Covid-19: Is the Pfizer vaccine less effective than Moderna’s against the Delta variant?

Researchers from the American federation Clinic Mayo have reportedly shown that the Pfizer vaccine would be less effective against the Delta variant than that of Moderna. The report of this study was published on 8 August on the MedRxiv platform, but the results have yet to be confirmed.

42% efficacy only against the Delta variant

To carry out this study, the team of scientists analyzed data from 50,000 vaccinated and unvaccinated patients over the period from January to July 2021, corresponding to the spread of the Alpha variant (or English), then that of the Delta variant (or Indian).

They first realized that the two messenger RNA vaccines were overall very effective against the Alpha variant: 86% effectiveness against infection for Moderna and 76% for Pfizer, and respectively 92% and 85% against the risk of hospitalization.

On the other hand, with the appearance of the Delta variant, the results are not at all the same. In July, the Pfizer vaccine had only a 42% efficacy against infection (but 75% against hospitalization anyway), while the Moderna vaccine remains 76% effective.

Results to take with caution

This study could revive lively debates around the effectiveness of vaccines, but the researchers make it clear that these data need to be further deepened and confirmed. And several reasons could explain this difference in effectiveness between the two vaccines, yet based on the same technology. Venky Soundararajan, the study’s lead author, explains:

On the basis of the data available to us so far, this is a combination of the two factors. The Moderna vaccine is probably – most likely – more effective than the Pfizer vaccine in regions where Delta is the dominant strain, and the Pfizer vaccine appears to have a lower efficacy durability,

A difference in dosage between the two vaccines could also explain these results: one injection of Moderna is stronger than one of Pfizer.

But epidemiologist Mircea Sofonea notes, however, that the serological status of patients was not taken into account in this study: “This means that, in the group of unvaccinated, it cannot be excluded that some have been previously in contact with the virus and have thus [already] developed immunity,” he stresses.

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