Cite as:

Alexander F. Siegenfeld and Yaneer Bar-Yam, The impact of travel and timing in eliminating COVID-19, Communications Physics (November 6, 2020).

Abstract

While the spread of communicable diseases such as coronavirus disease 2019 (COVID-19) is often analyzed assuming a well-mixed population, more realistic models distinguish between transmission within and between geographic regions. A disease can be eliminated if the region-to-region reproductive number—i.e. the average number of other regions to which a single infected region will transmit the disease—is reduced to less than one. Here we show that this region-to-region reproductive number is proportional to the travel rate between regions and exponential in the length of the time-delay before region-level control measures are imposed. If, on average, infected regions (including those that become re-infected in the future) impose social distancing measures shortly after experiencing community transmission, the number of infected regions, and thus the number of regions in which such measures are required, will exponentially decrease over time. Elimination will in this case be a stable fixed point even after the social distancing measures have been lifted from most of the regions.

How to eliminate COVID-19 for good, without unnecessary lockdowns

CAMBRIDGE (November 6, 2020) — Is it possible to eliminate COVID and return to what was before? If this controversial idea is possible, then most of the strategies of suppression, "living with the virus,” and herd immunity become clearly inferior to elimination. And a stable elimination is possible according to a new paper in Communications Physics from the New England Complex Systems Institute and MIT.

The key is to use a red and green color zone designation of local regions to rapidly suppress local outbreaks while reducing travel out of the regions in which they occur, an idea rooted in the successful elimination of the 2014 Ebola outbreak. Importantly, there is some room for error: the travel reductions need not be airtight nor the outbreak suppression immediate.

The authors show not only why countries such as Australia, New Zealand, Taiwan, South Korea, China, and Thailand have been successful using outbreak suppression and travel restrictions in getting to normal, where people can go about their lives and attend gatherings and sporting events, but also how to expand that success to Europe, the Americas, and Africa.

A key concept in combatting diseases like COVID-19 is the reproductive number: the average number of people who will be infected by a single individual. If that number is greater than one, the disease spreads exponentially. Below one, infections will exponentially decrease to zero.  When combatting a pandemic, the authors show that the analogous region-to-region reproductive number must also be considered. 

Just as the reproductive number measures the average number of people to whom an infected individual will spread the disease, the region-to-region reproductive number measures the average number of regions to which an infected region will spread the disease. If this number is above one in a group of regions, such as a state or country, the pandemic will continue; below one, the disease cannot sustainably spread among the regions and outbreaks will die out. Thus, understanding how to reduce the region-to-region reproductive number below one shows us how COVID-19 can be eliminated and remain eliminated while regions fully open up their economies. 

In the U.S., regions might be commuting zones or counties. If there is community spread of a disease within a region, then the region as a whole is considered infected. Only regions with community spread (red zones) need to restrict social and economic activity; regions without community spread (green zones) can open back up.  

Red zones can control their outbreaks with social distancing, mask usage, and contact tracing. The sooner these measures are implemented after community spread is discovered, the smaller the region-to-region reproductive number will be, the fewer the number of people who will be infected, and, crucially, the shorter the duration for which restrictions will be needed. A speedy response is better for the health and economy of a region; if there is no community spread, the region is safe to open up businesses and schools. Travel reductions and border control, even if imperfect, can also greatly reduce the region-to-region reproductive number.

The researchers point out that lowering region-to-region reproductive numbers below one is a stable way to eliminate the coronavirus. A region with no community spread may still see infections from travelers, but these cases can be contained with contact tracing or, failing that, with local and sporadic social distancing measures. The whole state or country will not have to shut down again. This has been seen in New Zealand and China, where the initial wave of the disease was eliminated and subsequent regional outbreaks were quickly brought under control without nation-wide lockdowns.

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