Incidence and risk factors for acute respiratory illnesses and influenza virus infections in Australian travellers to Asia

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Abstract

Background

Respiratory infections including influenza are a common cause of acute short-term morbidity in travellers and yet the risk of these infections is poorly defined.

Objectives

To estimate the incidence density of and risk factors for acute respiratory infections (ARIs) and influenza in Australian travellers to Asia.

Study design

Travel-clinic attendees were prospectively identified and completed questionnaires (demographic data, travel itinerary, health and vaccination history) and also provided pre and post-travel serological samples for Influenza A and B (complement fixation test). Returned travellers with an ARI provided nasopharyngeal specimens for RT-PCR identification of respiratory viruses.

Results

In this cohort (n = 387) of predominantly (72%) short-term travellers, 58% were female, the median age was 37 years and 69% were tourists. ARIs occurred in 109 travellers (28%) translating to an incidence of 106.4 ARIs per 10,000 traveller days (95% confidence interval CI 88.6–126.7). The traveller type of missionary or aid worker was a risk factor for acquiring an ARI (p = 0.03) and ARIs occurred early (< 30 days) in the travel period (p = 0.001). Four travellers (1%) acquired influenza A during travel translating to an incidence density of 3.4 infections per 10,000 days of travel (95% CI 1.4–8.6). Influenza vaccination was reported in 49% of travellers with a 3.5-fold higher incidence of influenza in unvaccinated travellers compared to vaccinated travellers (p = 0.883).

Conclusions

This is one of the largest prospective studies estimating the incidence of respiratory infections in travellers. These findings have important implications for practitioners advising prospective travellers and for public health authorities.

Section snippets

Background

The spread of influenza and other viral respiratory tract infections has been at the forefront of public health priorities in recent years. Travel is a key factor in the rapidity and ease of transmission of respiratory viruses across continents as illustrated by the recent H1N1 pandemic and SARS outbreak.

Acute respiratory illnesses (ARIs) are common in international travellers and are diagnosed in up to 20% of febrile travellers who attend a health-care facility after travel.1, 2, 3 The

Objectives

Our primary objectives were to (i) estimate the overall incidence density of ARIs (ii) estimate the incidence density of confirmed influenza virus infections in Australian Travellers to Asia and (iii) identify the risk factors associated with ARIs.

Study population and questionnaires

From August 2007–January 2010, we performed a prospective cohort study involving three travel clinics (Royal Melbourne Hospital [RMH]; The Travel Doctor/Travellers Medication and Vaccination Centre, Melbourne CBD [TMVC]; and Monash Medical Centre [MMC]). Travellers over the age of 16 years and intending to travel to countries within Asia (only) for a minimum duration of 7 days were eligible to participate.

Validated pre and post-travel questionnaires were provided to travellers and blood samples

Statistical analysis

The data were analysed with SPSS statistical software, version 19. Because the periods of travel varied the incidences of respiratory and influenza infections were calculated as incidence densities – the numbers of ARIs/influenza infections per 10,000 traveller days. For assessment of predictors for developing the first ARI, Cox's proportional hazards regression approach was used. In this analysis, subjects are removed from being at risk when they experience their first ARI. The Cox regression

Results

From August 2007–February 2010, 681 eligible travellers were invited to participate in the study. Of the 467 travellers who agreed to participate, 58 (12.4%) were lost to follow-up, 4 had missing post-travel sera and 18 were later excluded as they did not meet the study criteria at a time after enrolment into the study, leaving 387 travellers with complete demographic data and paired serological assays.

Discussion

In this multi-centre prospective study, we have defined the risk of acute respiratory infections and influenza in travellers to Asia. The high incidence of 106.4 acute respiratory infections per 10,000 travellers days is the first incidence estimate available for general travellers. This finding confirms the frequency of ARIs in travellers as indicated by observations from surveillance based and single-centre studies of ARIs in febrile returned travellers.2, 3, 9, 10, 11 The incidence of

Funding

This work was supported by an unrestricted investigator initiated grant from Sanofi-Pasteur.

Competing interest

Not applicable.

Ethical approval

All participants provided written informed consent as required by the Health and Research Ethics committees (Melbourne HREC No. 2007.112; Monash HREC No. 08125A).

Acknowledgement

We would like to thank the travel clinic staff for their assistance in the study.

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