🧡US cases are declining significantly. Should we be surprised? No. New infections arise from this formula for R:
R = D x O x T x S
D = Duration (time infectious)
O = Opportunities (contacts/day)
T = Transmission (probability infect contact)
S = Susceptible (proportion of popn)
2/ One of those factors, S, has been changing significantly over the past year because the US has had many more infections per capita than Canada.
In fact, previously infected are now about 30% of the population. Vaccinated (2 dose) are now ~ 6%.
3/ If we now go back to R = DOTS, we get
R = DOT x (1 - .3 -.06)
If DOT stays about the same, then R drops to 64% of what is was when everyone was susceptible. An original R of 1.25 becomes .8. A "new R" of .8 is going to drive cases down because 100 infections cause 80 new ones.
4/ What about the B117 variant that has a much higher transmission rate? Modelling suggests that even it might only muster a small new wave in the US. Because increasing population immunity means a lower R.
5/ What is the learning for Canada? This is the future but it is NOT now. Most of our population remains susceptible. Our 830K total cases are perhaps 1/4 of those infected (3.3M). We now have 415K vaccinated. Those add up to 10% of our popn. So 90% are still susceptible.
6/ At 90%, an R of 1 will drop to .9. That may help a little but the variant has a 40-50% higher "T". We are far behind the US on popn immunity. That's why we need to continue reducing "O", opportunities, by limiting contacts until vaccinations reduce "S" enough.
7/ The timing of variant growth and slower vaccination growth in Canada have given the virus an opportunity for one last "perfect storm" third wave. We can outsmart it by doing what we do better than the US - working together to limit contacts. /end
#Covid19 #CovidZero #cdnpoli

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