The extent and impact of vaccine status miscategorisation on covid-19 vaccine efficacy studies

It is recognised that many observational studies and randomised control trials reporting high efficacy for Covid-19 vaccines suffer from various biases. Systematic review identified thirty-seven studies that suffered from one particular and serious form of bias called miscategorisation bias, whereby study participants who have been vaccinated are categorised as unvaccinated up to and until some arbitrarily defined time after vaccination occurred. Simulation demonstrates that this miscategorisation bias artificially boosts vaccine efficacy and infection rates even when a vaccine has zero or negative efficacy. Furthermore, simulation demonstrates that repeated boosters, given every few months, are needed to maintain this misleading impression of efficacy. Given this, any claims of Covid-19 vaccine efficacy based on these studies are likely to be a statistical illusion or are exaggerated.