In a recently published study medRxiv* Preprint servers, researchers developed a model to reconstruct estimates of how many travelers would have tested positive for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the country of departure. They used data from French Polynesia, where they conducted systematic SARS-CoV-2 screening upon arrival at the airport.
Travelers arriving at the airport have been screened and tested for coronavirus disease 2019 (COVID-19) in several countries. COVID-19 positive people were subsequently isolated to contain the spread of the disease. Although some countries reported the number of infected people detected upon arrival over time, this data was primarily limited to small testing programs that were short-lived. Therefore, there is a global lack of systematically reported long-term SARS-CoV-2 test data from travelers arriving at an airport.
Despite the inconsistency in reporting COVID-19 test data at airports, this presents a unique opportunity for SARS-CoV-2 surveillance across multiple countries. The challenge, however, is that the infections detected in arriving travelers reflect only a subset of all individuals infected with SARS-CoV-2.
About the study
In the present study, researchers used data from over 220,000 COVID-19 tests administered to travelers arriving in French Polynesia between July 2020 and March 2022 to estimate SARS-CoV-2 prevalence on departures from airports in assess several countries.
French Polynesia experienced three COVID-19 pandemic waves caused by wild-type, delta and omicron SARS-CoV-2 variants. For inbound travelers, 15% of the measured cycle threshold (CT) values were below 20, with a smaller spread in health worker self-tests. Most travelers entering French Polynesia had either the United States or mainland France as their starting point for their journey.
During the first phase of surveillance, travelers adhered to the COV-CHECK self-test protocol, which was conducted four days after arrival. In addition, travelers performed a reverse transcription polymerase chain reaction (RT-PCR) test within 72 hours prior to departure. Later in February 2021, quarantine also became mandatory in this protocol.
Researchers also assessed whether arrival screening can reliably assess international SARS-CoV-2 dynamics. To do this, they converted SARS-CoV-2 prevalence upon departure, flipping current positivity into an estimate of daily new infections. They also compared the cumulative SARS-CoV-2 incidence over this period with repeated serological surveys in France and the USA.
The study dataset contained 1341 positive arrival tests with significant variation in weekly prevalence throughout the pandemic. The researchers found that the growth and decline of the epidemic affected the detection of COVID-19 cases. For example, they estimated the detection rate of SARS-CoV-2 infected people to be 60% by RT-PCR testing upon arrival on day 4 of arrival, within the first ten days after their infection when the epidemic at the point of departure increased to 10% per day. That rate would have dropped to 40% during an epidemic that was declining by 10% a day.
Interestingly, measured SARS-CoV-2 prevalence upon arrival in travelers from the US or France anticipated actual COVID-19 dynamics in the two countries. Indeed, travel tests served as a leading indicator, showing that spikes in SARS-CoV-2 prevalence occurred just before spikes in reported cases. It also took into account delays in COVID-19 symptom emergence, testing and reporting of the case(s) in the country of origin.
Taking into account travelers’ testing protocols, the study model estimated a peak infection prevalence on departure in France and the US in late 2020/early 2021 at 2.8% and 1.1%, respectively. The Omicron BA.1 wave caused SARS-CoV-2 prevalences of 5.4% and 5.5% in France and the United States, respectively, in early 2022. In France, the estimate of the cumulative COVID-19 incidence at the end of 2020 was slightly higher than shown in serological studies. There are several explanations for the observed discrepancies. First, travelers from France declined during this period, which may have affected the representativeness of travelers.
While risk-averse individuals are less likely to travel, risk-takers must be more likely to have contracted COVID-19 in transit. Then there is a possibility that some travelers may have contracted SARS-CoV-2 upon arrival. In French Polynesia, they tested incoming travelers for SARS-CoV-2 four days after arrival. In contrast, study estimates for the US were consistent with increases in seroprevalence during the COVID-19 pandemic in late 2020 and 2021.
The study analysis highlighted the usefulness of routine traveler testing at airports. Study results provided a proof-of-concept for ongoing COVID-19 management and future pandemic planning. It stressed the importance of collecting test data systematically with minimal linkage to enable real-time estimation of the underlying dynamics of the COVID-19 pandemic in multiple countries. These insights would stay the same internationally or at key global travel hubs. In addition, this effort would enable the synthesis of SARS-CoV-2 prevalence estimates across datasets, reducing uncertainty and significantly expanding the network of countries covered.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be relied upon as conclusive, guide clinical practice/health behavior, or be treated as established information.