Authors: Caitlin Owens
A new preprint study that raises concerns about the mRNA vaccines’ effectiveness against Delta — particularly Pfizer’s — has already grabbed the attention of top Biden administration officials.
What they’re saying: The study found the Pfizer vaccine was only 42% effective against infection in July, when the Delta variant was dominant. “If that’s not a wakeup call, I don’t know what is,” a senior Biden official told Axios.
Driving the news: 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.
Why it matters: Although it has yet to be peer-reviewed, the study raises serious questions about both vaccines’ long-term effectiveness, particularly Pfizer’s.
- It’s unclear whether the results signify a reduction in effectiveness over time, a reduced effectiveness against Delta, or a combination of both.
- “Based on the data that we have so far, it is a combination of both factors,” said Venky Soundararajan,a lead author of the study. “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.”
- He added that his team is working on a follow-up study that will try to differentiate between the durability of the two vaccines and their effectiveness against Delta.