Epidemiology of travel-associated pandemic (H1N1) 2009 infection in 116 patients, Singapore

Abstract

In June 2009, during Singapore's pandemic influenza plan containment phase, pandemic (H1N1) 2009 was introduced into the country through imported cases. To understand how travel patterns affected the initial outbreak, we examined epidemiologic and travel data for the first 116 case-patients admitted to Tan Tock Seng Hospital, Singapore, with travel-associated infection. Sixty-one percent and 54% of patients, respectively, met US Centers for Disease Control and Prevention and World Health Organization temperature criteria for influenza-like illness. One fourth of the case-patients traveled after illness onset, and 15% became ill while traveling. Regions of exposure for imported infections changed rapidly; case-patients initially arrived from North America, followed by Australasia and Southeast Asia. Case-patients on longer flights were more likely to become ill before arrival; those with shorter flights tended to become ill after arrival. Thermal scanners detected fevers in 12% of the arriving case-patients, resulting in a shorter time to isolation.

Figure 1

Sources of exposure, by region and country, among 116 patients in Singapore infected with pandemic (H1N1) 2009 virus identified during epidemiologic weeks 21–25, 2009. A) Asia compared with other regions; B) the Philippines; C) Thailand; D) Indonesia; E) other Asian countries. Week 21, May 24–30; week 22, May 30–June 6; week 23, June 7–13; week 24, June 14–20; week 25, June 21–27.

Figure 2

Travel duration and illness onset relative to arrival in Singapore for 116 patients infected with pandemic (H1N1) 2009 virus identified during epidemiologic weeks 21–25, 2009. A) Distribution of travel duration by epidemiologic week; B–E) timing of illness onset by travel duration in case-patients who did not have symptoms before embarkation (n = 87). Black, travel duration <3 h; white, 3–5.9 h; red, 6–14.9 h; blue, >15 h. Mean time from arrival to illness onset was 3.5 days (95% confidence interval [CI] 1.9–5.2), 1.7 days (95% CI 0.9–2.4), 1.0 days (95% CI 0.4–1.6), and 0.8 days (95% CI 0.0–1.5), respectively. The percentage of patients with symptom onset before arrival was 0%, 14%, 29%, and 33%, respectively. Week 21, May 24–30; week 22, May 30–June 6; week 23, June 7–13; week 24, June 14–20; week 25, June 21–27.

Figure 3

A) Source of referral to Tan Tock Seng Hospital (TTSH) and B) time to isolation for 116 patients infected with pandemic (H1N1) 2009 virus, Singapore. Week 21, May 24–30; week 22, May 30–June 6; week 23, June 7–13; week 24, June 14–20; week 25, June 21–27. GP, general practitioner.