Social mixing patterns relevant to infectious diseases spread by close contact in urban Blantyre, Malawi

Abstract

Introduction: Understanding human mixing patterns relevant to infectious diseases spread through close contact is vital for modelling transmission dynamics and optimisation of disease control strategies. Mixing patterns in low-income countries like Malawi are not well known.

Methodology: We conducted a social mixing survey in urban Blantyre, Malawi between April and July 2021 (between the 2nd and 3rd wave of COVID-19 infections). Participants living in densely-populated neighbourhoods were randomly sampled and, if they consented, reported their physical and non-physical contacts within and outside homes lasting at least 5 min during the previous day. Age-specific mixing rates were calculated, and a negative binomial mixed effects model was used to estimate determinants of contact behaviour.

Results: Of 1201 individuals enroled, 702 (58.5%) were female, the median age was 15 years (interquartile range [IQR] 5-32) and 127 (10.6%) were HIV-positive. On average, participants reported 10.3 contacts per day (range: 1-25). Mixing patterns were highly age-assortative, particularly those within the community and with skin-to-skin contact. Adults aged 20-49 y reported the most contacts (median:11, IQR: 8-15) of all age groups; 38% (95%CI: 16-63) more than infants (median: 8, IQR: 5-10), who had the least contacts. Household contact frequency increased by 3% (95%CI: 2-5) per additional household member. Unemployed participants had 15% (95%CI: 9-21) fewer contacts than other adults. Among long range (>30 m away from home) contacts, secondary school children had the largest median contact distance from home (257 m, IQR 78-761). HIV-positive status in adults >=18 years-old was not associated with changed contact patterns (rate ratio: 1.01, 95%CI: (0.91-1.12)). During this period of relatively low COVID-19 incidence in Malawi, 301 (25.1%) individuals stated that they had limited their contact with others due to COVID-19 precautions; however, their reported contacts were 8% (95%CI: 1-13) higher.

Conclusion: In urban Malawi, contact rates, are high and age-assortative, with little reported behavioural change due to either HIV-status or COVID-19 circulation. This highlights the limits of contact-restriction-based mitigation strategies in such settings and the need for pandemic preparedness to better understand how contact reductions can be enabled and motivated.

Keywords: Africa; Infectious disease; Malawi; Mixing data; Social contacts; Transmission.

Fig. 1

The probability distribution, number and proportion of study participants and reported contact events during the social contact patterns study in urban Blantyre, Malawi between April and July 2021. The frequency and probability distribution of participants, with proportion of participants in age groups of infants (<1 years-old), preschool (1–4 years-old), primary school (5–14 years-old), secondary school (15–19 years-old), adults (20–49 years-old) and older adults (50 + years-old) (insert) (A); The age frequency and probability distribution of contacts, with proportion of contacts by age groups (insert) (B); The number of contacts (bold fontface) and participants (roman fontface), and proportion of participants by day of the week when the contact event occurred (C); and the total number and probability distribution of reported contact events for a given number of contacts reported by the participant (D). Throughout the plots, the black line represents the probability density with values on secondary y-axis whereas yellow and purple colour represent participants and contacts, respectively. The 95% confidence interval for each bar is shown as vertical line.

Fig. 2

Characteristics of reported physical and non-physical social contacts in urban Blantyre, Malawi between April and July 2021. The proportion of physical and non-physical contacts by the duration of contacts (A), frequency of contacts (B), relationship of participant to contacts (C), the exact location of contacts (D), contacts within or outside community (E) and contact sex (F). The number in each bar plot indicates absolute number of reported contacts. Physical contact refers to participant’s skin to skin touch with a contact whereas non-physical contact refers to participant’s two-way close verbal conversation lasting for ≥ 5 min and with ≥ 3 words exchanged. The inverse cumulative distance distribution showing the proportion of contacts in relation to the distance (in metres) further away from the participant home by physical and non-physical contact type (G), for physical contact events by age group (H), and for non-physical contact events by age group (I).

Fig. 3

The daily mean number of reported contacts between age groups, in urban Blantyre, Malawi between April and July 2021. The number in each cell represents the daily mean number of contacts between two age groups from 1000 bootstrap replicates, corrected for reciprocity between participants and contacts, and weighted for day of the week. The matrices show the daily mean number of all contacts (A), physical contacts (participant’s skin to skin touch with a contact) (B) and non-physical contacts (participant’s two-way close verbal conversation lasting for ≥5 min and with ≥3 words exchanged with a contact) (C).

Fig. 4

Daily mean number of reported contacts between age groups, in urban Blantyre, Malawi between April and July 2021. The number in each cell represents the daily mean number of contacts between two age groups from 1000 bootstrap replicates, corrected for reciprocity between participants and contacts, and weighted for day of the week. The top and bottom matrices show the daily mean number of contacts respectively stratified by male and female sex (A), within and outside household contacts (B), within and outside community contacts (C), Human Immunodeficiency Virus (HIV) positive adults (20 + years old) and HIV-negative adults (D). The assortativity index Q quantifies the weight of mixing between individuals of the same age groups and is estimated on the square matrix. For the HIV-stratified matrices, the index Q is estimated on the square matrix between pairs of adult participants and adult contacts aged at least 20 years old.