Estimating Changes in Contact Patterns in China Over the First Year of the COVID-19 Pandemic: Implications for SARS-CoV-2 Spread - Four Cities, China, 2020

Abstract

Introduction: Previous studies have demonstrated significant changes in social contacts during the first-wave coronavirus disease 2019 (COVID-19) in Chinese mainland. The purpose of this study was to quantify the time-varying contact patterns by age in Chinese mainland in 2020 and evaluate their impact on the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Methods: Diary-based contact surveys were performed for four periods: baseline (prior to 2020), outbreak (February 2020), post-lockdown (March-May 2020), and post-epidemic (September-November 2020). We built a Susceptible-Infected-Recovered (SIR) model to evaluate the effect of reducing contacts on transmission.

Results: During the post-epidemic period, daily contacts resumed to 26.7%, 14.8%, 46.8%, and 44.2% of the pre-COVID levels in Wuhan, Shanghai, Shenzhen, and Changsha, respectively. This suggests a moderate risk of resurgence in Changsha, Shenzhen, and Wuhan, and a low risk in Shanghai. School closure alone was not enough to interrupt transmission of SARS-CoV-2 Omicron BA.5, but with the addition of a 75% reduction of contacts at the workplace, it could lead to a 16.8% reduction of the attack rate. To control an outbreak, concerted strategies that target schools, workplaces, and community contacts are needed.

Discussion: Monitoring contact patterns by age is key to quantifying the risk of COVID-19 outbreaks and evaluating the impact of intervention strategies.

Keywords: COVID-19; Contact pattern; SARS-CoV-2; Social contact.

Figure 1

Number of reported locally transmitted (red) and imported (green) COVID-19 cases, summary of the main interventions performed over time, and timeline of the surveys in Wuhan (A), Shanghai (B), Shenzhen (C), and Changsha (D).

Figure 2

Contact matrices by age group for the four study locations for the baseline period in (A–D), outbreak period (E–H), post-lockdown period (I–L), and post-epidemic period (M–P). Each cell of the matrix represents the mean number of contacts that an individual in a given age group has with other individuals, stratified by age groups. The color intensity represents the number of contacts.

Figure 3

Changes in the reproduction number (mean and 95% confidence interval) considering the estimated contact patterns in the different periods in Wuhan (A), Shanghai (B), Shenzhen (C), and Changsha (D).

Figure 4

The effect of social distancing on the spread of SARS-CoV-2 in Shanghai was examined, assuming an effective reproduction number of 4 and considering baseline (pre-COVID) contact patterns.

Figure S1

Mean number of reported contacts by participant age during the baseline, outbreak, post-lockdown, and post-epidemic periods in (A) Wuhan, (B) Shanghai, (C) Shenzhen, and (D) Changsha.

Figure S2

Contact type, contacts with face masks, and contact environment by age. (A) The proportion of physical contacts, non-physical contacts without 1m distancing, and non-physical contacts with 1m distancing by age for Wuhan. (B–D) The same proportions for Shanghai, Shenzhen, and Changsha, respectively. (E–H) The same proportions as (A–D), but for whether the participants and contacts were wearing face masks. (I–L) The same proportions as (A–D), but for the proportion of contacts that occurred indoors, outdoors, and both.