Why are Delta infections falling sharply in the UK?
Some things are clear, and some things are
not. It appears that the easing of restrictions on July 19 did not lead to a dramatic shift in people’s
behavior, as we had expected. According to the Google mobility data nothing much has changed in the past
week, and anecdotal evidence suggests that adherence to NPIs such as mask wearing is still widespread. So
the 0.3-point positive shock to the effective reproduction number that we were expecting last week did
not happen. Another thing that is clear is that testing has fallen by around 15%, which could account for
a decline of around 6,000 daily new infections. But, these two developments cannot explain why the
effective reproduction number has apparently fallen below one, leading to a sharp fall in the absolute
level of infections. Possible additional explanations are a seasonal weather effect and an early arrival
of the school vacation effect. But, it is hard to fully explain the dramatic collapse in new infections.
Some commentators point to a football effect. Clearly, super spreading events do occur, but we are not
persuaded that football can explain much of the recent up and down pattern in new infections. If people
were infected at gatherings connected to football, they would then disperse infecting others in their
communities. This does not appear to have happened. We are not the only people struggling to explain
recent developments in new infections. Epidemiologists across the land are scratching their heads. If the
sharp rise in new infections was due to the spread of the Delta variant, it is difficult to explain such
a sharp turnaround without an equally dramatic cause. The Delta variant has not gone away. The higher
basic reproduction number of the Delta variant should have a persistent effect on the effective
reproduction number. We are reluctant to abandon the idea that the Delta variant will be a problem
over time, due to a high basic reproduction number and the modest level of vaccine efficacy regarding
onward transmission, and we are very cognizant of lags in the system which means that the situation with
infections could change again in the near future. But, it is hard to argue against the idea that the
current Delta wave in the UK is turning out to be much, much milder than we anticipated.