Using high-resolution contact networks to evaluate SARS-CoV-2 transmission and control in large-scale multi-day events

Abstract

The emergence of highly transmissible SARS-CoV-2 variants has created a need to reassess the risk posed by increasing social contacts as countries resume pre-pandemic activities, particularly in the context of resuming large-scale events over multiple days. To examine how social contacts formed in different activity settings influences interventions required to control Delta variant outbreaks, we collected high-resolution data on contacts among passengers and crew on cruise ships and combined the data with network transmission models. We found passengers had a median of 20 (IQR 10-36) unique close contacts per day, and over 60% of their contact episodes were made in dining or sports areas where mask wearing is typically limited. In simulated outbreaks, we found that vaccination coverage and rapid antigen tests had a larger effect than mask mandates alone, indicating the importance of combined interventions against Delta to reduce event risk in the vaccine era.

Fig. 1. Distribution of cruise ship contacts.

a Illustrative short-term network dynamics, showing the cumulative network of all contacts that began between 12.00 to 12.05 pm on the second day of a sail and lasted till the end of their contact episode. Edge width and colour intensity are a function of the type of contact (i.e. close, casual and transient). Intra- and inter-cohort contacts are represented by the connection of nodes with the same and different colour respectively. b Number of unique close, casual, transient contacts made per day by passenger and crew. c Number of close contacts per day for both crew and passengers using different thresholds for the cumulative duration of interaction. Median (shapes), 50% (dark lines), and 95% intervals (light lines) of contacts from 5216 passengers and 4197 crew across four sailings are shown in (b) and (c).

Fig. 2. Number of contacts made over time in respective locations.

Contacts made between passengers from different travelling groups per visit to a type of location (a–e) and a snapshot of contact network at respective locations for 2 h intervals on the second day of the sailing (f–j). Type of locations are: F&B (a, f), entertainment (b, g), sports (c, h), shops (d, i) and public areas (e, j). Median (shapes), 50% (dark lines) and 95% intervals (light lines) of contacts from 5216 passenger and 4197 crew across four sailings are shown in (a–e). Nodes of different colour intensity represent the time spent in the location by respective passengers in (f–j).

Fig. 3. Type of contact by location of interaction and throughout the sailing.

a Proportion of all close, casual and transient contact episodes between passengers of different travelling groups by the location of interaction, b proportion of all close, casual, and transient contact episodes between passengers of different travelling groups by respective F&B locations. Median (shapes), 50% (dark lines) and 95% intervals (light lines) are shown.

Fig. 4. Outbreak size under respective interventions.

a Cases and contacts in one outbreak simulation with cases represented by an enlarged node and red curved arrows depicting disease transmission, b proportion of simulations by respective outbreak size under different interventions, c average outbreak size and d 95th percentile of outbreak size for different interventions and varying vaccination coverage.