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Two doses needed to combat the Delta variant effectively

Since March 2021, India has suffered an explosion in cases of COVID-19, with more than 400 000 new infections and 4 000 deaths per day at the height of the crisis at the beginning of May. Genome sequencing of the virus uncovered a new strain, the Delta (B1.617 or Indian) variant which, since May, has been detected in 43 countries across all continents. The most affected country has been the UK, with its large Indian community and close travel connections; previously, the dominant strain here was the Alpha (B1.1.7 or UK) variant. Vaccines against this latter strain of SARS-CoV-2 seem to protect against symptoms of COVID-19. However, there is little actual data on their effectiveness. UK researchers at the London School of Hygiene and Tropical Medicine have carried out evaluative studies on the Pfizer-BioNTech and AstraZeneca vaccines.

They carried out 2 types of analysis using UK data. Firstly, they analysed the vaccination status of people having been infected by COVID-19 who had had symptoms. This approach allowed a reduction of bias related to testing access. They also estimated the number of cases arising from the Indian variant, compared to the dominant circulating strain (the Alpha variant), and according to the vaccination status of individuals. Data relating to the 2 vaccines were analysed: the Pfizer-BioNTech vaccine, based on mRNA technology, and the AstraZeneca, a viral vector vaccine.

The scientists firstly considered the vaccines without distinguishing between them: it seemed that after the 1st dose, efficacy was significantly less against the Delta variant (31% per cent effectiveness) as against the Alpha variant (49% effectiveness). The figures were similar for the Pfizer-BioNTech and AstraZeneca vaccines. After the 2nd dose, the difference in efficacy between the 2 strains was reduced: globally, effectiveness is 80% against the Delta variant and 87% against the Alpha variant.

If we distinguish between the 2 vaccines, the effectiveness of the Pfizer-BioNTech vaccine is 88% against the Delta variant, and 94% against the Alpha variant. The AstraZeneca vaccine is globally less effective, with, respectively, 67% and 74% efficacy. In a context where the Delta variant seems to be more transmissible (due to mutations in the spike surface protein which render this strain’s replication more efficient) and has become dominant in numerous countries, these results appear reassuring.

The Pfizer-BioNTech and AstraZeneca vaccines seem effective against the Delta variant after 2 injections. The loss in effectiveness of these 2 vaccines against the Delta variant, compared with the Alpha variant, is weak after 2 doses, but between 12 and 19% after just one dose. These results demonstrate the importance of being fully vaccinated, especially for vulnerable people.

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