Demographic and Clinical Characteristics of Pandemic Influenza A (H1N1) 2009 Outbreak in Kerala, Southern India

Aims: To study the clinical and epidemiological features in the affected individuals from different areas of Kerala, India.

Study design: Population based cross sectional study.

Place and Duration of Study: Regional Facility for Molecular Diagnostics, Rajiv Gandhi Center for Biotechnology and Directorate of Health Services, Kerala, between August 2009 and September 2010.

Methodology: We conducted active surveillance for referral hospitals with specialist in-patient care in Kerala during pandemic periods. Oropharyngeal or nasopharyngeal swabs were tested for influenza viruses by Real time reverse transcriptase PCR.

Results: A total of 4252 samples were tested for H1N1 influenza virus, of which, 30.17% were positive for pandemic influenza A H1N1 and 10.49% were positive for Influenza A (seasonal flu). Severe disease and mortality in the pandemic influenza A (H1N1) 2009 infection predominantly affected relatively healthy adolescents and adults between the age of 10 and 50 years. Both Males (29.28%) and Females (31.15%) were equally effected even though we observed a significant difference (P=.02). 141 cases exhibited lower respiratory tract symptoms. Pneumonia alone accounted for 28% of complicated cases. It was observed that the majority of cases (29.28%) during the first outbreak season were imported from affected overseas regions.

Conclusion: In this study, prevalence of Influenza A H1N1 was high in the healthy younger population and there wasn’t any sex related susceptibility for Influenza infection. Majority of districts showed a positivity of approximately 10-30%, few with high positivity of >30%. Our findings highlight the importance of regular influenza immunization as it is significant to understand that the H1N1 (2009) virus may still circulate for many years with similar high severity.