| | | You're asking all the right questions, and you are getting into a lot of complexities involved in this. I am a bedside doctor, I am not an epidemiologist, so keep that in mind. The R value is not a fixed constant for a given virus, but is the result of the interaction of the virus, social behaviors, and percent already immune. For example, as the number of persons immune climbs, the R gradually declines, such that when you reach the herd immunity threshold the R drops below 1. It's a gradual decline, not a quantum event.
The R value of 2.5 is what the virus displayed in virgin populations in the spring with no social distancing. The effective R in the US for the last three months has probably been between 1.1 and 1.4, as a result of social distancing and masking etc, but since it is above 1 we have inexorably built a wave that is now washing over the entire country.
The second question is actually extremely important. If we do push R below 1, that doesnt magically make the disease disappear. The prevalence of the disease is hugely important. If we reach herd immunity and at the same time there are only 100k active infections in the country, the pandemic peters out fairly quickly. With 10 day infection cycles, even if R is just .9, that would result in the total number of infections dropping about 30% per month. But if we reach herd immunity and R goes to .9, but there are 3 million active infections, it will be a long time before the pandemic burns out entirely. We can speed up the process by pushing R even lower, either by pushing the vaccinated fraction much higher, or by social distancing. Back in February, the Chinese were able to get R below .5 in Wuhan with their incredibly hard lockdowns. Those aren't really viable in the US, but it does highlight the fact that we need to do the viral suppression measures now, so that the total disease burden is also going down while vaccines are rolling out.
Third question is something we speculated about back in March. But it turns out that Arizona, Florida, and Texas also had major surges in the summer, and the experience of Brazil certainly suggests that warm humid climates with plenty of UV doesnt help much. I do think the current US wave was driven by pandemic fatigue and by cooling fall weather pushing more people indoors.
Can the baseline value of R change? Maybe, it would require the virus to mutate in such a way that an infected person sheds a lot more virus for a longer period, or the virus can enter human cells more reliably or some other such change. That typically doesnt happen. The UK strain is a mysterious at this point, I'm not even sure what the basis is for claiming it transmits 70% more, does that mean the R is 70% higher than the wild strain? Don't know.
Vaccination can get us back to normal through a second mechanism beside herd immunity. As the risk of serious illness and death is concentrated in about 50-100 million Americans (depending on how you define high risk), just getting that group largely vaccinated will make a huge difference. How important is it to vaccinate healthy people under 30, at least initially? |
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