Sailing the high Cs – Cruise ships Contact networks for COVID-19 transmission

Combining Bluetooth contact tracing devices with location data during cruise sailings, new research provides insights into social interactions on cruise ships, and highlights the potential implications for disease control in similar large-scale events.

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Date of expiry, 6 Mar 2024. Phew, no need for renewal. It was Nov 2020, and my passport could finally be put to good use again during the slated ‘shakedown cruise’ sailing. As a then public health officer at the Ministry of Health, Singapore, I was part of the team onboard to evaluate the risk of COVID-19 transmission in cruises as the industry prepared to resume operations after an 8-month hiatus. The COVID-19 pandemic was approaching its one-year anniversary and travel-starved Singapore residents were waiting fervently for the reopening of countries' borders. With the ongoing surge in COVID-19 Alpha variant cases worldwide, this was unlikely to happen anytime soon as the country took strict precautions to limit the influx of imported cases infected with Variants of Concern.

But fortunately there was a means of travelling out of the country and returning without a net increase in imported cases – a cruise to nowhere. These cruise sailings offered a short voyage away from the usual inland activities without calling on any ports and so travellers are sheltered from the external risk of SARS-CoV-2 infection from countries with ongoing outbreaks. However, memories of the surge in COVID-19 cases onboard the Diamond Princess cruise ship early during the pandemic was a forlorn reminder that internal risks should not be underestimated.

Cruise ships have long been notorious for infectious disease transmission, largely involving food-borne outbreaks such as norovirus. Outbreaks on board were also reported during the 2009 H1N1 pandemic and in other seasonal influenza epidemics. Thus, even before the pandemic, public health measures such as case isolation, closure of contaminated areas, promotion of personal hygiene were already implemented to reduce the risks.

Outbreaks onboard are driven by a myriad of factors. In a typical cruise sailing, there are about 3,000 – 6,000 passengers onboard, translating to a space to passenger ratio of 30 – 50 ft3/passenger in commercial cruises (similar capacity to a large sedan in the US carrying 4 passengers). The interaction between passengers and crew members at service touchpoints provides opportunity for diseases to pass back and forth from a population of 1,000 – 2,000 crew, who have interrelated work functions and live in tight spaces.

More than half of the facilities in commercial cruises are categorised as food and beverages or sports, and these areas involve high movement activity (e.g. at buffets, at outdoor water features) where mask wearing is not feasible – the perfect recipe for epidemiological turbulence. Thus, it comes to no surprise when cruise ships were viewed as “floating Petri dishes” which could aggravate the COVID-19 case incidence. Sailing against the viral winds meant a lot of preparation: we needed to establish pre- and post-cruise testing in passengers and routine testing in crew, inspect and enforce physical distancing rules onboard, plan the conveyance of suspect cases to isolation sites, train and operationalise contact tracing efforts onboard, scrutinise legal boundaries for data collection and analysis during pilot sailings…

Fig 1. Contact tracing device (rightmost, mint green) used onboard . 

Disease like COVID spread through social interactions, so to understand transmission and to design effective strategies, we really need to measure these networks of contacts. Fortunately, the pandemic has accelerated the use of contact tracing devices to measure the intensity and duration of social interactions between individuals using Bluetooth signals. These devices can store the encrypted data locally before uploading to a secure server after several sailings. Crucially, data on an entire cruise contact network allows us to track an individual’s close, casual and passing contacts and determine the potential logistics that the Ministry of Health would need to coordinate for successful contact tracing and quarantine of close contacts. In addition, as a mathematical epidemiologist, this novel, high-resolution network data piqued my interest on the potential transmission of SARS-CoV-2 onboard.

 Cruises involve many activities with multiple people in the contact network, and the outcomes of the interventions could potentially translate to other settings of large-scale events such as conferences sometimes lasting for more than three days. The low case incidence in Singapore meant that there is a very small chance of encountering an infected individual onboard but the risk of a large outbreak remains in the absence of interventions for safe sailing. Using the empirical contact data, we could study how the risk of infection vary with the duration of contact and under different assumptions on the criteria of an effective contact (i.e. how different is a 30 minute vs a 1 hour contact). Our recent paper shows how we incorporated the contact data into a mathematical model to study the potential number of new infections that might arise from a SARS-CoV-2 infected person in the presence of control measures implemented under realistic conditions.

Fig 2. Network of all contacts lasting between 1200 and 1205 on a sail day. Edge width and colour intensity illustrate the duration of the contact. Intra- and inter-cohort contacts are represented by the connection of nodes with the same and different colour respectively

More fundamentally, the tracing devices allow for seamless data collection to document the characteristics of the contact network on cruises. This helps to overcomes the challenge of participants forgetting their number of contacts made and the duration of contact in self-reported contact history. Inaccurate data would bias our modelled risk of COVID-19 transmission. 

Furthermore, physical distancing and gathering size limits onboard were new public health measures. While passengers were constantly reminded of the need to maintain at least one metre distance from other travel groups during the cruises, occasional excitement when enjoying the facilities may let the rules slip. On the other hand, crew working areas are usually out of bounds to the public and physical distancing is largely dependent on self-discipline. Subsequent analysis of contact patterns can help check whether rules are being adhered to. When potential non-compliances were detected, further investigations are necessary. Where did the prolonged gathering occur? Is the gathering intentional or a bottleneck onboard? Who are the people in these groupings?

In the pilot cruise sailings, as the Bluetooth devices do not capture location data, alternative data sources were utilised to piece together the location and profile of persons gathering in large clusters and allowed us to better augment human traffic flow. Furthermore, the deployment of Bluetooth contact tracing devices in a steel ship raised several curious questions. Could the Bluetooth signals reflect off the steel interiors of the crew working environment? Does the current calibration detect signals from other devices in the neighbouring cabin? How long does it take to upload the contact data to the servers on land, decrypt and process the data for contact tracing efforts? Again, the shakedown cruises provided a platform to test the feasibility of data collection.

Overall, designing a holistic public health outbreak response, collecting essential data and testing a variety of scenarios was made possible through the close working relationship between the public health specialist, cruise business personnel and operators, border health officials, medical professionals, data engineers and scientists to address public health, business, legislation and technological considerations. While the conceptualisation to execution took less than 6 months, this draws on the years of experience in managing infectious disease outbreaks and keeping abreast of the rapid developments in epidemiology and behavioural science during this pandemic. 

“Ok, take a deep breath and recall the happy experiences onboard the cruise”, my COVID-19 swabber said as he inserts the nasal swab up my nostrils as part of the post-cruise testing and made me shed tears (of joy, perhaps) within seconds. We disembarked after two working days and headed straight to our onshore office tower for an after action review.  As the conversations between the cruise operators and the government representatives continued to flow, my mind was occasionally distracted by the view of the majestic cruise with LED lights from my office window, my body was attempting to recover from the swaying motion and my fingers subconsciously found the keys to type out meeting notes. We worked through the remaining operational issues and discussed how new modalities such as vaccination and different testing methods would potentially shape the experience of cruising in the subsequent months. In no time, travellers were back to the high seas.

Rachael Pung

Student, London School of Hygiene & Tropical Medicine