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
- n=Date of Infection
- Total Cases (TC)= Daily Reported Total Cumulative Confirmed Cases
- New Cases (NC) = TC(n)-TC(n-1)
- Average Infectious group (AI) = \(\sum_{n-5}^{n}\text{NC}\) / 5
Infectious days
- Infection Ratio (R) = NC (n+5 Incubation Days)/ AI(n)
- RADIR(n)= \(\sum_{\mathbf{n-6}}^{\mathbf{n}}\mathbf{R}\) / 7 days in
week
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
- Location: Urban/Rural, Latitude/Longitude,
occupants/Household taken from the Office of National Statistics
- Demographics: % Individual with Age>60, Social
Deprivation, % in full-time employment or education, Ethnicity, were
taken from NHS GP Practice profiles
- Health: % population with Longer term conditions (including
hypertension and diabetes), % confident in their own health
management were taken from GP Practice Profiles
- COVID-19: Total Reported cases/1,000 population taken from
this study 8 April 2020
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.