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Open Access Research article

Differing clinical characteristics between influenza strains among young healthy adults in the tropics

Jonathan Yap1, Chi Hsien Tan1, Alex R Cook23, Jin Phang Loh4, Paul A Tambyah5, Boon Huan Tan4 and Vernon J Lee167*

Author Affiliations

1 Biodefence Centre, Ministry of Defence, Singapore

2 Department of Statistics and Applied Probability, National University of Singapore, Singapore

3 Program in Health Services and Systems Research, Duke-NUS Graduate Medical School, Singapore

4 Defence Medical and Environmental Research Institute, DSO National Laboratories, Singapore

5 Department of Medicine, National University Health System, Singapore

6 Saw Swee Hock School of Public Health, National University of Singapore, Singapore

7 National Centre for Epidemiology and Population Health, Australian National University, Canberra, Australia

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BMC Infectious Diseases 2012, 12:12  doi:10.1186/1471-2334-12-12

Published: 20 January 2012

Abstract

Background

Influenza infections may result in different clinical presentations. This study aims to determine the clinical differences between circulating influenza strains in a young healthy adult population in the tropics.

Methods

A febrile respiratory illness (FRI) (fever ≥ 37.5°C with cough and/or sore throat) surveillance program was started in 4 large military camps in Singapore on May 2009. Personnel with FRI who visited the camp clinics from 11 May 2009 to 25 June 2010 were recruited. Nasal washes and interviewer-administered questionnaires on demographic information and clinical features were obtained from consenting participants. All personnel who tested positive for influenza were included in the study. Overall symptom load was quantified by counting the symptoms or signs, and differences between strains evaluated using linear models.

Results

There were 434 (52.9%) pandemic H1N1-2009, 58 (7.1%) seasonal H3N2, 269 (32.8%) influenza B, and 10 (1.2%) seasonal H1N1 cases. Few seasonal influenza A (H1N1) infections were detected and were therefore excluded from analyses, together with undetermined influenza subtypes (44 (1.5%)), or more than 1 co-infecting subtype (6 (0.2%)). Pandemic H1N1-2009 cases had significantly fewer symptoms or signs (mean 7.2, 95%CI 6.9-7.4, difference 1.6, 95%CI 1.2-2.0, p < 0.001) than the other two subtypes (mean 8.7, 95%CI 8.5-9.0). There were no statistical differences between H3N2 and influenza B (p = 0.58). Those with nasal congestion, rash, eye symptoms, injected pharynx or fever were more likely to have H3N2; and those with sore throat, fever, injected pharynx or rhinorrhoea were more likely to have influenza B than H1N1-2009.

Conclusions

Influenza cases have different clinical presentations in the young adult population. Pandemic H1N1 influenza cases had fewer and milder clinical symptoms than seasonal influenza. As we only included febrile cases and had no information on the proportion of afebrile infections, further research is needed to confirm whether the relatively milder presentation of pandemic versus seasonal influenza infections applies to all infections or only febrile illnesses.