If I'm interpreting this series of graphs correctly, it's also assuming that social contact is uniformly distributed across the population (i.e. every person ha

If I'm interpreting this series of graphs correctly, it's also assuming that social contact is uniformly distributed across the population (i.e. every person has an equal number of daily contacts). The less true that is, the fewer people need to be infected for herd immunity effects to become significant. We don't have good numbers on what the real distribution is, but there is growing evidence this is a significant effect.