Role of case introductions in the
community spread of infectious diseases

Winnipeg 23 May 2023

Julien Arino (Julien.Arino@umanitoba.ca)

Department of Mathematics & Data Science Nexus
University of Manitoba*

Canadian Centre for Disease Modelling (CCDM/CCMM)
NSERC-PHAC EID Modelling Consortium

*The University of Manitoba campuses are located on original lands of Anishinaabeg, Cree, Oji-Cree, Dakota and Dene peoples, and on the homeland of the Métis Nation.
We respect the Treaties that were made on these territories, we acknowledge the harms and mistakes of the past, and we dedicate ourselves to move forward in partnership with Indigenous communities in a spirit of reconciliation and collaboration.

Thanks ..

  • Nicolas Bajeux
  • Pierre-Yves Boëlle (IPLESP, Sorbonne Université)
  • Amy Hurford (Memorial University of Newfoundland)
  • Evan Milliken (University of Louisville, Kentucky, USA)
  • Stéphanie Portet (University of Manitoba)
  • James Watmough (University of New Brunswick)

Funders

  • Canadian Institutes for Health Research (CIHR), NSERC EIDM & University of Manitoba Faculty of Science Research Chairs program
  • (& logistical support) Public Health Agency of Canada

Outline

  1. Waves of COVID-19 & Case introductions
  2. Modelling introductions
  3. Measures to control introductions
    a. Role of transport restrictions
    b. Role of quarantine

Waves of COVID-19
& Case introductions

Why so much heterogeneity?

  • Some jurisdictions (health regions/municipios/counties) had cases throughout
  • Others either had silent transmission chains for more than 3 weeks or had reimportation of cases
  • Critical phase is importation/establishment in locations: sometimes things pick up, other times they don't...

Heterogeneity in "lived epidemics" increases
at the micro-scale

center

Case introductions

  • In Ecology, introductions have been studied for a while, because they are one of the drivers of evolution and, more recently, because of invasive species

  • In epi, introduction/importation occurs when an individual who acquired the pathogen in a jurisdiction makes their way to other jurisdiction while still harbouring the pathogen

  • Geographies greatly influence reasoning

    • Country level: importations quickly become less relevant
    • Consider an isolated location of 500 people.. disease may become extinct then be reimported

Modelling introductions

Investigating outbreak types using a simple CTMC SIS with a twist

Regular chain of this type has as sole absorbing state

We add another absorbing state: if , then the chain has left the stochastic phase and is in a quasi-deterministic phase with exponential growth

Doing this, time to absorption measures become usable additionally to first passage time ones

And the question becomes: how long does the chain "linger on" before it is absorbed? We define the inter-absorption trajectory as the stochastic phase

Measures to control introductions

Almost exclusively attacked from the perspective of would-be importer

In practice:

  • Travel restrictions/interruptions
  • Quarantine

Plague control measures

  • Lazzarettos of Dubrovnik 1377
  • Quarantena of Venice 1448
  • Isolation of known or suspected cases as well as contacts, first for 14 days and gradually 40 days
  • Improvement of sanitation: pure water supplies, garbage and sewage disposal, food inspection
  • .. Find and kill a snake, chop it into pieces and rub the various parts over swollen buboes. (Snake Satan, and evil is drawn to evil)

Effect of quarantine

Lazzaretto vecchio

Effect of quarantine on introduction rates

the mean time between case introductions, the mean quarantine-regulated time between case introductions, the efficacy of quarantine (in %). Then

Suppose 5 days and efficacy of quarantine is 90% at 7 days and 98% at 14 days, respectively

Then 50 and 250 days, respectively

Role of travel restrictions

Mur de la Peste in Cabrières-d’Avignon

Untangling the contribution
of introduced cases

Time and time again, top jurisdictional level PH authorities take travel interruption measures

What is really the contribution of introductions to overall spread within a jurisdiction?

Use an importation layer separating introduced (imported) cases from locally generated ones

Introductions in a metapopulation model
Left: low movement rate. Right: higher movement rate

Interruption of travel for COVID-19

Country Date travel suspension Date first case
Seychelles 2020-03-03 2020-03-14
El Salvador 2020-03-17 2020-03-18
Cape Verde 2020-03-17 2020-03-20
Sudan 2020-03-17 2020-04-05
Marshall Islands 2020-04-22 2020-10-29
Vanuatu 2020-03-20 2020-11-11
North Korea 2020-01-21 Unreported
Turkmenistan 2020-03-20 Unreported
Tuvalu 2020-03-26 2022-05-20 (2021-11-02?)

To conclude

[..] a powerful expression of state's sovereignty, immigration control provides a typical avenue for governments to reassure their citizens and bolster a national sense of belonging, while providing an ideal scapegoat for their own failure or negligence.

V. Chetail. Crisis without borders: What does international law say about border closure in the context of Covid-19? Frontiers in Political Science, 2 (12) (2020)

Merci / Miigwech / Thank you

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