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The efficacy of anti-COVID-19 vaccines: a study on American veterans

Current vaccines, notably the Janssen vaccine, and the Moderna and Pfizer-BioNTech vaccines which use mRNA technology, are effective in the prevention of symptomatic COVID-19. These vaccines also reduce infection by the SARS-CoV-2 virus. In common with other countries, the United States began its vaccination campaign at the end of 2020. But over time, and because the Delta (Indian) variant, which can partially evade antibody responses, had become dominant, the protection provided by the vaccines had weakened by the summer of 2021. Researchers in California have analysed this lowering of protection.

The researchers used data on infections and deaths connected to SARS-CoV-2 from more than 780 000 American army veterans during the period from the 1st of February to the 1st of October 2021. This period covered the appearance of the Delta variant which became dominant. It should be noted that the type of vaccine varied with age: the youngest patients had received more Janssen vaccines while older age groups had predominantly been given an mRNA vaccine.  

The scientists observed that the vaccine’s efficacy diminished over time, and this was true for all types of vaccine: effectiveness against infection by SARS-CoV-2 fell from 88% to 48%. This was particularly the case with the Janssen vaccine.

But while the efficacy of the vaccines diminishes, they nevertheless protect against death, even though the scientists did note differences according to age and the type of vaccine: there was an average of 82% efficacy for people younger than 65 years, against 72% for those older than 65. The Janssen vaccine gives less protection, with 73% efficacy for those younger than 65 years, and 52% in people older than 65 (against 84% and 70% for the Pfizer-BioNTech vaccine).

In conclusion, the protection acquired through vaccination diminishes over time, and the Delta variant does partially escape immune responses. These phenomena have been observed in various countries, notably in Israel, where the population was massively and rapidly vaccinated. In the light of all this data, the FDA (the US Food and Drink Administration) began to authorise the administration of a 3rd dose to reinforce immunity and give improved efficacy against the Delta variant from September 2021.

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