Now a different sort of question: Why could we do this study in the first place?
2 reasons.
4/ First, we had DATA from millions of people.
We worked with high-quality data from @ClalitHealth, a health services organization that covers >50% of the Israeli population.
In contrast, looks like many countries aren't taking investment in healthcare databases seriously.
5/
Maintaining population health databases, available for immediate use in a public health crisis, is a matter of national security. How else can you make timely policy decisions?
Israel, UK (@Opensafely), and a few others are making great progress. How is everybody else doing?
6/ Countries/states with fragmented, unlinked, or not readily available databases will keep relying on others.
What happens if your questions are different? e.g., what's the safety of a different vaccine in a different population?
But good DATA weren't enough for our study...
7/ Second, we had health data EXPERTS
This study was led by a team of epidemiologists @ClalitResearch, an institute specifically created to make Clalit's data useful.
Governments: This is a good model
(also used by non-governmental organizations like @KPDOR and @KPWaResearch).
8/ Governments: Together with your health data resource, create/support a research institute. Then encourage external collaborations.
@ProfMattFox 1/
The odds ratio from a case-control study is an unbiased estimator of the
a. odds ratio in the underlying cohort when we sample controls among non-cases
b. rate ratio in the underlying cohort when we use with incidence density sampling
No rare outcome assumption required.
@ProfMattFox 2/
Because the odds ratio is approximately equal to the risk ratio when the outcome is rare, the odds ratio from a case-control study approximates the risk ratio in the underlying cohort when we sample controls among non-cases and the outcome is rare.
But...
@ProfMattFox 3/
... for an unbiased estimator of the risk ratio (regardless of the outcome being rare), we need a case-base design, not a classical case-control design.
Of course, all of the above only applies to time-fixed treatments or exposures.
1/ We recently confirmed the effectiveness of the Pfizer-BioNTech vaccine outside of randomized trials @NEJM. nejm.org/doi/full/10.10…
Studies like ours are being used to promote a vaccine passport to travel in the US, UK, and European Union.
A few clarifications are in order.
2/
Before we start, a disclaimer:
Vaccine passports involve complex ethical, economic, and political considerations.
Here I talk exclusively about scientific issues. The goal is that those making decisions have a better understanding of what we do and do not know as of today.
3/ Based on our study, we can say confidently that the vaccine is highly effective in preventing you from getting sick with #COVID19.
Based on our study, we can't say confidently that the vaccine is highly effective in preventing you from getting infected and infecting others.