A new Mayo Clinic preprint looked at the effectiveness of mRNA vaccines between January and July.
It found Pfizer's effectiveness dropped to 42% in Minnesota in July, when Delta became dominant.
Nonetheless, vaccines help protect against severe COVID-19 cases and deaths.
Pfizer's coronavirus vaccine might not be as good at preventing infection of the Delta variant as Moderna's, according to a new preprint from the Mayo Clinic and the analytics company Nference.
The study, which has not yet been peer-reviewed, tracked the effectiveness of the mRNA vaccines between January and July. All vaccines can protect against severe COVID-19 infections and death, even in the face of the Delta variant, but the data on the vaccines' efficacy has been mixed.
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. Some 40% of the people in the study were also over 65 years old.
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%.
An unnamed senior Biden official told Axios of the study: "If that's not a wakeup call, I don't know what is."
It was not clear whether or how many of those breakthrough infections were of the Delta variant because the researchers did not sequence the breakthrough cases to see if they were caused by Delta.
The study also did not say whether the drop in effectiveness is due to vaccines losing their potency over time or because they aren't as effective against Delta, or a combination of both. It's possible the study's results might not hold up in other locations or situations, and the authors said additional research is needed.
Venky Soundararajan, a lead author of the study, told Axios he believes it's a combination of both but said his team is planning a follow-up study to answer these questions.
"The Moderna vaccine is likely - very likely - more effective than the Pfizer vaccine in areas where Delta is the dominant strain, and the Pfizer vaccine appears to have a lower durability of effectiveness," Soundararajan told Axios.
The data on these vaccines' efficacy against the Delta variant has been mixed: A UK study published last month found when patients were fully vaccinated, with both shots given at least two weeks to take effect, Pfizer's COVID-19 vaccine was about 88% effective against symptomatic COVID-19 from the Delta variant.
Pfizer and Moderna did not immediately respond to Insider's requests for comment on Thursday. Pfizer told Axios in a statement that it expected "to be able to develop and produce a tailor-made vaccine against" the Delta variant "in approximately 100 days after a decision to do so, subject to regulatory approval."
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