Coronavirus disease (COVID-19) technical guidance by WHO Household transmission investigation protocol | Page 9

2.
Methods
2.1 Design
This household transmission investigation is a prospective case-ascertained study of all identified
household contacts of a laboratory-confirmed COVID-19 infection (see Section 2.2). Participants are
identified from those with laboratory-confirmed infection, which is distinct from a cohort study in
which a group of disease-free households are recruited and then followed over time (see Fig. 1).
Case-ascertained transmission studies are more efficient than cohort studies when interest is in
early ascertainment of the clinical, epidemiological and virological characteristics of an emerging
virus. This is because the risk of primary or secondary infection in a “sleeping” cohort would be
expected to be low during the early stage of the pandemic before widespread community
transmission is established.
This household transmission investigation should be established following identification of the first
laboratory-confirmed cases of COVID-19 infection in any country. It should also ideally be conducted
before widespread community transmission occurs, that is, within the early phases of the COVID-19
epidemic in the country. The household transmission protocol aims to identify key clinical,
epidemiological and virological characteristics of infection with this novel virus and its transmission
in near real-time.
Fig. 1. The chain of transmission in a household transmission study
Contact
HH A 1
Primary case
(confirmed
case)
(Household A)
Contact
HH A 2
...
2.2 Population
The population under investigation consists of the confirmed cases of COVID-19 and their close
contacts in their households. Households will be enrolled in the study once a confirmed COVID-19
case is identified in at least one member of the household. Households are subsequently followed
up to observe secondary infections. If there is a large number of eligible primary cases it may not be
feasible to follow up all households, because of limitations in resources and capacity. Therefore, it
may be necessary in Country X to predetermine and agree upon a sampling strategy for the inclusion
of households to remove possible sources of bias.
Every effort should be made to include all identified household contacts of cases of laboratory-
confirmed COVID-19.
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