As some countries rapidly roll out vaccination programmes, there have been concerns that increases in infection rates amongst vaccinated groups mean vaccines are less effective than we hoped, especially in the face of the feared Delta variant. Epidemiologist Dr Katelyn Jetelina from the University of Texas Health Science Centre School of Public Health explains why this isn’t what the numbers show – rather than decreasing vaccine effectiveness, increasing rates can be explained by a statistical phenomenon known as ‘base rate fallacy’. Presenter: Charlotte McDonald Producer: Nathan Gower