Pfizer’s vaccine efficacy dropped to 42% while Moderna’s was at 76% when the Delta variant became the dominant strain

Pfizer’s coronavirus vaccine may not be as good at preventing infection with the Delta variant as Moderna’s, according to a new preprint from the Mayo Clinic and analytics company nference.

The study, which has not yet been peer reviewed, tracked the effectiveness of the mRNA vaccines between January and July.

The study used PCR testing data from the Mayo Clinic Health System’s sites in Minnesota, Wisconsin, Arizona, Florida, and Iowa, to look for breakthrough coronavirus cases among 35,902 people fully vaccinated with Moderna and 37,573 people fully vaccinated with Pfizer. A majority of the people in the study were also over 60.

Over the course of the study in Minnesota, Moderna’s vaccine was found to be 86% effective against a COVID-19 infection while Pfizer’s was 76% effective. Both were also highly effective against hospitalization (Moderna 91.6%, Pfizer 85%), ICU admission (Moderna 93.3%, Pfizer 87%), and death from COVID-19 (no cases found).

But what has caused concern is the data from July, when the Delta variant became the dominant strain in the US – and accounted for more than 70% of cases in Minnesota.

During that month in Minnesota, Pfizer’s effectiveness against COVID-19 infection dropped to 42% while Moderna’s dropped less dramatically to 76% effectiveness.

An unnamed senior Biden official told Axios of the study: “If that’s not a wakeup call, I don’t know what is.”

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