Thread by @awgaffney

The important COVID-19 vaccine effectiveness metric is: how well does it prevent *severe disease*?

With a respiratory virus, severe disease is what matters.

Severe disease = pneumonia & acute respiratory distress syndrome (ARDS), which is why and how COVID-19 kills.

Vaccine effectiveness in preventing mild disease matters *mostly* insofar as it prevents us from spreading the virus & thereby causing severe disease in others.

Because, mild respiratory tract infections themselves = colds & flu-like syndromes. Misery for a bit, but we survive.

So fact that we're talking about 3rd booster shots, when so far Pfizer vaccine w/ 2 shots has retained same effectiveness against severe disease from delta as in the original trials, doesn't make sense, particularly when 1% in low-income nations haven't been vaccinated at all.

This virus will likely be endemic. The point is to maximize immunity with global as-close-to-universal-as-possible vaccination (+ effective public health interventions) to minimize loss of life prior to that being an established fact.

reuters.com/business/healt…

In any event, observational data on the relative effectiveness of a particular vaccine against a particular variant to prevent cases alone is likely so variable because it can be so heavily confounded by the particular *testing* strategies employed in a given nation, AND ...

.... how that differs by someone's vaccination status.

Anyway, pre-vaccine, it didn't make as much sense to delink (from an epidemiological perspective) mild and severe COVID-19, because an increase in the former invariably produced a horrifying increase in the latter. But mass vaccination changes the equation.

No one knows for sure, and we will see, but I do believe that most likely gradual accumulation of immunity will make SARS-CoV-2 seem (clinically) more and more like other seasonal respiratory viruses, and less and less like the pathogen of the century.

The point of public health interventions (i.e. lockdowns) is to buy time until we mass vaccinate. The point of mass vaccination is to leap-frog ahead temporally so as to achieve mass immunity without mass death, or (having lost that opportunity) to minimize loss of life.

Conceptual clarity here is key.