The COVID Vaccines From Pfizer and Moderna May Be Less Effective Against the Delta Variant, as New Data Suggests

The COVID Vaccines From Pfizer and Moderna May Be Less Effective Against the Delta Variant, as New Data Suggests

The new Delta strain of the SARS-CoV-2 virus is still ravaging the world. The current majority of the COVID infections from the US are mostly caused by the Delta variant.

Thanks to Yahoo! News, we know that a new preprint study raises concerns regarding the effectiveness of the mRNA vaccines against the Delta variant of COVID. Pfizer’s vaccine is particularly raising concerns.

Pfizer’s vaccine was only 42% effective against COVID infection in July

The new study found that the coronavirus vaccine developed by Pfizer had shown only a 42% effectiveness against COVID infection back in July when the new Delta strain was dominating.

As a senior Biden official said for Axios, as cited by Yahoo! News:

If that’s not a wakeup call, I don’t know what is.

The new study compared the effectiveness of the vaccines developed by Pfizer and Moderna within the Mayo Clinic Health System between January and July.

Although the effectiveness of the two vaccines dropped significantly in July, things look better if we look at the overall stats.

Credit:, Gerd Altmann
Credit:, Gerd Altmann

Pfizer’s vaccine was 76% effective against COVID infection, while Moderna’s was 86% effective. As when it comes to preventing hospitalizations, things also look in Moderna’s favour. While Pfizer’s drug reached effectiveness of 85%, Moderna had 92% effectiveness.

The study is yet to be peer-reviewed. At this point, it’s not clear whether the results are warning about a reduction in vaccine effectiveness over time, a reduced effectiveness against the Delta variant, or even a combination of both things.

Venky Soundararajan, who is a lead author of the new study, declared as quoted by Yahoo! News:

Based on the data that we have so far, it is a combination of both factors,

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.


Even since he was a child, Cristian was staring curiously at the stars, wondering about the Universe and our place in it. Today he's seeing his dream come true by writing about the latest news in astronomy. Cristian is also glad to be covering health and other science topics, having significant experience in writing about such fields.

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