There follows a guest post by Daily Sceptic reader ‘Amanuensis’, as he is known in the comments section below the line. He is an ex-academic and senior Government researcher/scientist with experience in the field, who says he is “a bit cross about how science has been killed by Covid”. It was originally posted on his Substack page, but I thought it was such an excellent analysis of the UKHSA’s favoured test-negative case-control approach and its problems – especially why it seems consistently to exaggerate vaccine effectiveness – that Daily Sceptic readers should be treated to it too.
There has been much consideration in recent months about the effectiveness of the Covid vaccines, and this leads to thoughts about how vaccine effectiveness is calculated in the first place. The trouble with any attempt to calculate vaccine effectiveness is bias – that is, are the vaccinated and unvaccinated similar enough to make the calculation, or, rather, can we remove any bias to get an unbiased estimate.
As an example of bias, in the early days of the Covid vaccinations the majority of the vaccinated were old, and the unvaccinated were young – so if there was an effect of age then we’d get a biased result simply by comparing overall case rates (per 100,000) in the vaccinated versus the unvaccinated groups. In this case the bias might be resolved by splitting the analysis into different age groups, but what about other factors? Most of all, what is the bias associated with willingness to become vaccinated (maybe the vaccinated are in general more likely to be the healthy ones, say)?
Some time ago, statisticians came up with a really great way to remove rather a lot of the ‘difficult’ bias – it is called the Test Negative Case Control approach (TNCC). With this approach you don’t simply count infections, but compare the rates of infections amongst those who get tested – more specifically, you compare the ratio of positive to negative results in the vaccinated against the positive-to-negative ratio in the unvaccinated groups.
The great thing about this method is that it automatically compensates for many behavioural effects in the vaccinated compared with unvaccinated groups – so, say the unvaccinated are half as likely to go and get tested compared with the vaccinated, the TNCC should remove most of this effect. Of course, many demographic things are of interest (particularly the impact of age and gender), so you’ll usually separate out these variables, but the advantages of the TNCC method remain.
Anyway, pretty much every study on Covid vaccine effectiveness makes use of TNCC – it gives such a powerful and unbiased estimate. You can read more about it in this review article.
Oh, but what’s this I see in that paper?
The Flaw at the Heart of the UKHSA’s Vaccine Effectiveness StudyRead More