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A study from Canada showed a single AZ dose was 82% effective in preventing hospitalization and death among the elderly against the Gamma variant, while a Brazilian study with the first dose of the same vaccine showed a 55 - 61% efficacy against hospitalization and death among the elderly for the same variant. So, which one is it?
The hidden factor likely at play here: control groups of the unvaccinated are increasingly seropositive. Even if you screen everyone who had a positive test, that still leaves 50 - 95% of people who actually had Covid as a part of your control group depending on the country. And since both vaccine and convalescence offer high levels of protection, that will lead to a downward bias in VE estimates.
Recent data shows the convalescent were 84% protected in SA during the Beta variant wave, and in the UK the chance of reinfection for Delta compared to Alpha is just 1.5x higher (so, 85 - 92% instead of 90% - 95%).
