I did not compute the odds precisely before writing up that section.
The two cases cited have attack rates of 53⁄61 and 104⁄122.
For a 25% cross immunity rate that would correspond to ~2% and .4% probability respectively.
For a 33% cross immunity rate that would correspond to a .02% and a .0001% probability respectively.
The actual claim of what percentage of the population is immune is fairly nebulous but anything beyond 25% would be hard to justify.
I did not compute the odds precisely before writing up that section.
The two cases cited have attack rates of 53⁄61 and 104⁄122.
For a 25% cross immunity rate that would correspond to ~2% and .4% probability respectively. For a 33% cross immunity rate that would correspond to a .02% and a .0001% probability respectively.
The actual claim of what percentage of the population is immune is fairly nebulous but anything beyond 25% would be hard to justify.