Changes in the Epidemiology of Influenza and Respiratory Syncytial Virus During 2020-2022 Relative to the Pre-COVID-19 Pandemic Period (2017-2020) From Systematic Sentinel Syndromic Surveillance in South Africa.

BACKGROUND: Nonpharmaceutical interventions, implemented during the COVID-19 pandemic, affected the transmission of other respiratory pathogens. METHODS: Systematically collected respiratory illness surveillance data and consistent case definitions were used to describe changes in influenza and respiratory syncytial virus (RSV)-associated outpatient visits and hospitalisations in South Africa during the first 3 years of the COVID-19 pandemic relative to a pre-COVID-19 pandemic period (2017-2019). RESULTS: In 2020, influenza circulation almost ceased. In 2021 an out-of-season circulation was observed with a return to prepandemic timing, albeit with a higher peak in 2022. During the pandemic period, influenza-associated influenza-like illness (ILI) was more common in those aged ≥ 5 years compared to < 6 months. Patients with influenza-associated severe respiratory illness (SRI) were less likely to be ≥ 45 years versus < 6 months and less likely to be admitted to ICU (aOR 0.2, 95% CI 0.04-0.8). RSV circulation declined at the start of the pandemic, with an out-of-season spring resurgence in 2020 followed by a return to prepandemic timing in 2021 and a higher peak in 2022. During the pandemic, compared to the prepandemic period, patients with RSV-associated SRI were more likely to be aged 1-4 years (aOR 1.5, 95% CI 1.2-1.8) versus < 6 months and less likely to be admitted to ICU (aOR 0.5, 95% CI 0.3-0.8). CONCLUSION: We report low levels of influenza circulation and out-of-season RSV circulation in 2020 with changes in the age distribution of cases and risk of ICU admission. Return to prepandemic timing was earlier for RSV, with higher seasonal peaks for influenza-associated ILI and RSV-associated SRI in 2022.