Coronavirus disease (COVID-19) technical guidance by WHO Household transmission investigation protocol | Page 7

1.
Background
1.1 Introduction
The detection and spread of an emerging respiratory pathogen are accompanied by uncertainty over
the key epidemiological, clinical and virological characteristics of the novel pathogen and particularly
its ability to spread in the human population and its virulence (case-severity). This is the situation for
coronavirus disease 2019 (COVID-19), first detected in Wuhan city, China in December 2019 (14).
Closed settings, such as households, have a defined population that may not mix readily with the
larger surrounding community, and therefore such settings can provide a strategic way to track
emerging respiratory infections and characterize virus transmission patterns because the
denominator can be well defined. Also, exposure is within the setting, and follow-up of household
contacts is generally more feasible in this well-defined setting as compared to an undefined one.
Studies in household settings allow determination of the transmission dynamics (reproduction
number and serial interval) of the virus, as well as aiding understanding of the clinical spectrum of
illness in secondary cases (15). Closed settings are also useful to observe chains of transmission in an
epidemic, as the pool of susceptible, exposed individuals is larger. Therefore, in the case of multiple
waves of infection through the closed setting, unique insight into transmission dynamics can be
derived in the early epidemic stages.
To date, initial surveillance has focused primarily on patients with severe disease, and, as such, the
full spectrum of the disease, including the extent and fraction of mild or asymptomatic infection that
does not require medical attention, is not clear. Infections identified in close contacts are potentially
generalizable to naturally acquired infections (in contrast to cases presenting for emergency care,
among which there would be fewer mild cases). Following close contacts with similar levels of
exposure to infection from primary cases can also permit identification of the asymptomatic
fraction. Principally, follow-up and testing of respiratory specimens and serum of close contacts can
provide useful information about newly identified cases, as well as the spectrum of illness and
frequency (by, for example, age) of asymptomatic and symptomatic infection.
With the emergence of a novel coronavirus, the initial seroprevalence in the population will be low,
due to the virus being new in origin. Therefore, surveillance of antibody seroprevalence in a
population can allow inferences to be made about the cumulative incidence of infection in the
population. Household transmission studies also can provide the opportunity to follow up confirmed
cases, to understand antibody kinetics.
The following protocol has been designed to investigate household transmission of the virus
responsible for COVID-19 in any country in which COVID-19 infection has been reported and
households are exposed. Each country may need to tailor some aspects of this protocol to align with
public health, laboratory and clinical systems, according to their country capacity and availability of
resources, as well as the cultural appropriateness of the protocol. However, by using a standardized
protocol such as the one described here, epidemiological exposure data and biological samples can
be systematically collected and shared rapidly in a format that can be easily aggregated, tabulated
and analysed across many different settings globally. This will facilitate timely estimates of the
severity and transmissibility of COVID-19 infection, as well as informing public health responses and
policy decisions. This is particularly important in the context of a novel respiratory pathogen, such as
the virus responsible for COVID-19.
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