People who fly often can affect the spread of epidemics, but only if infected early on in a slowly expanding outbreak, according to a mathematical model

On the face of it, the chances that an infected person will get on a plane and spread disease to a new area will be higher for those who are frequent fliers. So should this group be targeted with interventions in an outbreak? The impacts of frequent travel on the spread of disease are more subtle than previously supposed, suggest Déirdre Hollingsworth and colleagues. Estimates of the rate of international spread of respiratory infections could suffer if different patterns of travel behaviour are not taken into account.

“These frequent fliers tend to travel for business purposes and mix predominantly with other business travelers, stay in particular hotels, and use specific airport lounges,” they explain in Emerging Infectious Diseases. “This form of assortative (like with like) mixing means a respiratory infection could potentially spread quickly within this group and thus be disseminated rapidly between countries.”

During the 2003 SARS epidemic in Asia, an infected person stayed in a hotel in Hong Kong passing the virus on to 16 other guests. Some of these people then boarded flights to Australia, Canada, Singapore, the Philippines, and Vietnam, leading to secondary outbreaks.

Modelling of epidemic spread has so far not included frequent fliers’ behaviour, Hollingsworth and colleagues point out. They used a simple model to examine how a small proportion of frequent fliers would affect the spread of SARS-like and flu-like outbreaks in a population.

“Our simulations show that frequent travelers accelerate international spread of epidemics only if they are infected early in an outbreak and the outbreak does not expand rapidly,” they report.

In a SARS-like outbreak, where a frequent flier becomes infected early on, including different travel behaviours (assortative mixing) into the model made a big difference to how fast the infection spread. But influenza probably has a shorter latent asymptomatic period than SARS, meaning that infected people are less likely to travel, and flu spreads faster through the general population. So the effect of the frequent fliers was not so noticeable in the flu simulation.

“...any increased rate of export caused by early concentration of [flu] infection among the high-frequency fliers will be quickly overcome by the number of cases being exported from the general population, which indicates that heterogeneities in travel have little effect,” the authors conclude.