Household transmission of seasonal coronavirus infections: Results from the Flu Watch cohort study

Study design and procedures

Data were drawn from the Flu Watch prospective cohort study of acute respiratory infections in English households¹¹. We included the three winter seasons (2006–2007, 2007–2008, 2008–2009), during which all samples were systematically tested for coronaviruses. The Flu Watch study methodology and cohort profile are described in detail elsewhere^11,12.

Whole households were recruited annually to the study following random selection of a household member from GP practice lists. From 2008–2009, participants from the previous cohort were also re-invited to participate. Inclusion criteria were that all household members agreed to participate, and all members over 16 agreed to provide blood samples for other Flu Watch research. Exclusion criteria were household size >6 members, severely incapacitating or terminal illness in any household member, and heavy involvement in other research¹².

Participants provided demographic data at seasonal baseline following recruitment, and were followed-up weekly via telephone or online throughout each season to report any symptoms of acute respiratory infection. If participants experienced any symptoms, they were requested to provide a nasal swab on the second day of illness and to provide a daily diary of symptoms from the first day of illness until the symptoms resolved. Real-time PCR was carried out to screen nasal swabs for a panel of viruses, including three circulating seasonal coronavirus strains (229-E, NL63, and OC43)¹¹.

Definitions and analyses

Guided by an estimated incubation period of 2–5 days and further 2–18 days of symptomatic illness with viral shedding for seasonal coronaviruses^13,14, we defined index cases as the first PCR-confirmed infection in a household or >23-days following a prior case, co-primary cases as potential index cases of the same strain arising within ≤2 days, and secondary cases as infections with the same strain occurring >2 days and ≤23 days from exposure to the index case(s). We assumed that household transmission to any secondary cases had occurred if the strain was unknown, but criteria for transmission were otherwise met.

We estimated the proportions of total PCR-confirmed coronavirus cases in the study likely acquired in the community (i.e. index, co-primary, or single-person household cases) and acquired in the household (i.e. secondary cases). The proportions were weighted to the English national structure of age and region¹¹.

For households in which secondary transmission was possible (i.e. excluding single-person households and episodes of co-primary infection affecting all household members), we calculated the symptomatic household transmission risk– the proportion of households that experienced at least one secondary case – and the household secondary attack risk – the proportion of total exposed participants who became secondary cases. To avoid multiple inclusions of the same episode where there were co-primary cases, transmission risk and secondary attack risk were estimated based on a randomly-selected single index case. Due to the testing protocol, it was not possible to detect asymptomatic primary or secondary cases. We calculated the clinical-onset serial interval as the time in days from onset of reported symptoms in the index case(s) to onset of reported symptoms of the secondary case(s). All co-primary or multiple secondary cases in this cohort reported symptom onset on the same day.

We stratified the household symptomatic transmission risk by the following household features potentially relevant to transmission: the age structure of the household (adults between 16-64 only, older adult(s), adult(s) and children), age of index case (child <16, adult 16-64, older adult 65+) for households with children (not stratified for other household structures due to limited variation), the number of people and number of children in the household, whether the household contained current smoker(s), index of multiple deprivation, long-term health conditions in household contact(s), and the index sex, index healthcare worker status and index transmission-preventive hygiene behaviour. Index case hygiene behaviour was classified as a binary variable according to adherence to prevention guidelines^15,16 i.e. covering the mouth while coughing or sneezing, using a single-use tissue, and washing the hands habitually after coughing or sneezing and at least moderately-frequently throughout the day (>=5 times according to median split and previous literature¹⁷). Due to the small number of household transmissions, we present descriptive stratified analyses only and do not describe by strain sub-groups. Where features of the index case or index-exposed dyads were investigated, we included only those households where there was a clear index case rather than co-primary cases. Any instances of missing demographic data are noted in Table 1.

To estimate potential non-detection of household transmissions due to non-adherence to swabbing protocol, we measured the proportion of exposed participants who reported symptoms within one month of an index case but did not submit a nasal swab. Analyses were performed in Stata Version 15.

Proportion of infections acquired in the households and features of transmission

Table 2 reports the proportion of infections acquired within and outside of the household and the symptomatic transmission risk, secondary attack risk, and clinical-onset serial interval by strain and overall.

There were a total of 195 coronavirus cases during the three seasons in households in which transmission was possible, of which a weighted proportion of 90.66% (95% CI 84.60-94.49, 173/195) were index or co-primary cases presumably acquired in the community, and 9.34% (5.51-15.40, 22/195) were in exposed household members and presumed to be acquired through household transmission. The proportion of presumed household-acquired infections ranged across strains from 6.18% (1.92-18.12, 5/60) for NL63, to 10.59% (4.71-22.09, 10/80) for OC43, and 12.15% (5.01-26.60, 7/52) for 229E. Strain data was unavailable for three cases.

There were 134 potential household outbreaks with a coronavirus index case or co-primary cases. All 22 co-primary cases from 10 households occurred on the same day. Of the 134 potential outbreaks, 22 had transmission to at least one other household member giving an overall household transmission risk of 14.93% (95% CI: 9.78-22.11). Across strains, household transmission risk ranged from 11.90% (4.84-26.36, 5/42) for NL63, 13.21% (6.27-25.72, 7/53) for OC43, and 19.44% (9.21-36.49, 7/36) for 229E.

A total of 22 exposed participants contracted a coronavirus infection, out of 275 participants at risk (excluding index and co-primary cases), yielding a household secondary attack risk of 8.00% (95% CI: 5.31-11.88, 22/275) overall. Secondary attack risks by strain were 5.10% (2.11-11.84, 5/98) for NL63, 8.73% (4.56-16.10, 9/103) for OC43, and 10.14% (4.82-20.11, 7/69) for 229E. Due to unknown index strain, outbreak strain was unavailable for five exposed participants.

The median clinical-onset serial interval was 7 days (IQR = 6-9.5 days, range 5-21 days). This median value was consistent for 229E and NL63, with some between-strain variation in range, and similar to the OC43 median serial interval of 6.5 days. There were two households for which the exact date of symptom onset of the exposed case could not be traced (only the beginning date of the week of illness), which were excluded from the serial interval calculation.

Demographic features and transmission Risk

Table 1 reports household symptomatic transmission risk stratified by demographic features of households. Transmission risks were highest for households containing older adults (25.00%, 95% CI 10.47, 48.73), households with children (15.71%; 8.84, 26.40), then adult-only households (9.09%; 3.38, 22.21). Households with 3+ children (36.36%; 13.34, 67.96), those with smokers (28.57%; 10.61, 57.42), those in deprived areas (23.08%; 10.50, 43.41), and those in which 1+ household contact had a chronic health condition (16.67%; 7.54, 21.51) had the highest transmission risks within their categories. In households with children, child index cases demonstrated higher transmission risk (17.24%; 7.11, 36.10) than adult index cases (12.90%; 4.75, 30.58). Male index cases also demonstrated higher transmission risk (18.46; 10.69, 29.99) than female index cases (10.17%; 4.57, 21.11). There was no evidence of onward transmission if the index case was a healthcare worker or if they practiced good hand and respiratory hygiene, though the number of index cases in these categories were low (n= 7 and 13 respectively).

Adherence to swabbing protocol during household outbreaks

Among the 275 exposed household members, there were 75 distinct episodes of respiratory symptoms within 23 days of a household coronavirus index case. For 4 of these episodes (5.33%), the participant did not provide a swab at any point during their illness.

Ethics statement

The protocol was approved by the Oxford Multi-Centre Research Ethics Committee (06/Q1604/103).

Underlying data

University College London: Household Transmission of Seasonal Coronavirus Infections: Results from the Flu Watch cohort study. https://doi.org/10.5522/04/12383873.v1

This project contains the following underlying data:

Data are available under the terms of the Creative Commons Attribution 4.0 International license (CC-BY 4.0).