pic.twitter.com/FuFe35Kexy

— simonthong aka kitty poo (@KittyPo80176717) July 18, 2022

1/
We estimated the effectiveness of a 4th dose of #COVID19 vaccine using observational data.

Compared with 3 doses, short-term effectiveness was
68% for hospitalization, 74% for death in persons aged 60+https://t.co/tKmFgKDAqp

And then we received an interesting criticism…

— Miguel Hernán (@_MiguelHernan) July 18, 2022

3/
What a wonderful opportunity to highlight the advantages of #TargetTrial emulation using observational data.

Note the analysis of randomized trials of vaccines is NOT restricted to people who get infected during the study.

Why should the analysis of observational studies be?

— Miguel Hernán (@_MiguelHernan) July 18, 2022

5/
Second, restricting the analysis to infected persons introduces selection bias if infection is a #collider.

And infection is likely to be a collider,

as #JacobWaxman, @OriMagen89, and I explain in our replyhttps://t.co/h57DYECIsB

The causal diagram would look like this 👇 pic.twitter.com/SbaUimuYn6

— Miguel Hernán (@_MiguelHernan) July 18, 2022

7/
If you are looking for a gentle introduction to #causaldiagrams, #selectionbias, and colliders

take a look at thishttps://t.co/mM4DTNtbDh

— Miguel Hernán (@_MiguelHernan) July 18, 2022

Once this is absent and risk of acquiring infection becomes very similar, helpful to know what will then happen to risk of severe disease, when the difference between groups will only be what happens after you've been infected

2/

— Alasdair Munro (@apsmunro) July 18, 2022

Identifying the direct effect of the vaccine that is not mediated by infection can be achieved

if infection (the mediator) is randomly assigned, or

if we adjust for confounders for the effect of infection,

not simply by CONDITIONING on the mediator

2/https://t.co/3qKpcvCfVx

— Miguel Hernán (@_MiguelHernan) July 18, 2022

Only 30 days of follow-up? 😲

— Frederik Feys (@FFeys) July 18, 2022

🤣"Safe and effective" It's all a paradise in papers paid by investors. But in "real world"… pic.twitter.com/kbIOQIHTOL

— M.A.C (@manuelcl10) July 18, 2022