Absolute risk of venous thrombosis calculated as 1 in 4,656 long-haul flights

The authors surveyed 8,755 employees of international companies collecting data on their travel and whether or not they developed thromboses (deep vein thromboses and pulmonary emboli) afterwards. The employees were followed up for a total of 38,910 person-years, during 6,872 of which they were exposed to a long-haul flight. In the follow-up period, 53 thromboses occurred, 22 within 8 wk of a long-haul flight.

The researchers then calculated that there was an incidence rate of 3.2/1,000 thromboses per year exposed to long haul travel compared with 1.0/1,000 per year in individuals not exposed to air travel; this rate is equivalent to a risk of one event per 4,656 long-haul flights.

Venous thrombosis has been linked to air travel since 1951 but despite a number of previous studies the absolute risk of symptomatic venous thrombosis after long-haul travel has not previously been calculated. Knowledge of the absolute risk of symptomatic thrombosis after air travel is important so that travellers can accurately assess their risk of developing thromboses and decide whether prophylactic measures against thromboses are worthwhile.

The authors found that the risk of thromboses increased with exposure to more flights within a short time frame and with increasing duration of flights. The risk was particularly high in employees under age 30 y, women who used oral contraceptives, and individuals who were particularly short, tall, or overweight. The incidence of thromboses was highest in the first 2 weeks after travel and gradually decreased to baseline after 8 weeks.

Although the authors note that this study was performed in a working population with a mean age of 40 y and go on to say that the absolute risk of venous thrombosis in the general population is likely to be higher than the risk found here, they conclude however that “The results of our study do not justify the use of potentially dangerous prophylaxis such as anticoagulant therapy for all long-haul air travellers, since this may do more harm than good. However, for some subgroups of people with a highly increased risk, the risk–benefit ratio may favour the use of prophylactic measures.”

The study was conducted as part of the WRIGHT project (World Health Organization Research In to Global Hazards of Travel), an international research project under the auspices of the World Health Organization.