The role of vaccine status homophily in the COVID-19 pandemic: a cross-sectional survey with modelling

Abstract

Background: Vaccine homophily describes non-heterogeneous vaccine uptake within contact networks. This study was performed to determine observable patterns of vaccine homophily, as well as the impact of vaccine homophily on disease transmission within and between vaccination groups under conditions of high and low vaccine efficacy.

Methods: Residents of British Columbia, Canada, aged ≥ 16 years, were recruited via online advertisements between February and March 2022, and provided information about vaccination status, perceived vaccination status of household and non-household contacts, compliance with COVID-19 prevention guidelines, and history of COVID-19. A deterministic mathematical model was used to assess transmission dynamics between vaccine status groups under conditions of high and low vaccine efficacy.

Results: Vaccine homophily was observed among those with 0, 2, or 3 doses of the vaccine. Greater homophily was observed among those who had more doses of the vaccine (p < 0.0001). Those with fewer vaccine doses had larger contact networks (p < 0.0001), were more likely to report prior COVID-19 (p < 0.0001), and reported lower compliance with COVID-19 prevention guidelines (p < 0.0001). Mathematical modelling showed that vaccine homophily plays a considerable role in epidemic growth under conditions of high and low vaccine efficacy. Furthermore, vaccine homophily contributes to a high force of infection among unvaccinated individuals under conditions of high vaccine efficacy, as well as to an elevated force of infection from unvaccinated to suboptimally vaccinated individuals under conditions of low vaccine efficacy.

Interpretation: The uneven uptake of COVID-19 vaccines and the nature of the contact network in the population play important roles in shaping COVID-19 transmission dynamics.

Keywords: COVID-19; Contact network; Homophily; Mathematical model; Transmission; Vaccine.

Fig. 1

Example meta advertisement used for participant recruitment

Fig. 2

Distribution of vaccine homophily scores, by participant vaccination status

Fig. 3

Homophily and Contact Network Size by Quantile. Figure Note: Each box in the boxplot spans from the first quartile (Q1) to the third quartile (Q3) of the data, representing the middle 50% of the data points for each vaccine status category. The black horizontal bar within each box represents the median of the data. In the context of this boxplot, it indicates the median network size for participants within each vaccine status category. The vertical lines, known as whiskers, extend from the upper and lower edges of the box to the highest and lowest values within a specific range. This range is typically defined as 1.5 times the interquartile range (IQR) above the upper quartile and below the lower quartile. Values outside this range are considered outliers and are not included in the whiskers. In this plot, the whiskers represent the spread of the network size data points, excluding outliers, for each vaccine status category

Fig. 4

Number of Infections and Force of Infection: Assessment of the Impact of Homophily Under Scenarios of Low and High Vaccine Efficacy. A Number of infections under a scenario with vaccine homophily and high vaccine efficacy. The trajectory is colour-coded by vaccination status. Heat maps P1 and P2 show the force of infection at 15 and 45 days, respectively. B Number of infections under a scenario without vaccine homophily and with high vaccine efficacy. C The number of infections per day for various vaccination groups under a scenario with vaccine homophily and low vaccine efficacy. D Daily number of infections according to vaccination status under a scenario without homophily and low vaccine efficacy. The following parameter values were used under conditions with vaccine homophily. The horizontal and vertical axes on the heat maps represent vaccination status. The numbers within the heat maps indicate which group is transmitting infection to which other group: “2 to 1” indicates that individuals with 2 doses transmit to those with only 1 dose on that grid, and “3 to 0” indicates that those with 3 doses transmit to unvaccinated individuals on that grid