How much protection do the vaccines provide against severe Covid? Some people claim that getting vaccinated reduces your risk of death by as much as 90%. They point to charts showing massive disparities in hospitalisation and death rates between the vaccinated and unvaccinated.
However, these crude comparisons may overstate vaccine effectiveness. A study published last year by the U.S. CDC found that vaccinated people were less likely to die of non-Covid causes, suggesting that they’re inherently healthier and/or more risk averse than unvaccinated people.
This ‘healthy vaccinee’ effect means that part of the disparity in death rates between the vaccinated and unvaccinated is due to factors other than vaccination. So even if currently-unvaccinated people got vaccinated, they’d still die from Covid at higher rates than currently-vaccinated people.
Now, I’m not claiming that all or even most of the disparity in death rates between the vaccinated and unvaccinated is due to the ‘healthy vaccinee’ effect. I’m simply claiming that crude comparisons overstate vaccine effectiveness – which is probably less than 90%.
Okay, but is there any evidence for this form of ‘selection bias’ – as it’s known in the jargon – other than the CDC study? Yes: the ONS documented the very same phenomenon in a report published last year. See the table below, which shows age-standardised mortality rates by vaccination status:
Looking at the second column, unvaccinated people have a higher non-Covid death rate than all four groups of vaccinated people.
Note, however, that the ratios in the first column are substantially greater than those in the second column. This implies that the ‘healthy vaccinee’ cannot explain all or even most of the disparity in Covid death rates between the vaccinated and unvaccinated.
On the other hand, the ratios would presumably be smaller if the comparison were restricted to vaccinated people who’d received their second dose more than, say, six months ago. We know that vaccine effectiveness wanes over time – for both infection and serious illness.
The ‘healthy vaccinee’ effect turned up again in a study published in Science. Barbara Cohn and colleagues analysed a large dataset on U.S. veterans, and found that survival rates among over 65s were higher among vaccinated individuals who tested positive than among unvaccinated individuals who tested negative:
Note: the left-hand chart corresponds to those aged under 65; the right-hand chart to those aged over 65. The researchers did acknowledge that selection bias may have impacted their results, writing:
Patterns of survival for those with a negative PCR test by vaccination status suggests there are underlying differences in unvaccinated compared to vaccinated persons, and that we did not measure or account for in our analysis; these differences may contribute to the different risks of death we observed.
There’s now fairly robust evidence for the ‘healthy vaccinee’ effect, at least in Britain and the U.S. The next step would be estimating how much of the disparity in death rates between the vaccinated and unvaccinated it can account for. Hopefully someone with the requisite data will attempt this soon enough.
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