Estimating the Risk of Outbreaks of COVID-19 Associated with Shore Leave by Merchant Ship Crews: Simulation Studies for a Case Country

  1. Nick Wilson
  2. Tony Blakely
  3. Michael Baker
  4. Martin Eichner

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Abstract

Aim

We aimed to estimate the risk of COVID-19 outbreaks in a case study COVID-free destination country, associated with shore leave for merchant ship crews.

Methods

A stochastic version of the SEIR model CovidSIM v1.1, designed specifically for COVID-19 was utilised. It was populated with parameters for SARS-CoV-2 transmission, shipping characteristics, and plausible control measures.

Results

When no control interventions were in place, an outbreak of COVID-19 in our case study destination country (New Zealand; NZ) was estimated to occur after a median time of 23 days (assuming a global average for source country incidence of 2.66 new infections per 1000 population per week, a crew of 20, a voyage length of 10 days, 1 day of shore leave both in NZ and abroad, and 108 port visits by international merchant ships per week). For this example the uncertainty around when outbreaks occur is wide (an outbreak occurs with 95% probability between 1 and 124 days). The combined use of a PCR test on arrival, self-reporting of symptoms with contact tracing, and mask use during shore leave, increased this median time to 1.0 year (14 days to 5.4 years). Scenario analyses found that onboard infection chains could persist for well over 4 weeks even with crews of only 5 members.

Conclusion

Introduction of SARS-CoV-2 through shore leave from international shipping crews is likely, even after long voyages. The risk can be substantially mitigated by control measures such as PCR testing and mask use.

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  1. ScreenIT

    SciScore for 10.1101/2020.09.08.20190769: (What is this?)

    Please note, not all rigor criteria are appropriate for all manuscripts.

    Table 1: Rigor

    NIH rigor criteria are not applicable to paper type.

    Table 2: Resources

    No key resources detected.

    Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).

    Results from LimitationRecognizer: We detected the following sentences addressing limitations in the study:
    Study strengths and limitations: This appears to be the first modelling study to explore the risk of COVID-19 outbreaks arising from shore leave of maritime ship crews (based on our search of PubMed and preprint sites in August 2020). Another strength is that the work builds on an established model that has been used to also study air transport and other aspects of SARS-CoV-2 transmission (see Methods). But as with all modelling there are important limitations. Some of these relate to parameters, with a particularly critical one being the daily incidence of SARS-CoV-2 infection in the source country that the ship leaves from. We just used a global average for this incidence to account for the diverse maritime trading patterns that New Zealand has and also because the crews are also internationally diverse (often flying in from another country just prior to the ship’s departure). Nevertheless, there are likely to be highly variable risks by source country and countries that the crew come from. Another example of parameter limitations are the Reff onboard such vessels and also the Reff for shore leave by crew. The former is likely to vary by different designs of merchant vessels (container ships vs. tankers vs. bulk carriers etc.) and also by size (e.g. it is likely that in vessels of under 3000 gross tonnage the crew are in shared sleeping rooms). However, we did not have sufficient data to model such heterogeneity. We also didn’t account for potential immunity amongst crew fr...

    Results from TrialIdentifier: No clinical trial numbers were referenced.

    Results from Barzooka: We did not find any issues relating to the usage of bar graphs.

    Results from JetFighter: We did not find any issues relating to colormaps.

    Results from rtransparent:
    • Thank you for including a conflict of interest statement. Authors are encouraged to include this statement when submitting to a journal.
    • Thank you for including a funding statement. Authors are encouraged to include this statement when submitting to a journal.
    • No protocol registration statement was detected.

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  2. Version published to 10.1101/2020.09.08.20190769v1 on medRxiv