Truth to be told, I have gotten pretty bored with Covid. But things are picking up again, so maybe it makes sense to write down some idle speculation. Interestingly, we have mostly learned so far that we don’t have much of a clue of what’s going on. One of the reasons seems to be, that there isn’t really a binary distinction between „still vulnerable“ and „immune“. Instead there seems to be a spectrum of immunity.
If you just had a full blown infection, you are probably 99.9% immune. Reinfection might happen occasionally, but seems to be quite rare. But if you did not have a full blown infection, you might still have been exposed to the virus in small doses. This exposure already leads to some protection and when the day comes and you get enough viral particles up your nose to start the illness, you will have a milder case than without earlier exposure. If you have never been exposed to Covid19, you might have some protection because of earlier infections with other Corona-viruses. Karl Friston calls these phenomena „immunological dark matter“.
In the current second wave the case fatality rate is significantly lower than in spring. There are several possible reasons: One is the „dry tinder“ effect, which postulates that there is a certain number of old and frail people who are ready to go and are just waiting for the next severe flu. If the flu season has been mild, dry tinder accumulates and when Covid comes along it burns up. Now, we have burned through the tinder and death rates will stay low. In the summer lull this was a popular argument by anti-lockdown activist. They combined it with the claim that we had basically already reached herd immunity.
Unfortunately full herd immunity is unlikely for most places, as even cities like Madrid that have been hit hard in the first wave have strong second waves now. And without herd immunity it seems very unlikely that we have actually burned through more the 20-30% of the dry tinder. More likely reasons for the lower case fatality rate is increased testing, partial immunity by minor exposure and better treatment protocols.
The increased testing part is a bit scary, because it implies that currently we are slowing the growth rate by testing, diagnosing, isolating and tracking. This seems to be a benefit that will run out if the number of cases becomes too high. So at a certain point we might see an acceleration when testing and isolating happens for a much smaller fraction of cases than right now.
Karl Friston predicts that the second wave will kill far fewer people. Given that he is some 30-50 IQ points smarter than the average epidemiologist, he is well worth listening to. Unfortunately this might well hold for his native UK, but it obviously doesn’t hold for Eastern Europe and I doubt it automatically holds for Germany. Incompetence in the spring might be the best protection in the fall.