What’s the relative risk?
Recent reports from the Victorian Department of Health find that unvaccinated people are ten times more likely to contract COVID than vaccinated people.
We also know that vaccinated people are less likely to transmit the disease even if they become infected. The Doherty modelling from August puts the reduction at around 65%, although more recent research has suggested a lower estimate for AstraZeneca. Hence for this thought experiment, we’ll take a lower value of 50%.
As the prevalence of COVID changes over time, it’s hard to estimate an absolute risk of exposure. So instead, we need to think about risks in a relative sense.
If I were spending time with an unvaccinated person, then there’s some probability they’re infected and will infect me. However, if they were vaccinated, they’re ten times less likely to be infected and half as likely to infect me, following the numbers above.
Hence we arrive at a 20-fold reduction in risk when hanging out with a vaccinated person compared to someone who’s not vaccinated.