Waves or Spikes?

Main points

• With this edition we refocus the tracker on local surges in infection to address the question whether sudden increases observed are local spikes or are indicative of the start of a new wave.  Given the divergence among local areas in case rates, and surge testing in areas with high case rates, it is less meaningful to monitor the reproduction number at the level of nations or regions. We focus on three local authorities which were the first to experience localised increases in case rates.
• Figure 1 provides R number estimates and the case rates per 100,000 population, for Bolton, Blackburn and Darwen and Bedford, based on specimen date data series released on 1st 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.
• At the end of our estimation sample on 28th May 2021, the R number was in the range 0.80 - 1.00 for Bolton (case rate 375) and 0.75 – 1.10 for Bedford (case rate 161).   Blackburn and Darwen (case rate 436) are the only area of concern, with an R number in the range 1.10 - 1.50.
• A sudden surge in infection in any specific location can lead to a sharp increase in the estimated reproduction number. A part of such an increase in R may be due to increased testing in the location. It may also be driven by the fact that the increase is from a low base. If the infection can be contained through vaccination the Reproduction numbers will fall.
• The trajectories of infection in Bedford and Bolton appear to fit this profile. The lower reproduction numbers in these areas may reflect the efficacy of the vaccine in limiting the susceptible population. However, a degree of transmission spill over to surrounding geographical areas in the North West is evident in the data.

“The combination of low case rates with localised outbreaks and associated surge testing make it less meaningful to monitor the reproduction number at the regional and national geographical levels. We consider three local authorities that first experienced the Indian, or Delta, variant. It appears that both Bolton and Bedford are past the peak, whilst cases are still increasing in Blackburn with Darwen.

"These findings suggest that localised outbreaks can be locally contained through high vaccination rates and changes in behaviour. But there is more to be done in this regard to prevent local spikes morphing into waves.”