In May, the blog showed that Covid-19 has a Herd Immunity threshold of 60%-75%. In other words, 60%-75% of the population must have immunity (from prior infection and vaccination) to prevent epidemic spread of the disease.
This was based on R0, the basic reproduction number, being 2.5 for Covid-19 (green bar on this graph).
With the increase of the Alpha and Delta variants becoming the major coronaviruses in many countries, including the UK, Canada and the U.S., this has now changed.
The Alpha variant is 1.5 times more transmissible than the initial coronavirus.
The Delta variant is 1.6 times more transmissible than the Alpha variant.
This increases R0 to 2.5 x 1.5 x 1.6 = 6.0
Reading up from 6.0 on the horizontal axis of the graph we get a Herd Immunity threshold of .85. This means that 85% of the population needs to be immune. This is a big part of the explanation for the sudden spike in cases in the UK and the U.S.
The other part of the explanation is that while single vaccination rates went up sufficiently to protect against the Alpha variant, protection against the Delta variant really requires full vaccination. The immunity level of fully vaccinated people has not reached the 85% level in any of these countries. Add 10% to the numbers in the following graph to include people with immunity from prior Covid-19 infections to get the total immunity level for the population.
The other big problem is that as the number of people vaccinated increases, there is a slower vaccination rate. People with vaccine hesitancy make up a larger proportion of the people who are unvaccinated. The graph above shows this as a decrease in the slope of the line. This has occurred in the UK and very significantly in the U.S. since mid-May. Canada does not show this slowdown yet.
A simple projection of the above chart, assuming these countries maintain their current vaccination rate, shows the following results for reaching herd immunity when about 80% of the population is fully vaccinated:
Canada in August
UK in November
U.S. in January 2022
not sure but when u re-calculate the R coefficient and multiply 2.5 x 1.5 and then again x 1.6 not sure this makes sense to me…seems to me its double-counting, but I am no epidemiologist, tho u seem to be becoming one!
R of 2.5 means on average each infected person infects 2.5 other people. If the alpha variant is 50% more transmissible, then R is 2.5 x 1.5 = 3.75. In other words each infected person infects 3.75 other people.
For delta which is 60% more transmisible than alpha, then R = 3.75 x 1.6 = 6.0.