Geographic Spill Over

Main points

• We comment on the structure and composition of the widely anticipated wave emerging in the UK. The advancing pattern is consistent with a race between, on the one hand, local surges that spill over geographically due to higher transmissibility of the now dominant Delta variant, and on the other, the vaccination programme that reduces the susceptible pool.
• Figure 1 provides R number estimates and the case rates per 100,000 population, for the nations of the UK and English regions, based on specimen date data series released on 29th June 2021. We discard data for the last 3 days due to data revisions in that time window. The estimates reported are to be read in the context of the policy of increased testing in local authorities with relatively high case rates.
• A sudden surge in infection in any specific location can lead to a sharp increase in the estimated reproduction number. A part of this increase in R may be due to increased testing in the location.  A part of it may be driven by the fact that the increase is from a low base. Importantly, if infection can be contained through a focussed and timely vaccination programme, the reproduction number will fall.
• At the same time, the more transmissible Delta variant was always likely to spill over and take hold in surrounding geographical areas. With lower base numbers the initial growth rates will be high.
• We should expect regional and national level incidence patterns to reflect an aggregation of a succession of local surges that spill over geographically even as they are contained through vaccination. As the unvaccinated proportion of the adult population diminishes the incipient wave is more likely to be contained.

“The current picture is one of localised outbreaks that have spilled over geographically due to the higher transmissibility of the Delta variant. As vaccination coverage becomes universal, shrinking the susceptible pool, local surges will tend to peak quickly and die out. However, the geographic spill over dominates the figures as the aggregation of numerous staggered local surges mount. Reassuringly, the forecast rise in hospital admissions relative to the earlier peak is very small in relative magnitude.