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The delta variant of SARS-CoV-2, the virus that causes COVID-19, first emerged in India and has since become a global concern. To be considered a variant, a virus must have sufficient mutations to change a portion of its genetic code. The delta variant carries seven mutations. A recent report describes the effectiveness of the Pfizer-BioNTech and AstraZeneca vaccines against the delta variant.

The Pfizer-BioNTech vaccine is mRNA-based. It contains the genetic instructions for synthesis of a single viral protein that stimulates the immune system to make antibodies against the SARS-CoV-2 spike protein. mRNA vaccine technology allows rapid scaling of vaccine production and facilitates modification if the virus mutates significantly. The AstraZeneca vaccine is adenoviral vector-based. It uses a modified version of the chimpanzee adenovirus (ChAdOx1) to deliver the genetic information that codes for the SARS CoV-2 spike protein. Adenovirus vector vaccines are currently being tested for numerous infectious agents, ranging from malaria to HIV. Both the Pfizer-BioNTech and AstraZeneca vaccines require two doses for optimal immunity.

The authors of the report took two approaches to gauge vaccine effectiveness against the delta variant versus the alpha variant (the first SARS-CoV-2 form). First, they compared vaccination status in people who had symptomatic COVID-19 with vaccination status in people who reported symptoms but had a negative test. Next, they estimated the proportion of people with cases caused by the delta variant relative to those caused by the alpha variant, based on vaccination status. Nearly 80,000 people were included in the analysis.

The authors found that the effectiveness of both vaccines was lower against the delta variant than against the alpha variant, but the Pfizer-BioNTech vaccine was at least 88 percent effective against delta after two doses, and the AstraZeneca vaccine was 67 percent effective after two doses. Notably, both vaccines were only 30 percent effective against the delta variant after only one dose.

These findings indicate that the Pfizer-BioNTech and AstraZeneca vaccines against COVID-19 are highly effective against the delta variant and underscore the importance of obtaining both doses for optimal immunity. Learn more about COVID-19 vaccines in this clip featuring Dr. Roger Seheult.

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