AstraZeneca Scientist Says Delta Variant Makes Herd Immunity Impossible(businessinsider.com)

    > Erin Mordecai, an assistant professor of biology at 
    Stanford University, agreed with Riley about the 
    miscalculation, and cautioned against the comparison. 

    > She said it’s impossible to accurately calculate a death 
    rate for each group — vaccinated versus vaccinated — 
    because demographics skew the data. 

    > “Those breakthrough cases may not be an average subset 
    of the population, they may be a more at risk group that's 
    more likely to have severe disease anyway,” Mordecai said. 
    “To better understand what's going on with the relative 
    risk, you would need to know the demographics of the 
    underlying health conditions of the people who experienced 
    those breakthrough cases and deaths.”