comment how country based mitrigation influence the course of the covid 19 pandemic
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How will country-based mitigation measures influence the
course of the COVID-19 epidemic?
Governments will not be able to minimise both deaths
from coronavirus disease 2019 (COVID-19) and the
economic impact of viral spread. Keeping mortality as
low as possible will be the highest priority for individuals;
hence governments must put in place measures to
ameliorate the inevitable economic downturn. In our
view, COVID-19 has developed into a pandemic, with
small chains of transmission in many countries and large
chains resulting in extensive spread in a few countries,
such as Italy, Iran, South Korea, and Japan.1
Most countries
are likely to have spread of COVID-19, at least in the early
stages, before any mitigation measures have an impact.
What has happened in China shows that quarantine,
social distancing, and isolation of infected populations
can contain the epidemic.1
This impact of the COVID-19
response in China is encouraging for the many countries
where COVID-19 is beginning to spread. However, it
is unclear whether other countries can implement the
stringent measures China eventually adopted. Singapore
and Hong Kong, both of which had severe acute
respiratory syndrome (SARS) epidemics in 2002–03,
provide hope and many lessons to other countries. In
both places, COVID-19 has been managed well to date,
despite early cases, by early government action and
through social distancing measures taken by individuals.
The course of an epidemic is defined by a series of
key factors, some of which are poorly understood at
present for COVID-19. The basic reproduction number
(R0), which defines the mean number of secondary cases
generated by one primary case when the population
is largely susceptible to infection, determines the
overall number of people who are likely to be infected,
or more precisely the area under the epidemic curve.
For an epidemic to take hold, the value of R0 must be
greater than unity in value. A simple calculation gives
the fraction likely to be infected without mitigation.
This fraction is roughly 1–1/R0. With R0 values for
COVID-19 in China around 2·5 in the early stages of the
epidemic,2
we calculate that approximately 60% of the
population would become infected. This is a very worst-
case scenario for a number of reasons. We are uncertain
about transmission in children, some communities are
remote and unlikely to be exposed, voluntary social
distancing by individuals and communities will have an
impact, and mitigation efforts, such as the measures
put in place in China, greatly reduce transmission.
As an epidemic progresses, the effective reproduction
number (R) declines until it falls below unity in value
when the epidemic peaks and then decays, either due to
the exhaustion of people susceptible to infection or the
impact of control measures.
The speed of the initial spread of the epidemic, its
doubling time, or the related serial interval (the mean
time it takes for an infected person to pass on the
infection to others), and the likely duration of the
epidemic are determined by factors such as the length
of time from infection to when a person is infectious
to others and the mean duration of infectiousness. For
the 2009 influenza A H1N1 pandemic, in most infected
people these epidemiological quantities were short with
a day or so to infectiousness and a few days of peak
infectiousness to others.3
By contrast, for COVID-19, the
serial interval is estimated at 4·4–7·5 days, which is more
similar to SARS.4
First among the important unknowns about COVID-19
is the case fatality rate (CFR), which requires information
on the denominator that defines the number infected.
We are unaware of any completed large-scale serology
surveys to detect specific antibodies to COVID-19.
Best estimates suggest a CFR for COVID-19 of about
0·3–1%,4
which is higher than the order of 0·1% CFR for
a moderate influenza A season.5
The second unknown is the whether infectiousness
starts before onset of symptoms. The incubation
period for COVID-19 is about 5–6 days.4,6 Combining
this time with a similar length serial interval suggests
there might be considerable presymptomatic infec-
tiousness (appendix 1). For reference, influenza A has a
presymptomatic infectiousness of about 1–2 days, whereas
SARS had little or no presymptomatic infectiousness.7
There have been few clinical studies to measure COVID-19
viraemia and how it changes over time in individuals. In
one study of 17 patients with COVID-19, peak viraemia
seems to be at the end of the incubation period,8
pointing
to the possibility that viraemia might be high enough
to trigger transmission for 1–2 days before onset of
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