Incidence of Respiratory Syncytial Virus-Associated Lower Respiratory Tract Illness in Infants in Low- and Middle-Income Regions During the Coronavirus Disease 2019 Pandemic

Background Incidence data of respiratory syncytial virus-associated lower respiratory tract illness (RSV-LRTI) are sparse in low- and middle-income countries (LMICs). We estimated RSV-LRTI incidence rates (IRs) in infants in LMICs using World Health Organization case definitions.Methods This prospective cohort study, conducted in 10 LMICs from May 2019 to October 2021 (largely overlapping with the coronavirus disease 2019 [COVID-19] pandemic), followed infants born to women with low-risk pregnancies for 1 year from birth using active and passive surveillance to detect potential LRTIs, and quantitative reverse-transcription polymerase chain reaction on nasal swabs to detect RSV.Results Among 2094 infants, 32 (1.5%) experienced an RSV-LRTI (8 during their first 6 months of life, 24 thereafter). Seventeen (0.8%) infants had severe RSV-LRTI and 168 (8.0%) had all-cause LRTI. IRs (95% confidence intervals [CIs]) of first RSV-LRTI episode were 1.0 (.3-2.3), 0.8 (.3-1.5), and 1.6 (1.1-2.2) per 100 person-years for infants aged 0-2, 0-5, and 0-11 months, respectively. IRs (95% CIs) of the first all-cause LRTI episode were 10.7 (8.1-14.0), 11.7 (9.6-14.0), and 8.7 (7.5-10.2) per 100 person-years, respectively. IRs varied by country (RSV-LRTI: 0.0-8.3, all-cause LRTI: 0.0-49.6 per 100 person-years for 0- to 11-month-olds).Conclusions RSV-LRTI IRs in infants in this study were relatively low, likely due to reduced viral circulation caused by COVID-19-related nonpharmaceutical interventions.Clinical Trials Registration NCT03614676. Respiratory syncytial virus (RSV) can cause serious illnesses, such as pneumonia, especially in infants and young children in low- and middle-income countries. We evaluated the frequency of RSV-associated lower respiratory tract illness (RSV-LRTI) in infants in these countries. Our study largely overlapped with the COVID-19 pandemic, when measures like lockdowns, physical distancing, and masks strongly reduced the spread of respiratory viruses. We followed 2094 infants in 10 low- and middle-income countries in Asia, Africa, and South America from birth to their first birthday. In total, 168 infants had at least one LRTI episode in their first year of life, of whom 32 (19%) had RSV-LRTI; 17 RSV-LRTIs were severe. This corresponded to a rate of 8.7 all-cause LRTI, 1.5 RSV-LRTI, and 0.8 severe RSV-LRTI per 100 infants per year, lower than the rates seen before the COVID-19 pandemic. Most RSV-LRTI cases occurred in infants 6 months or older, later than what was usually reported before the COVID-19 pandemic. While our findings likely do not reflect a standard RSV season due to the low virus circulation because of COVID-19 measures, they provide important information on RSV illness during the COVID-19 pandemic. In this prospective cohort study, conducted in 10 low- and middle-income countries in 2019-2021, the incidence of respiratory syncytial virus-associated lower respiratory tract illness in infants was low due to reduced viral circulation caused by COVID-19-related nonpharmaceutical interventions.