Coronavirus disease (COVID-19) technical guidance by WHO Household transmission investigation protocol | Page 8

1.2 Objectives
The overall aim of this protocol is to gain an understanding of the transmission dynamics of
COVID-19 to household contacts of laboratory-confirmed cases of COVID-19, as well as rapid and
early information on key clinical, epidemiological and virological characteristics of COVID-19
infection.
The primary objectives of this household transmission study are to provide key epidemiological data
to complement and reinforce the findings of FFX (1), in the areas of:
 the proportion of asymptomatic cases and symptomatic cases;
 the incubation period of COVID-19 and the duration of infectiousness and of detectable
shedding;
 the serial interval of COVID-19 infection;
 the reproduction numbers: R 0 and R of COVID-19;
 clinical risk factors for COVID-19, and the clinical course and severity of disease;
 high-risk population subgroups;
 the secondary infection rate and secondary clinical attack rate of COVID-19 infection among
household contacts; and
 patterns of health-care seeking.
A reminder of some definitions of epidemiological terms:
 The incubation period is defined as the period of time between an exposure resulting in
COVID-19 infection and the onset of the first clinical symptoms of the disease (from infection
or exposure to disease).
 The serial interval is defined as the period of time from the onset of symptoms in the primary
case to the onset of symptoms in a contact case.
 The basic reproduction number R 0 is defined as the number of infections produced, on average, by
an infected individual in the early stages of the epidemic, when virtually all contacts are susceptible.
Note that it can be assumed that there will be very little to no immunity to COVID-19.
 In this context, the secondary infection rate is a measure of the frequency of new infections of
COVID-19 among contacts of confirmed cases in a defined period of time, as determined by a
positive COVID-19 result. In other words, it is the rate of contacts being infected, assessed
through polymerase chain reaction (PCR)/serological assays on paired samples.
 The secondary clinical attack rate is a measure of the frequency of new symptomatic cases of
COVID-19 infection among the contacts of confirmed cases in a defined period of time, as
determined by a positive COVID-19 result. In other words, it is the rate of clinical manifestation
of the infection in contacts.
 The duration of infectiousness is the time for which virus is shed and able to be transmitted,
regardless of clinical symptoms.
 It is currently not known how long detectable COVID-19 virus shedding lasts; information from
this study would help to clarify the duration of detectable shedding among individuals with
confirmed infection.
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