| Turns out a third dose of messenger RNA vaccine provides a key boost to immunity against Covid regardless of the original shot. That's what scientists from the Chinese University of Hong Kong found after scouring 53 studies that contained at least 24 different vaccine regimens. Three mRNA doses offered the best protection against infection, the researchers found, though just one of those shots as a booster after other types of Covid-19 vaccines worked almost as well. The findings are yet another confirmation of the efficacy of the mRNA shots developed by partners BioNTech and Pfizer as well as Moderna, which have become some of history's fastest-selling pharmaceuticals. "Our results imply that mRNA vaccines will continue to be the preferred vaccine type, either as primary vaccines or booster doses," the researchers wrote.  A vial of Moderna's mRNA vaccine Photographer: Michaela Handrek-Rehle/Bloomberg But the study also offers reassurance that a third mRNA shot can turbo-charge the effect of other vaccines. Specifically, the researchers said an mRNA booster after an initial regimen of viral vector vaccines like those developed by Johnson & Johnson and AstraZeneca with the University of Oxford was highly effective. The same was true after two doses of China's Sinovac Biotech vaccine. Vaccine regimens fared differently depending on whether the goal was to measure their protection against just infection, severe Covid or hospital admission. In terms of avoiding death, the scientists said they lacked reliable data. It's not the first time research points to the benefits of mixing and matching shots. But in a country like China, where the research originates, regulators have yet to approve an mRNA vaccine. — Marthe Fourcade and Clara Hernanz Lizarraga | 
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