Werner's Blog — Opinion, Analysis, Commentary
Can influenza really be stopped at the border?

In early 2003, severe acute respiratory syndrome (SARS) scared the world as this deadly form of pneumonia spread around the globe. This month, a new scare arose as reports from China's city of Wuhan emerged about a new type of coronavirus. Numerous deaths have already been reported in China, and the first infected air passenger was reported in Washington State. What have we learned from the SARS epidemic in 2003, if anything? Can we stop influenza-type viruses at the border? Media reports suggest that B.C. health officials prepare coronavirus diagnostic test and airport screening. But how effective is airport screening?

A recent research paper, Priest et al. (2011) suggests that the use of infrared thermal image scanners (ITIS) to screen arriving passengers at the airport may not be as effective as hoped. There are two key reasons for this. First, thermal imaging equipment only measures the body's surface temperature, not the body core temperature. Surface temperature can be affected by a number of factors, while core temperature is much more accurate for determining if someone has a fever. Travelers may also take antipyretics (e.g., ibuprofen, aspirin, acetaminophen) that reduce fever and mask the underlying infection. Second, not every influenza strain leads to fever. Influenza A strains are more likely than influenza B strains to be associated with fever. Travelers can be without symptoms also because the incubation time for the virus is several days.

Because of the problems identified above, ITIS screening may miss a significant share of those who may be infected. These are "type I errors". On the other hand, those who show elevated temperature may not have the particular virus, a "type II error". The latter can be identified if there are medical tests that can determine the presence of the particular strain conclusively and rapidly. Fortunately, this appears to be the case in the new coronavirus outbreak.

Airplanes are implicated in two ways in spreading influenza-like diseases. First, they can spread new diseases very quickly from one city to another and from one country to another. Second, new diseases may spread from one passenger to another during a flight, as people are in close proximity and air may be recirculated through the plane.

So what can be done to minimize the risk from airplane travel? Good sanitary practice (hand-washing) during a flight can prevent people from catching the virus through direct contact. Hand-washing is the single most important infection control measure, followed by avoding touching eyes and nose. Modern airplanes are also equipped with HEPA filters that prevent the spread of airborne particles, while only about half the air is fresh from the outside. However, these filters are not mandatory and their filtration is not perfect. Travelers who are coughing or sneezing should consider wearing (surgical) face masks to reduce the chance of spreading viruses. However, virologists are skeptical about their effectiveness against airborne viruses for everyone. Surgical face masks may be too loose and leave eyes exposed, but they may help reduce accidental hand-to-mouth contact. The US Centre for Disease Control (CDC) is not convinced that broad use is warranted and makes no recommendation about mask use in the community by asymptomatic persons to prevent exposure to influenza viruses. When an airline passenger has been identified as infected, rapid contact tracing can help limit the spread of the disease. This in turn requires that health agencies can access travel records quickly and track down who was sitting close to someone else on a plane.

Prevention and detection are the most useful strategies at this point. Quarantines may sometimes be effective (e.g., Nishiura et al. 2009), but can be difficult to implement logistically. Travel bans are rather drastic and come at a high cost, and should only be considered as a last resort. China has just imposed restrictions on travel to and from Wuhan, in light of the unusually high traffic that is expected for Lunar New Year celebrations. In this particular case, reducing travel volume may help, but in general the effectiveness of travel bans is far from assured. A 2014 study by researchers for the World Health Organisation found through mathematical modelling that travel bans would have to be extensive (over 90%) to have any meaningful effect on reducing the magnitude of epidemics. Mateus et al (2014) found that travel bans may delay the spread of pandemics but ultimately may do little to contain pandemics geographically.

If we cannot stop influenza viruses at the border, the next best thing is to prepare for them rigorously. BC has learned from the SARS outbreak in 2003, and since 2005 the province has a detailed (and updated in 2012) Pandemic Influenza Response Plan. And yes, please do as I do and get your seasonal flu shot. Even though the flu shot doesn't cover the coronavirus, it is flu season and other strains are going around too. Vaccination has been shown to have many benefits. While influenza vaccine varies in how well it works, every season influenza vaccines prevent millions of influenza illnesses, tens of thousands of hospitalizations and thousands of deaths.

Further readings and information sources:

Posted on Friday, January 24, 2020 at 08:00 — #Health
© 2020  Prof. Werner Antweiler, University of British Columbia.
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