Estimation of seasonal influenza disease burden using sentinel site data in Pakistan 2017-2019: A cross-sectional study.

Background: The influenza A(H1N1)pdm09 pandemic highlighted the need for reliable disease burden estimation from low- and middle-income countries like Pakistan. We designed retrospective age-stratified estimation of influenza-related severe acute respiratory infections (SARIs) incidence in Islamabad Pakistan 2017-2019. Materials and Methods: The catchment area was mapped on SARI data from one designated influenza sentinel site and other healthcare facilities in the Islamabad region. The incidence rate was calculated as per 100,000 for each age group with 95% confidence interval. Results: The catchment population for the sentinel site was 0.7 million against the total denominator of 1.015 million, and incidence rates were adjusted. During January 2017 to December 2019, among 13,905 hospitalizations, 6715 (48%) patients were enrolled; 1208 of these (18%) were positive for influenza. During 2017, influenza A/H3 dominated with 52% detections followed by A(H1N1)pdm09 (35%) and influenza B (13%). Furthermore, elderly 65+ years age group had highest hospitalizations and influenza positive. The incidence rates of all cause respiratory and influenza-related SARI were highest among children >5 years; highest incidence was found in 0 to 11 month/year group with 424/100,000 cases and lowest in 5-15 years 56/100,000. The estimated average annual influenza-associated hospitalization percentage was 29.3% during the study period. Conclusion: Influenza accounts for a significant proportion of respiratory morbidity and hospitalization. These estimates would enable governments for evidence-based decisions and priority allocation of health resources. It is necessary to test for other respiratory pathogens for more clear disease burden estimation.