Objectives: Globally, the circulation of influenza and other seasonal respiratory viruses changed dramatically during the COVID-19 pandemic. This study aims to determine the trends of acute respiratory infections (ARIs) caused by SARS-CoV-2, influenza A, influenza B and respiratory syncytial viruses (RSVs) in patients presenting to hospitals in the Lao People’s Democratic Republic (PDR) (Laos). Design: Prospective surveillance study. Setting: Four provincial hospitals across Laos between March 2021 and July 2023. Participants: Participants of all ages who met our case definition for an ARI (axillary temperature ≥37.5°C or history of fever AND cough or other respiratory symptoms/signs OR loss of smell and/or taste) presenting to the hospital less than 10 days after symptom onset were eligible to be enrolled in the study. Combined nasopharyngeal and throat swabs were tested for SARS-CoV-2, influenza A, influenza B and human RSV (hRSV) using probe-based real-time RT (Reverse transcription)-PCR assays. Primary outcome measure: The proportion of patients in whom SARS-CoV-2, influenza A, influenza B and hRSV was detected. Results: There were 4203 patients recruited, of whom 898 (21%) were children aged under 5 years. SARS-CoV-2 was detected in 16.9% of patients, followed by influenza A, influenza B and hRSV (8.4%, 7.2% and 4.7%, respectively). 98 patients (2.3%) were diagnosed with probable co-infection, with at least two viruses detected. After May 2022, the number of cases of influenza A, influenza B and hRSV increased rapidly. Six per cent of patients (263) had a quick Sequential Organ Failure Assessment score of ≥2, and 34 (0.8%) patients died, of whom 11 tested positive for a respiratory virus. Conclusions: During the COVID-19 pandemic in Laos, few respiratory viruses were detected by passive surveillance until the relaxation of non-pharmaceutical interventions implemented for infection control. After restrictions were lifted, influenza A, influenza B and hRSV emerged rapidly, showing the importance of continuous surveillance
