The U.S. Centers for Disease Control and Prevention (CDC) and the Department of Homeland Security have rolled out new measures over the past few days to screen for Ebola among travelers arriving from Guinea, Liberia and Sierra Leone. But some public health experts say such steps have done little to stop deadly viruses from crossing borders in past outbreaks, and may be more about reassuring the public that something is being done.
A handful of airports — including New York City’s John F. Kennedy, Chicago-O’Hare, Atlanta, Newark and Washington-Dulles — receive 94 percent of the travelers from three West African countries where the most recent outbreak of Ebola has killed more than 4,000 people, according to the CDC.
At those airports, all passengers coming from Ebola-stricken countries in West Africa are receiving an extra layer of screening starting this week. They will be pulled aside and observed for signs of illness, asked questions about their potential exposure to the Ebola virus and given information about symptoms. If they appear ill, medical staff will take their temperatures with a noncontact thermometer — and those who have a fever will be quarantined and tested for Ebola.
"We work to continuously increase the safety of Americans," CDC Director Tom Frieden said. "We believe these new measures will further protect the health of Americans, understanding that nothing we can do will get us to absolute zero risk until we end the Ebola epidemic in West Africa."
However, a number of public health experts say such fever screening in recent years has not been very effective at stopping the spread of diseases such as severe acute respiratory syndrome (SARS) and swine flu. And Ebola’s long incubation period makes the situation even more difficult, as those who have it often do not show symptoms like a fever for days or weeks.
“What the studies show is that [fever screening is] not very effective, though it occasionally picks up somebody who might have the infection,” said David Heymann, a professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine, who coordinated the World Health Organization’s global response to SARS, or severe acute respiratory syndrome, in 2003. “The problem is that many of these people cross the border when they’re in the incubation period” and don’t yet have fevers, he said, so the thermal screening might give health officials a false sense of security.
Thomas Eric Duncan, who died of Ebola last week at a Dallas hospital, came to Texas from Liberia after receiving the same screening that thousands of other West Africans did — but he did not show signs of the disease until days after he arrived.
“We’ve learned in the past that borders cannot stop all disease,” Heymann said.
While thermal scanning technology may be good at detecting fevers — though some studies have called into question their ability to do even that — a fever obviously does not necessarily indicate the presence of the Ebola virus.
In the past two months, exit screening of the estimated 36,000 air passengers who have traveled outside of Guinea, Liberia and Sierra Leone resulted in 77 people being barred from their flights. But none of those 77 had Ebola, according to the CDC. Many had malaria, which is only spread by mosquitoes.
Also, experiences with past epidemics have shown that efforts to contain them using thermal screening at airports were not entirely effective.
For example, in Canada — where there were 438 probable SARS cases, including 44 deaths, in 2003 — airports in Toronto and Vancouver used thermal scanners to take some 2.4 million people’s temperatures that year. While 832 were referred for further testing, none of them had SARS, Canada’s public health agency said in a paper assessing lessons learned from the epidemic.
“In other countries, the yields for airport screening measures were similarly low,” the paper said.
“Unfortunately, the experience with SARS was that it was pretty worthless,” said Arthur Reingold, professor and head of epidemiology at U.C. Berkeley’s School of public health. “Clearly, fever-screening will not pick up all the fevers. It only picks up those who aren’t taking any medications at all.”
In other words, sick people often take over-the-counter medications to lessen their symptoms when they are traveling, and people who may unknowingly have Ebola could do the same.
However, both Heymann and Reingold agree that airport screening is critically important for public health awareness purposes — it will help travelers be able to recognize whether they have Ebola symptoms, and instill the importance of seeking medical treatment and telling clinicians if they have been to West Africa.
“If it picks up one case and it keeps one case from spreading to others, it may be considered a good investment in a rich country,” Heymann said, adding that the crucial element will be what the screeners do with those who do have fevers.
“A lot of what’s done in various places you could say is overkill, or is erring on the side of caution,” Reingold said. “Decontaminating around someone’s home is almost certainly not of any benefit except in terms of public reassurance,” he said, referring to the cleanup of the area around the apartment where Duncan had been staying. “But is public reassurance important? Yes, public health reassurance is important.”