Methods
In England, local government is divided between an Upper Tier (county council) and a Lower Tier (district council). The data of COVID-19 case are published daily for each of the 149 Upper Tier Local Authorities (UTLA) (9) which vary in size from 1.6m to 97k. This study used the latest data download possible.
The UTLA population numbers were taken from GP practice patient numbers published by Lower Layer Super Output Area (LSOA) then aggregated up to their respective UTLA (10,11). The same method was used to aggregate the other population demographic and health characteristics to UTLA level.
Statistical Analysis
The new cases were calculated and plotted on a timeline with a simple polynomial trend analysis. An exponential curve based on the disease 3 days doubling characteristic linked to the starting data was included for reference.
Two further variables the Average Daily Infection Rate (ADIR) and the Rate of Change of Infection Rate (∆IR) were calculated and used to track the national and regional developments in infection rate.
The COVID-19 characteristics incorporated into the analysis are 5 Incubation days and 5 Infectious days. Similar assumptions were made by the Imperial College COVID-19 Response Team in their ‘Impact of non-pharmaceutical interventions (NPIs) to reduce COVID-19 mortality and healthcare demand’ (12). In that paper, the authors stated “We assumed an incubation period of 5.1 days. Infectiousness is assumed to occur from 12 hours before the onset of symptoms for those that are symptomatic and from 4.6 days after infection in those that are asymptomatic with an infectiousness profile over time that results in a 6.5-day mean generation time”.
Average Daily Infection Rate ( RADIR):The daily infection rate R on any given day is calculated by dividing the infected population i.e. the reported new cases 5 days ahead (corresponding to the incubation period), by the infectious population i.e. an average of new cases over the 5 previous days (corresponding to the infectious period). RADIR is taken as a rolling average of the R values over the previous 7-days to allow for variation in weekly administrative case count. Therefore
Two sensitivity analyses were considered, first where the condition is faster in incubation and infection (4 days for each) and second where the condition is slower (6 days for each).
Rate of Change of Infection Rate(ΔIR): is calculated by taking the slope of the least-squares fit line using the previous 7 days R values.
The relation between the RADIR and the ΔIR in all the UTLAs was determined and the impact of the disease progression was considered by including the total number of reported cases/,000 population, shown in quartiles.
A stepwise regression model linking the RADIR to the local community characteristics with weighting by population numbers was carried out with factors included
Local GP practice data taken from various sources were aggregated up to UTLA level.
The regression coefficients for the association between RADIR and the reported COVID-19 cases/population were used to determine the RADIR when an UTLA has no reported cases. This is the expected value that the lockdown and increased social distancing delivers on their own on this day. One can also extrapolate to a value of Cases/1,000 pop that would be needed to give a RADIR = 0 i.e. 100% immunity in the total population. This value can then be used to indicate the relation between reported and community infection levels. Linear extrapolation was used however there may be asymptotic effects that change this number.
The Office of National Statistics has reported a detailed analysis of the total mortality in March associated with COVID-19 (13). This total additional mortality can be related to the total end of March reported cases of COVID-19 which can be uprated by the total potential community infection rate calculated in this report to give an estimate of overall COVID mortality rate.
Patients or the public WERE NOT involved in the design, or conduct, or reporting, or dissemination plans of our research. Ethics permission was not required, as no individually identifiable data were included in the analysis nor were nay individuals contacted.