Table 2.

Observed and expected numbers of first venous thromboembolism events by time in days since most recent flight arrival

	Days since most recent flight arrival
Traveller category	0-14*	15-30	31-60	61-100	Total
Australian citizens:
Observed	46	23	32	52	153
Expected (A)†	102.6	113.2	212.3	283.0	711.1
Expected (B)‡	22.1	24.4	45.7	60.9	153.0
Non-citizens:
Observed	200	69	78	91	438
Expected (A)§	105.3	116.2	217.9	290.5	729.9
Expected (B)	63.2	69.7	130.7	174.3	438.0

^*χ²=30.3, df=1, P<0.001 when patients with venous thromboembolism in first 14 days are compared with expected (B).

^†Calculated from total number of arrivals of Australian citizens in Western Australia (4.8 million) and using age and sex specific rates for venous thromboembolism in Western Australian population. These expectations illustrate scale of “healthy traveller” effect.

^‡Calculated on assumption that 153 observed venous thromboembolism events of first 100 days are uniformly distributed in time (for example, 153×14.5/ 100.5=22.1). For non-citizens in 0-14 day interval, expectation (A) is biased upwards if anything, whereas expectation (B) is biased downwards because of early departures of visitors from Western Australia. Expectations for citizens are likely to be more robust.

^§χ²=85.2, df=1, P<0.001 when patients with venous thromboembolism in first 14 days are compared with expected (A).