Almost 90 percent of infectious travelers could be detected with SARS-CoV-2 rapid tests at the airport, and most imported infections could be prevented by a combination of tests before departure and a quarantine of five days after departure, which would only include one negative test result according to a computer simulation by researchers at UC San Francisco.
The study provides airlines and states that have struggled through a year of the pandemic with much-needed data with little guidance on how to make safe travel possible.
The problem becomes more pressing as states reopen and travelers return to heaven. Data from the Transportation Security Administration shows that air travel has been on the rise since the 2020-2021 holiday season, though it’s still only a third to half of what it was before the pandemic.
“This evidence could be useful in standardizing testing and quarantine guidelines for COVID-19 at the airline, city and state level for travelers,” he said Nathan Lo, MD, PhD, a resident physician and aspiring faculty fellow in infectious diseases at UCSF and senior author of the paper published in The Lancet Infectious Diseases on March 22, 2021. “Nothing is going to be completely safe, and travel always poses a risk to both individuals and states, but this is one way of minimizing the risk significantly.”
The data in the study comes from a large-scale computer simulation of how various combinations of testing and quarantine can prevent infectious travelers from spreading COVID-19. The model makes predictions based on what is already known about how many people are likely to be infected on a given day, how long they are likely to be infectious, and how likely different types of tests are to detect infectious people.
The study simulated the likelihood that 100,000 travelers would be infectious on any given day just before, during, and after their trips if a number of different quarantine-plus testing strategies were used before and after the trip.
“We know how the infection spreads from the published literature and we can simulate the likelihood of various testing and quarantine strategies to protect a person from infection and reduce their likelihood of being a source of transmission in the community” said Lo.
The study found that PCR-based tests three days before departure or rapid tests at the airport reduced the number of infectious travelers by almost 90 percent, while a five-day quarantine that could only be lifted with a negative PCR test reduced the number of infectious travelers Number of “contagious days” or days travelers could spread the virus by two-thirds.
The simulated quarantine had an added benefit as it caught people who tested negative before leaving but were actually infected. People can test negative in the early stages of infection and only become positive on a test a few days later. The quarantine would prevent these people from spreading the virus in the days following their arrival at their destination.
Scientists were impressed to find that rapid tests, such as the Abbott BinaxNOW antigen test administered to travelers on the day of travel, were about as effective in simulations as the pre-travel gold standard PCR. However, they found that many rapid tests performed poorly and that the study’s results were only applicable when a rapid test was performed, as did those like Abbott BinaxNOW studied at UCSF and found to be reliable.
“It is reasonable to assume that the more sensitive PCR is the most useful,” said Mathew Kiang, ScD, Fellow at the FXB Center for Health and Human Rights at Harvard TH Chan School of Public Health, first author of the paper. “But here we saw that the rapid test performed almost as well on the same day after taking into account the delay in the test results of a PCR.”
He added that the rapid tests have the advantage of being a quick and easy alternative for travelers, while PCR tests require them to take an extra trip a few days before departure to be tested.
“It may not be as feasible to ask people to get tested before the day of travel,” he said.
The researchers also found that a 5-day quarantine was about as effective as a 10- or 14-day quarantine and was also more enforceable.
One of the study’s key implications is the extent to which requiring a negative test before travel as well as five days after travel can reduce the importation of infections from high-risk areas to low-risk areas, and thus control the spread of the pandemic.
“Ultimately, it’s a value judgment,” said Lo. “These strategies aren’t perfect, but if we implement testing and travel strategies, we can drastically reduce the risk of infection while traveling.”
Authors: Lo and Kiang included Lloyd Chapman, PhD, Isabel Rodríguez-Barraquer, Bryan Greenhouse, George Rutherford, Kirsten Bibbins-Domingo, and Diane Havlir, all UCSF; Elizabeth Chin and Benjamin Huynh from Stanford University; and Sanjay Basu, MD, PhD, from Harvard Medical School.
Financing: The study was supported by the UCSF Department of Medicine.
Disclosure: None.
The University of California at San Francisco (UCSF) is solely focused on health sciences and is dedicated to worldwide health promotion through advanced biomedical research, higher education in life sciences and health professions, and excellent patient care. It includes UCSF Health, which includes three high-ranking hospitals, as well as affiliates across the Bay Area.
