1.5 How R_E changes over time

We have already noted that R_E changes over time due to variable susceptibility, public health measures and vaccination. The value at any one time is important, since if R_E >1 an epidemic is growing, whereas when R_E <1 it is shrinking. Figure 4 shows the estimated value of R_E at different time points between July 2020 and October 2022. The values are given as ranges, and values vary above or below the critical value, R_E = 1.

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Figure 4 Estimated values of RE in the UK.

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Bar graph showing Patients admitted to hospital (weekly).

Figure 4 Estimated values of RE in the UK.

It is important to understand that the values shown in Figure 4 are not the numbers of people affected by COVID-19, but they reflect whether numbers affected are increasing or decreasing. It is possible to have very few people affected but a high value of R_E, or many people affected and a low value of R_E. This type of data was fundamental for informing public health policies in the early phases of the pandemic, before vaccines were available. If R_E values were >1, then the numbers that would be infected over the following weeks or months could be projected. In order to prevent the huge numbers overwhelming medical services, public health measures were introduced to reduce contact rates and bring R_E <1.

As one example, look at the R_E values in the month before January 2021 (Figure 4). The sudden rise in R_E at this time corresponds with the government decision to end lockdown restrictions on December 2nd 2020. Once the effect of this policy became evident, the lockdown had to be reinstated on 21st December in London and Southeast England and 26th December in other parts of the UK (Institute for Government analysis, 2021). With public health measures back in force, the R_E value fell below 1.

Once vaccines became available the proportion of susceptible individuals was reduced and vaccination could be relied on to keep infection rates under control. Consequently, during 2021, there was a progressive shift from a reliance on public health measures (to reduce contacts) to the use of vaccines (to reduce susceptibles).

[Note that when R_E values are stated in relation to particular time-points or over a period of time they are often referred to as R_Tvalues also called the net reproduction number. In many publications values of R_T or R_E are just given as R-values.]