Epidemiology, trends and determinants of influenza and SARS-CoV-2 positivity among severe acute respiratory infection patients in Cambodia, 2020-2024: a hospital-based retrospective cross-sectional surveillance data analysis.

OBJECTIVE: To describe the epidemiological pattern, temporal trends of influenza and SARS-CoV-2, and determinants of severe acute respiratory infection (SARI)-confirmed positivity among patients hospitalised with SARI in Cambodia. DESIGN: A hospital-based retrospective cross-sectional surveillance data analysis using national sentinel SARI surveillance data collected from 2020 to 2024. SETTING: Nine SARI sentinel hospitals are located across Cambodia. PARTICIPANTS: A total of 16 739 hospitalised patients who met the SARI case definition. MEASURE: SARI-confirmed positivity defines all SARI cases with laboratory-confirmed influenza or SARS-CoV-2 infections as the main outcome measure. Descriptive and bivariate analyses were performed, followed by multivariate logistic regression to identify factors associated with SARI-confirmed positivity. RESULTS: Among 16 739 SARI cases, median age in years was 16 (IQR=59), with 54.6% being male; 13.5% were laboratory-confirmed for influenza or SARS-CoV-2. High mortality rate in 2021. A(H3N2) dominated in 2020, followed by widespread SARS-CoV-2 circulation in 2021. In 2022, SARS-CoV-2 co-circulated with A(H3N2) and influenza B (Victoria). A(H1N1)pdm became the predominant strain in 2023 as SARS-CoV-2 and A(H3N2) declined. In 2024, A(H3N2) again led circulation, with B(Victoria) and SARS-CoV-2 also detected. Sporadic A(H5N1) cases began appearing in late 2023. Multivariable analysis indicated higher odds of SARI-confirmed positivity among children aged 5-14 years (adjusted OR (aOR)=1.85; 95% CI 1.47 to 2.31) and adults 50-64 years (aOR=1.36; 95% CI 1.15 to 1.60) compared with adults 25-49 years, during the rainy season (aOR=2.09; 95% CI 1.89 to 2.32) and among patients in Phnom Penh compared with other regions (p<0.001). Influenza detection markedly increased from 2022 to 2024 following the decline observed during the COVID-19 pandemic. CONCLUSION: These results indicate that there was an increase in SARI trends over the periods and the changing influenza subtypes circulating each year, along with a high mortality rate during the Delta variant COVID-19 pandemic in Cambodia. Age, region, year and season were significant factors associated with SARI-confirmed positivity. Targeted interventions and preventive measures should be prioritised for specific high-risk groups.