If you have traveled internationally the past 2 months, you may have encountered them: health officers briefly pointing a thermometer gun at your forehead or watching as you go by to check for signs of a cough or difficulty breathing. Many countries are now watching arriving and departing air passengers who might suffer from the viral disease COVID-19; some require passengers to fill out health declarations. (Some also simply ban or quarantine those who have recently been in outbreak hot spots.)
Exit and entry screening may look reassuring, but experience with other diseases shows it’s exceedingly rare for screeners to detect infected passengers. Just last week, eight passengers who later tested positive for COVID-19 arrived in Shanghai from Italy and passed the airport screeners unnoticed, for example. And even if screeners do find the occasional case, it has almost no impact on the course of an outbreak.
“Ultimately, measures aimed at catching infections in travelers will only delay a local epidemic and not prevent it,” says Ben Cowling, an epidemiologist at the University of Hong Kong. He and others say screening is often instituted to show that a government is taking action, even if the impact is marginal.
Still, researchers say, there can be benefits. Evaluating and quizzing passengers before they board planes—exit screening—may prevent some who are sick or were exposed to a virus from traveling. Entry screening, done on arrival at the destination airport, can be an opportunity to gather contact information that is useful if it turns out an infection did spread during a flight and to give travelers guidance on what to do if they become ill.
Just this week, U.S. Vice President Mike Pence, who is leading the coronavirus response, pledged “100% screening” on direct flights from Italy and South Korea to the United States. China, which reported only 143 new cases yesterday, “will cooperate internationally to institute exit and entry screening with relevant regions suffering epidemics,” Liu Haitao, an official at China’s National Immigration Administration, said at a 1 March press conference in Beijing, according to the state broadcaster CCTV.
How many COVID-19 cases screening has detected worldwide so far is unclear. At least one New Zealander was prevented from boarding an evacuation flight from Wuhan, China, after failing a health check, The New Zealand Herald reported. The United States started entry screening of U.S. citizens, permanent residents, and their families who have been in China within the previous 14 days at 11 airports on 2 February. (Anybody else who has been in China within that period cannot enter the country.) By 23 February, 46,016 air travelers had been screened; only one tested positive and was isolated for treatment, according to a 24 February report from the U.S. Centers for Disease Control and Prevention (CDC). That clearly has not halted the spread of the virus in the United States, which as of this morning has 99 confirmed cases, according to CDC, plus 49 more among people repatriated from Wuhan and the Diamond Princess cruise ship in Yokohama, Japan.
There are many ways infected people can slip through the net. Thermal scanners and handheld thermometers aren’t perfect. The biggest shortcoming is that they measure skin temperature, which can be higher or lower than core body temperature, the key metric for fevers. The devices produce false positives as well as false negatives, according to the EU Health Programme. (Travelers flagged as feverish by scanners typically go through a secondary screening where oral, ear, or armpit thermometers are used to confirm the person’s temperature.)
Passengers can also take fever-suppressing drugs or lie about their symptoms and where they have been. Most importantly, infected people still in their incubation phase—meaning they don’t have symptoms—are often missed. For COVID-19, that period can be anywhere between 2 and 14 days.
One dramatic example of the failures of airport screening just played out in China after eight Chinese citizens, all employees at a restaurant in Bergamo, Italy, arrived at Shanghai Pudong International Airport on 27 and 29 February, according to information pieced together from details in the local media and terse announcements by the Health & Family Planning Committee of Lishui, a city in Zhejiang province, which borders Shanghai.
Pudong has had a policy to scan all arriving passengers for fever using “noncontact thermal imaging” since late January; it also requires passengers to report their health status on arrival. It’s unclear whether any of the eight restaurant workers had symptoms, or how they handled that reporting. But after taking chartered cars to Lishui, their hometown, one of the passengers fell ill; she tested positive for SARS-CoV-2, the virus that causes COVID-19, on 1 March. The next day, the remaining seven tested positive as well. They were the first confirmed cases in Zhejiang province in 1 week.
Ultimately measures aimed at catching infections in travelers will only delay a local epidemic and not prevent it.
Past experience doesn’t instill much confidence either. In a 2019 review in the International Journal of Environmental Research and Public Health, researchers scrutinized 114 scientific papers and reports on infectious disease screening published in the past 15 years. Most of the data are about Ebola, a serious viral disease whose incubation period is anywhere between 2 days and 3 weeks. Between August 2014 and January 2016, the review found, not a single Ebola case was detected among 300,000 passengers screened before boarding flights in Guinea, Liberia, and Sierra Leone, which all had big Ebola epidemics. But four infected passengers slipped through exit screening because they didn’t have symptoms yet.
Still, exit screening may have helped head off more draconian travel restrictions by showing that measures were being taken to protect nonaffected countries, said the paper, authored by Christos Hadjichristodoulou and Varvara Mouchtouri of the University of Thessaly and colleagues. Knowing they would have encountered exit screening may also have deterred some people exposed to Ebola from even trying to travel.
What about screening at the other end of the trip? Taiwan, Singapore, Australia, and Canada all implemented entry screening for severe acute respiratory syndrome (SARS), which is similar to COVID-19 and also caused by a coronavirus, during the 2002–03 outbreak; none intercepted any patients. However, the outbreak was largely contained by the time the screening was initiated, and it came too late to prevent introduction of SARS: All four countries or regions already had cases. During the 2014–16 Ebola epidemic, five countries asked incoming travelers about symptoms and possible exposure to patients and checked for fevers. They didn’t find a single case either. But two infected, asymptomatic passengers slipped through entry screening, one in the United States and one in the United Kingdom.
China and Japan mounted extensive entry screening programs during the H1N1 influenza pandemic of 2009, but studies found that the screenings captured small fractions of those actually infected with the virus and both countries had significant outbreaks anyway, the team reports in its review. Entry screening is “ineffective” in detecting infected travelers, Hadjichristodoulou and Mouchtouri tell Science. In the end, travelers with serious infectious diseases turn up at hospitals, clinics, and physicians’ offices rather than being caught at airports. And screening is costly: Canada spent an estimated $5.7 million on its SARS entry screening, and Australia spent $50,000 per detected H1N1 case in 2009, Hadjichristodoulou and Mouchtouri say.
Every infectious disease behaves differently, but the duo doesn’t expect airport screening for COVID-19 to be more effective than for SARS or pandemic flu. And it’s unlikely to have a significant impact on the course of the outbreak, Cowling says.
Two recent modeling studies call screening into question as well. Researchers at the European Centre for Disease Prevention and Control concluded that approximately 75% of passengers infected with COVID-19 and traveling from affected Chinese cities would not be detected by entry screening. A study by a group at the London School of Hygiene & Tropical Medicine concluded that exit and entry screening “is unlikely to prevent passage of infected travelers into new countries or regions where they may seed local transmission.”
For countries that nonetheless adopt screening, the World Health Organization emphasizes that it is not a matter of just holding up a thermometer gun. Exit screening should start with temperature and symptom checks and interviews of passengers for potential exposure to high-risk contacts. Symptomatic travelers should be given further medical examination and testing, and confirmed cases should be moved to isolation and treatment.
Entry screening should be paired with collecting data about the patient’s whereabouts over the past few weeks that can later help with tracing their contacts. Travelers should also be given information to increase disease awareness and encouraged to practice good personal hygiene, says epidemiologist Benjamin Anderson of Duke Kunshan University.