Boustrophedon wrote: ↑Thu Dec 17, 2020 1:06 am
Any update on this? What impact will the vaccine have on R0?
That depends on which vaccine you are using. In theory, a vaccine must protect over 1/R0 of the population to cause a disease to die out where "protect" means that a vaccinated person cannot infect anyone else. From
https://en.wikipedia.org/wiki/Basic_reproduction_number we find that COVID-19 has R0 >= 3.28. That means a vaccine must protect 70% of the population. That can be a vaccine which is 70% effective administered to everyone, or a 90% effective vaccine administered to 78% of the population, or a 95% vaccine administered to 74% of the population, or a 100% effective vaccine administered to 70% of the population. Note that getting R < 1 is very often stated as a goal, but that is not really adequate. If you get R=0.99 and infection takes two days (i.e. the 1000 cases turns into 990 after two days) then it takes just over 19 weeks for the number of cases to be halved. However, if you can get R down to 0.5, then the number of new cases halves every two days.
Of course that's an over-simplification for real-world use, because we can use vaccination and social distancing simultaneously, thus reducing the required net effectiveness. Based on what I have seen happening so far, what will happen as more and more people get vaccinated is that people (quite likely including those not yet vaccinated) will use this as an excuse to relax measures to the extent that we see a succession waves of infection as people relax, drive up infections, panic and get stricter, driving down infections, and then get complacent and we're back to relaxing.
And just to add more complexity, preventing someone being infected, infectious, or sick are three different things. A hypothetical vaccine which prevented someone getting sick would still be very useful even if it provided no barrier to being infectious - in principle we could administer it to the whole population and then the disease woul be harmless so we could ignore it and let anyone catch it. This is, of course, extremely unlikely as the COVID-19 vaccines are designed based on how infection works, so are very likely to prevent infection or infectiousness.