UK COVID-19 lockdown changes: How R is changing


We should be watching the R value from Monday 15 June.

There is concern over rises in the Reproductive Number "R" once some lockdown restrictions are lifted, particularly the most vulnerable people in care homes may be impacted.

However, according to one simple model there is some comfort, in that R may not need to be below 1.0 in order for infections to decline, we may have some scope to allow it to rise another 10% -due to the "herd immunity" effect.

Here, we Use a simple Excel based method* to match two sets of data:

  1. Hospital deaths excluding those of care home residents

  2. Care home resident’s deaths (occurring in care home as well as hospital etc)

Also, from end May onwards, prediction of infections per day, number of people who are infectious and deaths.

  1. Infections based on hospital deaths (excluding care home residents).

Results match the actual deaths by adjustment of the transmission rate or “R” value. Percentage of UK population who’ve caught the virus is shown now to reach about 13%. Antibody testing during May showed 6.8% nationally and 15.5% in London.

Currently infectious people are less than 0.1% (or 1 in a thousand). The previous predicted trend through May was confirmed by results of the ONS weekly infection survey.

UK hospital deaths (e.g. care home residents) are accurately reported to be 27469 as at 22nd May (updates from ONS awaited) the predicted number for 14th June is 29199.

Care home residents are particularly vulnerable, and the previous mortality data is shown to the end of May.

Hence, now there have been over 22,000 deaths of care home residents due to Covid (confirmed and suspected) based on data from Office of National Statistics, Care Quality Commission and the Scotland and Welsh health services. The model was matched to early data and uses infection rates from the general public to predict care home infections. The deaths will accelerate again if a second wave occurs in the population as a whole.

Predicted total for hospitals plus care homes at 14th June is around 53,000.

Lifting the Lockdown

Below is the base case which assumes continuation of the lockdown and the R number staying at 0.9.

Currently there are about 5000 new infections per day, which should allow track and trace to be effective. Infectious numbers are steadily declining assuming R stays constant.

The likely situation that R will rise is shown in the next two graphs.

For R =1.1 after 15th June there is still a slight decline in the numbers of infections and infectious people, despite what we hear that R has to be less than one. This is due to the “herd immunity effect” (13% of people already infected and assumed immune).

If R rises to about 1.3, as it was just before the lockdown (but following closing of schools and pubs etc.) the infections will rise, causing a second but more gradual wave. Track and Trace should make an impact however, on the R rate and perhaps hospital admissions will stabilise at low rates. If R can be reduced toward 1.1, then lockdown restrictions need not be re-applied.

*Note: a simple form of “SEIR” model calculates numbers of people who are in each stage of the sequence Susceptible: Exposed: Infectious: Recovered. This model assumes that people who have not got antibodies, once exposed are infectious after 4 days, then are recovered after a further 5 days (or hospitalised), for just 0.35% of those infected, death occurs after 21 days.

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