Letter to the Editor
Dear Editor,
Determining the transmission of SARS-CoV-2 in households is crucial for
understanding the heterogeneities in the age-related risk of infection,
as the exposures will likely be high given the strong familial
interactions. The outcome from such analysis is important to guide
containment measures for the ongoing pandemic such as school closures
and healthcare resource allocation.
Close household contacts of a laboratory-confirmed COVID-19 case in
Singapore, a city-state in South-East Asia with a population of 5.7
million, were contact traced and interviewed for demographic
characteristics. We limit our analysis to contacts of the first 400
COVID-19 cases in Singapore, i.e. those identified before March 21, 2020
as the number of cases in the population with unidentified source of
infection was less than 40 at that point and the likelihood of household
cases being exposed to the population was low. This is also prior to the
tightening of precautionary measures introduced by the government to
minimise the spread of COVID-191,2 that may have
modified the risk of acquiring infection in the home.
Contacts with recent or active symptoms compatible with COVID-19 were
isolated and tested at least twice for SARS-CoV-2. Those testing
negative and other well household contacts were quarantined at home or
in government quarantine facilities and monitored daily for development
of symptoms for 14 days from their last exposure to a case. From March
2020, respiratory samples were also collected from all household
contacts who were aged 12 and below and tested for SARS-CoV-2, as this
age group may not report symptoms.
Multivariate logistic-regression model was used to examine the
relationship between infection outcome of a household contact and age,
adjusted by the duration of exposure. Exposure between each case and
household contact is assumed to be independent and measured from the
case’s symptom onset to last date of exposure with the household contact
or to symptoms onset in the infected household contact, whichever comes
first. The duration of exposure in each household contact is the sum of
their exposures in the household. Sensitivity analysis was performed to
adjust for potential demographic confounders such as gender and
household size. Household contacts with the same exposure outside of the
household as the household index case and cases who practice
self-isolation from their household contacts were omitted from the
analysis.
We computed the 95% confidence intervals (CI) for secondary clinical
attack rate (SCAR) among household contacts3. One-way
ANOVA was used to evaluate the difference in the mean secondary
infection rate in household of different sizes. We also report the
serial interval – defined as time from symptom onset in an infector to
symptom onset in an infectee, using data from households with only one
newly infected household contact (i.e. no tertiary transmission to
confound the serial interval analysis). All analysis was done using R
version 3.5.14.
Of the 400 cases identified before March 21, 2020, 46 cases and their
households were omitted due to common exposure between cases and
household contacts or cases reported to self-isolate from their
household contacts. Of the remaining cases, 34 had no household
contacts, 277 were primary or co-primary cases in their household and 43
household contacts were tested positive for SARS-CoV-2. A total of 265
households and 875 household contacts were identified, and the overall
median household size was 4 (IQR 3-6).
After adjusting for the duration of exposure, risk of infection was
estimated to increase with age (Figure 1). This association was
strengthened after further adjustment by gender and number of household
contacts (Supplementary Info). The mean SCAR was 3.8% (95% CI
2.1–6.9) and did not vary significantly with household size. The mean
serial interval from 36 pairs of cases and the earliest infected
household contacts was 6.1 days (IQR 3.8-8.0).
Our study suggests that the risk of COVID-19 infection in the household
setting increases with age. This finding corroborates with other studies
on the demographic characteristics of COVID-19 cases in the
population5. It also supports local observations from
two preschool outbreaks, one involving 16 adult staff who acquired
infection within the preschool through one of the staffs who continued
to work when ill, while none of the 77 out of 112 students who were
close contacts and whose parents consented their being tested were
positive for SARS-CoV-2. In another preschool outbreak involving 2 adult
staff and a 2-year old preschooler taught by one of the infected staffs,
17 (out of 30) students were close contacts and were tested, but none
were positive for SARS-CoV-2 (unpublished data). These data suggest that
children are less likely to be infected and hence not the major drivers
of transmission unlike in other respiratory diseases such as influenza
and RSV6. As such, it appears inadvisable that school
closures be the main community-based measure taken to reduce community
transmission. However, school closures have been rolled out in many
countries as part of physical distancing measures. This could serve to
minimise interactions between adult and elderly workers in schools, and
to ensure that parents remain home to care for their children.
Outcomes of this study were not affected by censoring of the data as
contacts were monitored until the end of their quarantine period. The
mean SCAR of 3.8% was similar to 2009 influenza A H1N1
pandemic7 but lower than that of seasonal
influenza8. However, the lack of children index cases
(zero case) limits our ability to evaluate infectiousness of COVID-19 by
age group.
While symptomatic contacts and asymptomatic children aged 12 and below
were referred for testing, respiratory samples were not collected from
other healthy household contacts. Thus, we were unable to ascertain the
true proportion of asymptomatic cases and hence secondary infection
rate. Furthermore, we did not interview the household contacts for
clinical risk factors for COVID-19 and this could confound the analysis.
With ongoing clinical trials for post-exposure prophylaxis of SARS-CoV-2
infection, the findings from the current study would serve as a
comparison to other household studies with the administration of such
intervention in the future.