In a recent article, I noted that vaccine effectiveness against death may have been overestimated due to the ‘healthy vaccinee’ effect – the tendency for people who get vaccinated to be healthier and more risk-averse than those who don’t.
Likewise, Will Jones recently reported on a large Swedish study, which observed declining effectiveness against severe outcomes, particularly after six months. Discouragingly, the decline in effectiveness was most pronounced among older, frail individuals – the group most at risk from Covid.
Now a new study (which hasn’t yet been peer-reviewed) has made a similar finding. Maxime Taquet and colleagues analysed data from a large database of electronic health records in the U.S.
Their sample comprised ~19,000 individuals who’d had a confirmed SARS-CoV-2 infection between 1st January and 31st August 2021. There were two groups: those who had been vaccinated at least 14 days prior to infection, and those who had not been vaccinated prior to infection.
The two groups were matched not only on basic demographic characteristics, but also on a large number of medical risk factors. In addition, the unvaccinated individuals were selected from among those who’d ever received a flu vaccine. Overall, substantial efforts were made to ensure the two groups were comparable.