Coronavirus disease (COVID-19) technical guidance by WHO Investigation protocol for COVID-19 | Page 12

Symptomatic
proportion of cases
(asymptomatic
fraction) The proportion of
individuals who show
symptoms or signs of
COVID-19 infection
Serological response
to infection The change in serum level of
specific antibodies to
COVID-19 virus
(Increase in titre)
Population groups
most at risk The identification of groups
who are most vulnerable to
COVID-19 virus infection
(e.g. age groups, gender,
occupation)
Ratio of severe
disease The proportion of an age
group with severe infection,
divided by the probability
that an infection resulted in
a severe case, expressed as
a proportion of the total
number of people in that
age group
Case fatality ratio The proportion of
individuals with fatal
outcome for COVID-19
infection
*The numerators of
interest are the
numbers of those
individuals reporting
various signs and
symptoms of infection
(e.g. fever, cough) and
the number/proportion
of individuals reporting
no signs or symptoms
(i.e. the asymptomatic
fraction); the
denominator is the total
number of individuals
tested.
*May only be an early
signal, a nested case-
control study could be
conducted to evaluate
risk factors for infection
* May require extended
follow-up to determine
outcome of those with
COVID-19 infection
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