Covid-19: Do many people have pre-existing immunity?
Epidemiologist here, a few thoughts reading other comments:<p>-Death data is preferable to new cases in some ways, since it doesn&#x27;t rely on testing, which ramps up&#x2F;down from locality to locality and over time.<p>-To compare deaths from the Spring with deaths recently isn&#x27;t apples to apples: we know now remdesivir and dexamethasone are helpful treatments, especially the latter, and these may help reduce mortality rate.<p>-Mortality now may be lower partly due to: if the virus killed off the more vulnerable populations (nursing home) early on, it has less frail individuals to infect now. These communities got ravaged, we had something like 40% of deaths in my area in nursing homes in our 6-week case peak.<p>-Trying to tease out immunity is very tough as well, early on there were few good tests and their specificity and sensitivity were less well known. At my agency, there were multiple tests we were collating, with different false positive and negative %s, and different reporting times (which all can influence R0, the variable number our behavioral interventions seek to tamp down).<p>-Comparing different countries is very tricky. Sweden is a favorite example for non-lockdown approach, but there we see it&#x27;s very tricky to tease out the benefits. On the one hand, they didn&#x27;t do much better epidemiologically than their neighbors. On the other hand, their economy was surrounded by locked down ones, so that may have tamped down any benefits their non-lockdown would&#x27;ve garnered due to neighbors&#x27; activities. Did it pay off? The verdict is still out, IMO.<p>Things are very complex in infectious diseases epidemiology. It&#x27;s extremely hard to know anything for certain with this virus, it&#x27;s behavioral tangled up with comorbidity with genetics and demographics and evolving treatment and viral dosage and strain of virus...it will take a few years of unpacking the mountains of data before we truly have a grasp of what happened here.