ObjectivesGlobally, 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).DesignProspective surveillance study.SettingFour provincial hospitals across Laos between March 2021 and July 2023.ParticipantsParticipants 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 measureThe proportion of patients in whom SARS-CoV-2, influenza A, influenza B and hRSV was detected.ResultsThere 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.ConclusionsDuring 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.