Social encounter profiles of greater Melbourne residents, by location – a telephone survey

A total of 25,406 calls were made to 8567 numbers, of which 7129 were currently connected telephones in residential households (2755 of 3398 in Boroondara, 4374 of 5169 in Hume). Contact was made with an individual in 4580 of these households (1827 in Boroondara and 2753 in Hume), with a further 683 proving ineligible due to geographical location (out of area), communication difficulties or absence of an adult present at the time of call. Communication difficulties reported included language difficulty (n = 28) or other physical limitation such as hearing impairment or age (n = 131). Of the remaining 3897 eligible contacts, 1307 (650 in Boroondara, 657 in Hume) completed the survey, with 12 interviews conducted in Turkish. Responses were spread across days of the week such that the numbers are fairly even across weekdays (range: 191—211) and separately across weekends (range: 156—157). Response rates according to the ‘all contacts’ denominator were 36 % (650/1827) in Boroondara and 24 % (657/2753) in Hume. When calculated as all completed interviews over a denominator comprising completions, refusals and break-offs, the response rate overall was 37.6 %, remaining higher in Boroondara (46 %) than Hume (32 %).

While the final number in the sample was less than our initial target of 2000, a pragmatic decision was made to cease recruitment at 1307, given greater than anticipated time requirements per participant and finite budgetary constraints. Implications of study complexity related to the telephone interview format for missing and incomplete data are described in more detail below.

Compared with 2011 Australian census data, characteristics of the sample differed significantly from those of the areas surveyed across a range of demographic and socio-economic factors in both Hume and Boroondara. Important differences included an over-representation of individuals who were aged over 50 years, female, Australian born, English speaking, educated to completion of secondary school, and married. In keeping with these characteristics, smaller households were over-represented. The anticipated bias towards longer resident individuals was also observed. Given this disparity, weighted results are presented for all aggregated data, in order to present results more likely to be representative of the populations under study. For more details, see Sections 5 Biased sampling, raking and sampling weights and 6 Tables demonstrating bias in CATI data of Additional file 1.

Unique addresses visited by participants over the course of the survey day were categorised and distributed as shown in Fig. 2 (top left panel). All but six participants spent some time at their usual home, with retail/hospitality, and private transport being the next most common types of designated settings. Approximately one tenth of locations were assigned as ‘other’, of which about half were non-participant private homes, with the remainder comprising places such as medical centres and facilities, and places of worship. The weighted number of total listed encounters by location is reported in Fig. 2 (top right panel) (and Additional file 4: Figure S1 for physical contact). These data demonstrate the high median number of contacts made in school and daycare settings, but also the marked heterogeneity in encounter profiles within several location types including home, private transport, retail/hospitality and ‘other’. Figure 2 (bottom left panel) highlights the greater length of interactions with workplace colleagues over the course of a working day, relative to those in most other domains. Figure 2 (bottom right panel) similarly reports duration of encounters, but restricted to those involving any physical contact. This last figure strongly reasserts the importance of household settings in providing opportunities for close-contact transmission of infection.

Figure 3 (left panel) reports the unweighted total number of listed encounters for each participant in the survey, with a mean of 8.4 (95 % CI: 7.9–8.8) and median of 7. The weighted total number of listed encounters for each participant across all participants (mean 8.5, 95 % CI: 7.9–9.1) and across days of the week (mean range 8.0–9.3, median range 6–8) was similar. In addition, the distinction between weekend and weekday is not a significant predictor for the number of listed encounters (p = 0.9). Figure 3 (right panel) shows a boxplot of the weighted number of all encounters by age group. The highest reported number of median contacts was among individuals aged between 30 and 49 years. This trend was more evident for episodes involving physical contact (Additional file 4: Figures S2 and S3).

Respondents frequently reported more than one encounter with the same individual over a 24 h period (Additional file 4: Figure S4). The weighted mean number of contacts (with 95 % confidence intervals) between participants and uniquely nominated individuals is shown in Fig. 4. Results are reported across six participant age categories and ten contact age categories, for male (left panel) and female (right panel) respondents. The number of participants contributing to each cell varies, and both weighted and unweighted counts for each gender can be found in the supporting information (Additional file 4: Tables S1–S4 for all encounters and Tables S5–S8 for physical encounters). While a generally assortative pattern of mixing is observed, we also note that females record many more encounters with children less than 15 years of age than males. Males aged 18–29, on the other hand, have the highest recorded within-age group interactions. Similar trends are observed for episodes involving any physical contact (Additional file 4: Figures S7 and S8).

The duration of encounters, by age, is a further measure of mixing intensity, reported as mean hours (with 95 % confidence intervals) in Fig. 5. Again, values for men and women are reported separately. The striking difference between this figure and Fig. 4 is the dominance of interactions between women in the 18–29 years group and children of pre-school age, most likely in household settings (Fig. 5- right panel). These prolonged mixing episodes are also associated with physical contact (Additional file 4: Figure S10 (right panel)). Encounters between males aged 18–29 years, previously noted to be both frequent and close, are also prolonged (Fig. 5 (left panel), and Additional file 4: Figure S10 (left panel) for physical contact).

Heterogeneity of encounters was further assessed by 692 reports of mixing with social groups of six people or more. The median age of participants reporting group contacts was 52 years (IQR 40, 62). Median reported group size was 12 (interquartile range 9, 20), with 40 % of such contacts occurring in the workplace. Retail and hospitality, sport and recreational settings, and educational environments each accounted for approximately 10 % of listed group encounters. A large ‘other’ category included places of worship, clubs and private social gatherings. Mixing matrices for unweighted numbers of encounters and duration of encounters (in person-contact-hours) for participant contact made in a group setting are available in Additional file 4: Sections 1.10 and 1.11, where each group was categorised with one typical age (0–19 years, 20–69, 70 or more) by the participant. Given uncertain overlap between mixing groups and individually reported contacts, subsequent analyses report only on uniquely identified individuals.

Participants were asked to differentiate between encounters with individuals known to them and strangers. Young men, and women aged 30–49 years, reported many more known contacts than other respondents (Fig. 6 -left panel). In the former case, the majority of these contacts were non-household members, while among the women about half of the contacts involved family (Fig. 6 - middle). For both sexes, the total number of known contacts both inside and outside the home tended to increase with household size (Fig. 6 -bottom panel). (See Additional file 4: Figure S11 for participants’ total duration of contacts by contact type and household size.) The vast majority (75 %) of group contacts (see above) involved individuals known to participants.

Summary measures of encounters, as presented in the Figures above, were compared between the LGAs surveyed in an initial exploratory analysis. While social characteristics of participants in both regions were broadly similar, some suggestions of difference emerged, warranting further evaluation. Key findings are reported in Table 1, with caution recommended in the interpretation of estimates where sample sizes were <60. Boroondara (B) residents reported mixing with more people on average in public spaces than Hume (H) residents, and similarly in retail and hospitality environments, of which more were visited [B: 1.8 (1.6, 2.0) cf H: 1.4 (1.3, 1.6), p = 0.007]. Hume residents tended to spend longer periods of time (hours) with others, particularly in arts and culture settings, although this latter estimate was based on a small sample size [B: 15 cf H: 9].

Women less than 20 years of age in Hume had contact with twice as many known individuals than in Boroondara, based on a limited number of participant reports [B: 10 cf H: 9]. Young men (20–29 years) in Hume socialised more with non-household members than those in Boroondara, although again, few participated in the study [B: 18 cf H: 19]. At the other end of the age spectrum, contact with unknown individuals was more frequent in Boroondara than Hume among females aged 50–59 and older, and males from 60 years of age.