MONDAY, 11 MARCH 2024
A brief historyThe consequences of global connectivity are not a recent phenomenon; the spread of many devastating diseases of the past can be attributed to trade routes, conflict, and colonisation. Infectious disease has played a central role shaping world history, with diseases such as smallpox proving more deadly in historical wars than injury and claiming millions of lives.
The Spanish conquest of the Americas in the 16th century brought with it several diseases, including both smallpox and ‘cocoliztli’ (thought now to be either salmonella or viral haemorrhagic fever). These diseases devastated local communities, killing millions and, by 1600, Mexico’s population had fallen to only 2 million. Plague, an infamous bacterial disease responsible for the deaths of more than 200 million people throughout human history, also owes its spread to human migration and connection. The European outbreaks of the 14th century killed one-third of the continent's population, and likely arrived from central Asia via Genoese ships containing infected fleas.
The death tolls of these diseases demonstrate how human connection has served as a propagator of disease throughout history, and the catastrophic risk of contact between populations with differing levels of disease immunity.
The air revolution
In recent history, the nature of global transport has shifted: travel that would take weeks by sea now takes hours, and the proportion of the population able to access this rapid travel is expanding. Aviation passenger numbers have grown nearly 9% a year since 1960, with estimates suggesting that in high income countries human mobility has increased over 1000-fold since 1800. This doubtlessly impacts disease.
The threat posed by rapid international connection has already been realised, with the spread of many recent diseases being directly facilitated by air travel. For example, the rapid spread of HIV between 1984 and 1990 can be modelled accurately using air traffic flow between cities. Introduction of a pathogen into an area with an appropriate vector population can have devastating consequences in the spread of vector-borne diseases.
For example, in 2013, introduction of the Chikungunya virus to Saint Martin (likely by viraemic travellers) was devastating, due to ecological suitability and competent vectors. It led to rapid spread throughout Latin America, affecting 46 countries and 3 million people. Again, global travel proved predictive in its spread, with travel patterns from endemic locations predicting spread to Puerto Rico. This was realised one year later.
International travel is not only concerning in the emergence and spread of novel pathogens; it can also lead to the spread of antimicrobial resistance (AMR), with the rapid rise in broad spectrum AMR representing a growing global health threat. For example, the NDM-1 gene confers resistance to almost all cephalosporin and carbapenem antibiotic classes. After initial identification in India in 2008, by 2010 NDM-1 was detected in multiple countries across several continents, including the US, Japan and the UK. The Carriage of Multiresistant Bacteria After Travel Study found 35% of participating travellers returned colonised with antimicrobial resistant bacteria and, worryingly, there was an 8% onward transmission rate to AMR negative household members, showing the potential of travel in AMR spread. A particularly risky form of travel is medical tourism (travel abroad for medical treatment), where interaction with multiple health facilities may lead to transmission of highly resistant strains.
An individual pathogen does not present the same level of threat to all countries globally, and absolute traveller number alone does not always indicate risk. The Fragile States Index (FSI) is a measure used to determine a country's resilience and capacity to respond to crises, including public health emergencies. Countries fall into one of four categories: Sustainable, Stable, Warning and Alert. Currently, 84% of the world's population lives in countries categorised as Warning or Alert, and in recent years, Warning countries have seen the greatest increase in international travel. These countries have more vulnerable public health infrastructures and might have less power in surveillance and reporting of disease. This increases risk globally, not solely for these countries.
The role of the global health community
The increased potential for spread of infectious disease has prompted a global effort to improve disease surveillance, detection, and management through the International Health Regulations (IHR). Revision of the IHR regulations was triggered by the SARS epidemic in the early 2000s. Whilst the death toll was relatively low (around 800 people), rapid global spread highlighted the risk of a globalised world and led to action. Central to the IHR regulations was establishment of an IHR hub in each participating country, that would enable reliable information sharing about emerging diseases.
There were several other key changes that aimed to improve global capacity to respond to public health crises, such as enabling the World Health Organisation (WHO) to act on information obtained from non-governmental sources, and a general switch in strategy from one that was focused on a few pre-existing diseases, to a more flexible approach, focussed on detection and reporting of emerging diseases. The revised IHR regulations also stressed the importance of avoiding unnecessary or disproportionate interference with international travel, and enabled WHO to request cessation of measures if they were deemed excessive. The IHR regulations, however, are limited by the surveillance and reporting capacities of individual countries, with only around one-third of countries globally able to sufficiently detect and respond to public health crises.
Increased global connectivity, particularly rapid movement by air travel, is impacting the way in which disease spreads through the global population. International cooperation and appropriate regulation of this movement might be key in protecting from future pandemics.
Travel bans: a force for good?
In the face of international pandemics, the use of travel bans to limit international travel might seem a logical way to reduce disease spread. However, evidence about their effectiveness is seemingly unclear, rendering travel bans a controversial policy, and one that is currently not recommended by the IHR. They have the potential to impose significant economic burdens and may discourage countries from participating in disease surveillance and open reporting. They might also target specific countries, in line with pre-existing political divisions or biases. Does their capacity to slow disease spread justify their social and economic costs?
The COVID 19 pandemic saw the uptake of unprecedented travel restrictions by almost all IHR state parties, resulting in a 65% drop in travel in the first half of 2020. This occurred in the absence of WHO guidance, and with evidence from previous pandemics, such as the influenza pandemics of the 20th and 21st century, suggesting international and domestic travel bans might have limited impact. Whilst, clearly, these travel bans were ineffective in preventing global spread of COVID 19, they may have had some role in early disease dynamics, particularly in slowing the initial transmission of COVID 19 from Wuhan into the rest of mainland China and internationally. Crucially, the predicted positive impact of these bans decreased with delay, with travel restrictions proving less effective if implemented more than 6 weeks after the epidemic onset. As such, implementation of travel bans before full scientific understanding and before WHO had declared an international pandemic was essential. Whilst travel bans in the early stages of the pandemic seem to have had some impact, evidence remains limited and low quality.
COVID 19 related travel bans also had the potential to target singular countries and discourage disease reporting. After South Africa announced the detection of the Omicron variant, the UK applied a strict travel ban to South Africa and other African countries, with the US, Israel and other countries quickly following suit. Despite this ban, the Omicron was soon after detected globally. In fact, it is likely that spread had already occurred before Omicron had been reported, with a confirmed case in Turkey only 2 days after initial detection (and with no links to South Africa). If we are uncertain where a novel strain is currently circulating, selective travel bans often have limited success. It potentially undermines principles of surveillance, reporting and transparency, for fears of effective economic sanctions being imposed: South Africa's tourism industry contributes around £3.7 billion annually, and accounts for 1.5 million jobs, so threats to this industry by travel bans are significant.
It is unfair to say travel bans play no role in prevention of disease spread internationally, and it is likely that effectiveness is ultimately dependent on the specific pathogen and geographical location. For example, more isolated geographical communities, capable of more complete travel bans, might benefit more. There are also other ways to reduce the risks of international travel, without completely banning it, such as through screening and quarantine, though this is challenging at the early stages of a pandemic, where knowledge of non-symptomatic transmission is insufficient, and testing capacity is limited. Ultimately, more evidence is needed to ensure travel bans are used effectively and with the aim of minimising unwanted economic and social consequences.
Looking to the future
In the light of COVID 19, attention is focussed on how to increase our preparedness for potential future pandemics. The speed of modern transport and human mobility highlights the essential nature of sufficient surveillance, rapid detection, and testing. COVID 19 has shown us the challenges of scaling public health facilities and testing capacity in response to a disease outbreak, and further efforts are required to ensure compliance to the IHR core policies, to enable a more coordinated global response in an increasingly connected world.
Article by Alice Sparks.
Image credit: Various. Svenskbygderna (original author)
Image licence: CC BY 4.0 Deed | Attribution 4.0 International | Creative Commons
The original image has been cropped
