HomeMy WebLinkAboutIR 10440 INFORMAL REPORT TO CITY COUNCIL MEMBERS No. 20-10440
To the Mayor and Members of the City Council June 23, 2020
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SUBJECT: PANDEMIC METRICS
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Issue:
On June 16, 2020, City Council requested additional information on pandemic research and
modeling.
Pandemic Research
It's challenging to state conclusively how world events have shaped respiratory virus pandemics
or pandemics, in general. Rather, it is easier to conclude how world pandemics have shaped the
world — given their tragic impact to human health worldwide. During the most severe pandemic in
recent history, the 1918 Influenza Pandemic, it was estimated that 500 million individuals were
infected and 50 million deaths occurred despite its relatively short duration (1918-1920). Further,
duration of respiratory virus pandemic events have varied throughout history given the intrinsic
nature of respiratory viruses (virulence); the host (population); technological advances; the
availability of treatment and prophylaxis (vaccines); the community's level of preparedness; and
the hospital infrastructure.
Modern researchers use data from previous pandemics to predict future pandemics, to develop
strategies to reduce their occurrence, to address emerging health risks and to model the course
of a current event. This includes micro, macro, environmental, industrial and public health
variables. While the world changes quickly, a respiratory virus pandemic spreads most easily
through social contact and much of the research looks at how cultures interact on a daily basis,
how compliant they are with the orders of authorities, type/openness of economies,
age/generation types, etc., along with the medical/scientific analysis.
Reproduction Number (R Naught)
A Reproduction Number (Rn) is used to estimate the number of new cases that will stem from a
single case. While there is a relationship between RN and case doubling, case doubling
measures the actual infections as they are reported through lab tests.
If the RN is 2, then we would expect one positive person to infect two additional people and those
two would infect 4 (i.e., 2 each) with an estimated transmission period of 4-5 days (i.e., time it
takes for one person to infect others). At R2, a single positive person, not practicing social
distancing, wearing a mask, staying home, etc. could cause 31 people to be infected over a 14-
day period. At R1, it would be 4 additional people over the same period.
An RN below 1 would suggest less spread and we would then expect to see a larger doubling
number. An RN above 1 would suggest a greater spread and we would expect the doubling time
to be lower. As an example, in the middle of May we had an R value of .7 and over the next
several weeks the case doubling went from 14 to 50 days. We predicted less transmission and
saw less transmission.
ISSUED BY THE CITY MANAGER FORT WORTH, TEXAS
INFORMAL REPORT TO CITY COUNCIL MEMBERS No. 20-10440
To the Mayor and Members of the City Council June 23, 2020
Page 2 of 2
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SUBJECT: PANDEMIC METRICS
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The City has a partnership with UNT Health Science Center and UT Southwestern who both
create Reproduction Number Models for the City. These institutions bring a variety of specialists
together to build the models. This includes Epidemiologists, Infectious Disease Doctors, Public
Health Professionals, Researchers, Behaviorists, and others. They analyze hard science/medical
metrics, introduce forensic analysis, etc. and create complex mathematical models the combine
known research, predictable viral process and educated presumptions.
The RN changes daily, but, generally, it does not swing greatly from one day to the next (i.e.,
incrementally will increase or decrease). It tends to ebb and flow just this phase of a pandemic
and is impacted by time of year, how well the population uses social distancing, the amount of
viral activity already in the community, etc.
Case Doubling
Where the RN is a prediction of new numbers, case doubling (CD) is the number of days it takes
for positive results, hospitalizations, or deaths to double. When we talk about flattening the curve,
we are actually suggesting that we increase the number of days for these things to double. The
faster they double, the more active cases we have in the community and the more medical
resources that are required at one time to treat patients. CD for positive test results can include
both antibody blood test and antigen swab test.
Positive Case Reporting
There are two broad categories of testing: antibody and antigen. Antigen testing, sometimes
referred to as diagnostic testing, includes a swab in the nasal passage test and is used primarily
to test for a current infection of COVID-19. Its accuracy is 90% and higher. Antibody is a blood
test and it is primarily used for past infection. A current infection might produce a negative finding
(no antibodies yet), and that is why it should not be used for this type of screening. Various
studies have found antibody tests currently available are only about 50% accurate. When
accuracy is improved, the antibody test will help us determine the true community exposure rate.
Tarrant County Public Health does not use antibody results in reporting new cases or case
doubling; however, the State of Texas and other regional/national agencies will use these results.
While the use of antibody results could throw off the current infection rate, reproduction number
models do not rely on these numbers alone. True viral spread will also show up as an increase in
EMS calls, visits to the hospital, ICU/hospital bed occupancy, etc.
Additional information can be obtained from Code Compliance Director Brandon Bennett at
817.392.6322.
David Cooke
City Manager
ISSUED BY THE CITY MANAGER FORT WORTH, TEXAS