Coronavirus disease (COVID-19) technical guidance by WHO Household transmission investigation protocol | Page 21

Secondary clinical
attack rate.
Secondary infection
rate (also called
secondary infection
incidence).
Clinical presentation
A measure of the frequency
of new symptomatic cases
of COVID-19 infection that
occur among contacts within
the incubation period
(range) following exposure
to a primary confirmed case,
in relation to the total
number of exposed
contacts; the denominator is
restricted to susceptible
contacts when these can be
determined.
(The rate of clinical
manifestation of COVID-19
infection in contacts).
It is a good measure of
person-to-person spread of
disease after the disease has
been introduced into a
population.
A measure of the frequency
of new infections of
COVID-19 among contacts
within the incubation period
(range) following exposure
to a primary confirmed case,
in relation to the total
number of exposed
contacts; the denominator is
restricted to susceptible
contacts when these can be
determined.
(The rate of contacts being
infected, assessed through
serological
assays/polymerase chain
reaction on paired samples).
It is a good measure of
person-to-person spread of
the infection after the
infection has been
introduced into a
population.
The range of clinical
symptoms in cases and
contacts.
(Clinical symptoms and
severity).
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Symptoms and
dates of contact
with confirmed
cases of COVID-19
infection.
Form 1A: Q6
Form 1B: Q6
Form 2, 3, 4, 5
Symptom diary
Laboratory
confirmation
(serology)
Form 2, 3, 4, 5
Symptoms
Form 1A: Q6
Form 1B: Q6
 Note that early estimates
are likely to be biased
due to some cases being
able to more successfully
produce secondary cases.
• Note that these
estimates will be specific
to setting and contact
type.
• The numerator will be
determined as the
number of household
contacts with confirmed
COVID-19 infection, while
the denominator will be
determined as the total
number of household
contacts.
• Represents an overall risk
of infection among
household contacts for a
defined time period.
• In-hospital clinical studies
will enhance
understanding of the
clinical course, severity
and risk determinants, as
well as case fatality.