researchers said in their conclusion.

The results from the ongoing trial have not yet been peer reviewed and published in a medical journal. More detail about the study is expected to be disclosed Friday afternoon at a meeting of FDA advisory panel, where researchers conducting the trial are scheduled to give a presentation on their early findings.

Mixing and matching boosters has become an increasingly important issue. Many countries outside the US have used the method for in an attempt to maximize vaccine effectiveness or avoid rare side effects that have been associated with some shots.

In the US, some people who received the Johnson & Johnson shot may be interested in getting a messenger RNA shot as a booster. Also, mixing-and-matching could make it easier for officials to roll out boosters more broadly, as people getting boosters could receive any Covid vaccine on hand at their pharmacy and wouldn't have to seek out the specific shot they had gotten previously.

Outside the US, concerns about rare blood clots associated with AstraZeneca Plc's vaccine led to many countries in Europe to pause use of the shot and instead administer a different second dose. In the UK health authorities have rolled out a booster program for an extra dose of the Pfizer vaccine as the preferred option, and also offered a half dose of the Moderna shot as an alternative.