It'll probably take at least 50% vaccination rates before we even have a chance of achieving herd immunity but at least the daily increase in vaccinations is a step in the right direction.
Dr Fauci mentioned 75%. That suggests he believes that the uninhibited
R0 of COVID-19 is 4.0, or maybe
3.8 if you allow for a vaccine efficacy of
95%. That would be the center of the range that Chinese scientists originally calculated in early 2020.
It's pretty simple arithmetic. To avoid exponential growth, you need to reduce transmission by 3.8 times to offset the natural spreadability of the virus.
The logical consequence of that is that a vaccine with only
62% efficacy (AstraZeneca) leaves you with an
Re (effective reproductive factor) of
1.44, which is a doubling of active cases after two generations of infection = very bad. This is the one that the UK and Australia are going with.
It could be worse: the 3rd stage trial results in Brazil for CoronaVAC, the vaccine from Chinese company Sinovac, came in at just
50.4% efficacy - that's nearly 10 times less effective in a population than Pfizer-BioNTech (if real life effectiveness correlates with the large clinical trials).
Of course, social distancing and masks etc must be reducing transmission by at least 3.5 times (
70%), hence the numbers are increasing but not actually exploding. So you would expect doubling of actual cases to take two months or more. It's just that the restrictions that are effective are not sustainable - economically at least.
Unfortunately, there appears to be growing acceptance that some of the new virus mutations (UK, SA, etc) really are a lot more infectious. That could push
R0 up to
6.0 or even beyond. Countries are responding with some degree of
lockdown, which can be more than 80% effective with a reasonably compliant population (that's how Australia and New Zealand eradicated earlier waves).
A lockdown may need to be
83% effective to avoid exponential growth of a virus with R
0 = 6.0. Yikes.