Overall, 4297 participants were included in this study, Supplementary Table S1 presented detailed demographic characteristics of the participants. We recorded a total of 23,090 contacts from 2155 male respondents and 2142 female respondents, including 7519 (32.6%) acquaintance contacts and 15,571 (67.4%) stranger contacts. Young and middle-aged participants reported higher number of daily contacts and higher proportion of stranger contacts, with participants aged 30–34 years reporting the highest number of contacts, those aged 20–24 years reporting the highest proportion of stranger contacts and aged 0–4 years reporting the lowest proportion of stranger contacts (Fig. 1a, b). Similarly, young and middle-aged participants reported longer total contact duration and participants of all ages had a low proportion of total contact duration with strangers, with participants aged 30–34 years reporting the longest total contact duration and those aged 20–24 years reporting the highest proportion of total contact duration with strangers, but the highest proportion of total contact duration with strangers was still less than 20% (Fig. 1c, d). Participants generally had contact with acquaintances at home, teenagers also often had contact with acquaintances at classroom, young, and middle-aged people had a higher proportion of contact with acquaintances in the workplace, and the proportion of elderly people having contact with acquaintances in recreation places and other places (parks, food markets, etc.) obviously increased (Fig. 1e, f). The contact between children and strangers mainly occurred in mass transit, while the contact between teenagers and strangers mainly occurred in the classroom, the contact between middle-aged and young people and strangers mainly happened in the workplace, and as age increased, the proportion of contact with strangers in supermarkets increased (Fig. 1g, h). In the past year, the daily activities of participants per week were mainly cross villages or cross towns, with few cross provincial or cross national activities, the longest weekly activity was across counties (Fig. 1i, j).

The distribution of contact patterns between acquaintances and strangers (including the number of contacts (a, b), duration of contact (c, d), and contact location (e, f, g, h)). And the frequency (i) and duration (j) distribution of participants across various regions

For the Participant–Total (P–T) contact, the matrix presented typical features of age-mixing patterns. This matrix had three distinct diagonals, with the middle diagonal representing contacts between the same age group, with female being more pronounced than male. The other two diagonals indicated contact between family members. The middle part of the matrix represented more contact between young and middle-aged people, with male being more pronounced than female (Fig. 2a). In addition, men had more mean daily contact than women (5.80 V.S. 4.89), mainly because they had more contact with strangers (3.86 V.S. 2.96) (Table 1). For the participant–acquaintance (P–A) contact, regardless of gender, the matrix exhibited a three diagonals phenomenon, that was, the contact mainly occurred among peers and between family members. (Fig. 2b). For participant–stranger (P–S) contact, the matrix had a clear central clustering characteristic, especially for male, indicating more contact happened between young and middle-aged people (Fig. 2c). The difference matrix also showed that participants’ contact with strangers is mainly concentrated in young and middle-aged people, especially for men (Fig. 2d). The contact matrix of contact duration had similar features to P–T, P–A, and P–S (Fig. 2e, f, g) and the average contact duration of P–A was much longer than that of P–S (5.53 h V.S. 0.20 h).

Contact matrix between participants and different contacts: (a) total population; (b) acquaintance; (c) stranger; (d) differences in the contact matrix between acquaintance and stranger. Contact duration matrix between participants and different contacts: (e) total population; (f) acquaintance; (g) stranger

For P–T and P–A contact, weekday contact had more obvious contact characteristics among peers and between family members than weekend contact. For P–A contact, there was a very obvious feature of contacts among young children on weekends (Fig. 3a, b). For P–S contact, the weekday contact displayed a more obvious characteristic of contact between middle-aged and young people than weekend contact. On weekends, mainly participants aged 30–34 years were in contact with strangers (Fig. 3c). The difference matrix also showed more central clustering on weekdays than on weekends (Fig. 3d). In addition, weekdays had more mean number of contacts than weekend (5.47 V.S. 4.11), mainly with stranger (3.50 V.S. 2.25) (Table 1). For the P–T and P–A contact, the resident population presented more obvious contact characteristics among peers and between family members than that of floating population (Fig. 3e, f). For the P–S contact, the resident population exhibited a more obvious phenomenon of contact between middle-aged and young people than that of the floating population (Fig. 3g). The difference matrix also showed more central clustering in the resident population than the floating population (Fig. 3h). Moreover, the resident population had more mean number of contacts than the floating population (5.59 V.S. 3.73) (Table 1). Finally, the non-physical contact reported by participants generally exceeded physical contact, whether in P–A or P–S contact (Fig. 3i, j, k) (Table 1). The difference matrix of physical contact was mainly concentrated in middle-aged people and the elderly (Fig. 3l).

Contact matrix between participants and different contacts stratified by day of the week (a–d), population types (e–h) and contact types (i–l): (a, e, i) total population; (b, f, j) acquaintance; (c, g, k) stranger; (d, h, l) differences in the contact matrix between acquaintance and stranger

For P–T contact, contacts mainly occurred in workplaces, classrooms, mass transits and restaurants (Fig. 4a, Table 1). For P–A contact, there was obvious characteristic of family member contact in home. In the workplace, contacts mainly occurred between colleagues, and in the classroom, contacts mainly happened between classmates. Except for irregular contact pattern in hospitals, there was a clear feature of same-age contact in mass transits, recreation places, restaurants, and other places (Fig. 4b). For P–S contact, stranger contact commonly occurred in workplaces, classrooms, mass transits, restaurants, and supermarkets and mostly concentrated in the middle-aged groups (Fig. 4c, Table 1). For the difference matrix, in recreation places, there was an obvious same-age contact characteristic, while in workplaces, mass transits and restaurants, the differences were mainly concentrated among young and middle-aged people (Fig. 4d). Finally, the Wilcoxon rank sum test showed a statistically significant difference in Q of P–A and P–S contact (Table 1).

Contact matrix between participants and different contacts stratified by location of contacts: (a) total population; (b) acquaintance; (c) stranger; (d) differences in the contact matrix between acquaintance and stranger

In general, female (Odds rate (OR) = 0.67, 95% confidence interval (CI) 0.58–0.77), those with higher monthly income (OR = 0.82, 95% CI 0.77–0.88), and those who received influenza vaccines (OR = 0.65, 95% CI 0.57–0.74) had lower incidence rate of influenza, while social contact was the risk factor of the incidence rate of influenza, and the OR of stranger contact (OR = 2.63, 95% CI 2.29–3.02) was greater than that of acquaintance contact (OR = 1.82, 95% CI 1.56–2.13). In addition, monthly income level and influenza vaccination had impacts on the incidence rate of influenza in men, but not in women. Social contact, whether they are strangers or acquaintances, had a higher impact on the incidence rate of influenza in women than in men (Stranger 2.76 V.S. 2.46, Acquaintance 1.98 V.S. 1.70). On weekdays, women (OR = 0.65, 95% CI 0.57–0.75), high-income people (OR = 0.87, 95% CI 0.81–0.93), people with more co-habitations (OR = 0.89, 95% CI 0.84–0.94), and people who have received influenza vaccines (OR = 0.63, 95% CI 0.55–0.72) had a lower incidence rate of influenza. Social contact, especially with strangers (OR = 2.56, 95% CI 2.22–2.95), would increase the incidence rate of influenza on weekdays, which was more significant on weekends (OR = 4.31, 95% CI 2.31–8.01). In the floating population, higher monthly income level (OR = 0.48, 95% CI 0.35–0.65) would reduce the incidence rate of influenza. In the resident population, female (OR = 0.63, 95% CI 0.54–0.72), high monthly income (OR = 0.86, 95% CI 0.80–0.92), large family size (OR = 0.90, 95% CI 0.85–0.95), and influenza vaccination (OR = 0.63, 95% CI 0.54–0.72) could achieve the same goal, while social contact was the opposite (Stranger OR = 2.66, 95% CI 2.30–3.07, Acquaintance OR = 1.95, 95% CI 1.65–2.30). Similar phenomena could be found in different contact places. In general, social contact in workplaces (Stranger OR = 3.70, 95% CI 2.62–5.22, Acquaintance OR = 4.58, 95% CI 2.49–8.44), classrooms (Stranger OR = 3.61, 95% CI 2.30–5.66), recreation places (Stranger OR = 4.94, 95% CI 2.26–10.78), and home (Stranger OR = 2.54, 95% CI 2.09–3.07) would increase the incidence rate of influenza, while influenza vaccination could reduce the incidence rate of influenza in workplaces (OR = 0.27, 95% CI 0.21–0.35) and mass transit (OR = 0.41, 95% CI 0.27–0.63) (Fig. 5).

Factors influencing the incidence rate of influenza in different scenarios