Breakthrough COVID-19 infection risk may differ with vaccine type, early Mayo Clinic research suggests

People who received the Moderna COVID-19 vaccine may be less likely to experience a breakthrough infection compared to Pfizer vaccine recipients, findings published Aug. 8 in preprint server medRxiv suggest. 

The retrospective study, led by researchers at Mayo Clinic Health System, compared the rates of infection among fully vaccinated people who received either the Moderna or Pfizer shot. It involved adults who were tested for COVID-19 across Mayo Clinic Health System sites in Arizona, Florida, Iowa, Minnesota and Wisconsin. 

Overall, findings showed that both vaccines were highly effective against severe infection and hospitalization. 

In July, however, researchers found Moderna's vaccine was 76 percent effective at preventing infection among those fully vaccinated while Pfizer's was 46 percent. Researchers noted that in July, cases of the delta variant became prevalent in states involved in the study. 

Between fully matched individuals fully vaccinated with Moderna or Pfizer's vaccine, researchers observed Moderna's vaccine "conferred a two-fold risk reduction against breakthrough infection," compared to Pfizer's. 

The difference was most pronounced in Florida. In July in the state, which is experiencing its worst COVID-19 case surge yet, researchers found the risk of a breakthrough infection was about 60 percent lower among Moderna vaccine recipients compared to those who received Pfizer's shot. 

"Our observational study highlights that while both mRNA COVID-19 vaccines strongly protect against infection and severe disease, further evaluation of mechanisms underlying differences in their effectiveness such as dosing regimens and vaccine composition are warranted," the study said.


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