(Bloomberg) -- A third dose of messenger RNA Covid-19 vaccine provides a key boost to immunity against the coronavirus, regardless of the original type of immunization, researchers said. 

An mRNA booster following an initial course of two shots of the same type is the most effective way to prevent non-severe Covid infections, according to an analysis of studies published Wednesday in the BMJ medical journal. Adding a third mRNA shot to other primary vaccination regimens raises protection to almost the same level, the authors from the Chinese University of Hong Kong said.

The findings are yet another confirmation of the efficacy of the shots developed by vaccine partners BioNTech SE and Pfizer Inc. as well as Moderna Inc., which have become some of history’s fastest-selling pharmaceuticals. Through mid-April, Pfizer clinched $32 billion in 2022 sales contracts for its shot, Comirnaty, the company said in early May. 

“Our results imply that mRNA vaccines will continue to be the preferred vaccine type, either as primary vaccines or booster doses,” the researchers wrote. 

Getting an mRNA booster after an initial regimen of viral vector vaccines like those developed by Johnson & Johnson and AstraZeneca Plc with the University of Oxford also offered good protection, they said. 

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