The study, conducted by nference and the Mayo Clinic, compared the effectiveness of the Pfizer and Moderna vaccines in the Mayo Clinic Health System over time from January to July.

Overall, it found that the Moderna vaccine was 86% effective against infection over the study period, and Pfizer’s was 76%. Moderna’s vaccine was 92% effective against hospitalization and Pfizer’s was 85%.

But the vaccines’ effectiveness against infection dropped sharply in July, when the Delta variant’s prevalence in Minnesota had risen to over 70%.

Moderna was 76% effective against infection, and Pfizer was only 42% effective.

The study found similar results in other states. For example, in Florida, the risk of infection in July for people fully vaccinated with Moderna was about 60% lower than for people fully vaccinated with Pfizer.