I’ve heard from a number of you about this post last night about updated data from Oregon. I realize that I was writing in shorthand and referencing points made or context discussed in earlier posts. So let me clarify a bit about what I’m trying to do here.
A big question in this uncanny period in the COVID pandemic is how well vaccines are holding up against the latest versions of the disease, particularly the Delta variant. But there are a host of different factors which play into the numbers we’re seeing. And by numbers here I’m mainly referencing breakdowns of key COVID metrics by vaccination status. Some are statistical, really artifacts of the statistics themselves. The most obvious is that as the percentage of the vaccinated population rises the percentage of cases among the vaccinated will also rise. That has nothing to do with vaccine efficacy. That’s an artifact of the statistics themselves.
There are various other potential factors: possible waning of the immune response to the vaccines, reduced efficacy of the vaccines against newer versions of the virus. Finally there are factors which are not directly tied to the vaccine or virology at all – most importantly, behavior. The period of surging infections we’re seeing coincides not only with the dominance of the Delta variant but the discontinuation of almost all mitigation in May and June.
One point I referenced yesterday is about what should count as an infection at all. Next week we’re going to be discussing this in an Inside Briefing with Dr. Monica Gandhi, a physician and infectious disease specialist at UC San Francisco. If you’re an Inside subscriber you should already have gotten an invite to the discussion. If you’re a subscriber we’ll be publishing the entire interview on the site for all subscribers later in the week. The central point we’re going to discuss is just what constitutes an infection, both from a clinical perspective and what should ‘count’ when it comes to understanding disease prevalence for public health and public policy. You can have COVID colonize your nasal passages or upper respiratory tract and have your vaccine immunity stop it in its tracks there. You may test positive for COVID. But should that count as an infection? That’s one thing we’ll discuss.
In any case, in this series of posts, there are a host of factors which play into these breakdowns of cases by vaccination status. I’ve referenced some of them above. What I am trying to do is pull together data on this core metric because that data is actually very hard to find. Most jurisdictions aren’t tracking it closely or at least not publishing the data. Cumulative data for all of 2021 is basically useless since we’re interested in possible trends over time. It doesn’t ‘solve’ the vaccine efficacy question. But it’s the first data point that allows you to know whether there’s change over time unconnected to the statistical artifacts of base rate fallacies.