This study depicted variations in the prevalence, subtypes, and lineages of respiratory syncytial virus (RSV) in Myanmar before and after the COVID- 19 pandemic. The years 2019 to 2023 witnessed significant fluctuations in RSV activity in Myanmar, characterized by the absence of a notable epidemic in 2020, a temporal shift with an RSV-A surge during the 2021 outbreak, a lack of expected cases in 2022, and a substantial resurgence of RSV-B in 2023. Our G-gene ectodomain sequencing revealed that RSV-A consistently belonged to lineage A.D.3 (GA2.3.5) both before and after the pandemic, while RSV-B retained the previously circulating lineage B.D.4.1.1 (GB5.0.5a), and introduced a new lineage, B.D.E.1 (GB5.0.5a), in 2023. However, detailed temporal phylogenetic analysis revealed that RSV-A and RSV-B exhibited different surviving patterns in Myanmar. The estimated evolutionary rate at the G-ectodomain for RSV-A was 7.76 × 10⁻³, and RSV-B was 5.67 × 10⁻³ substitutions/site/year.

This study highlights that the infection prevention and control measures implemented during the COVID- 19 pandemic significantly impacted the incidence of RSV cases in Myanmar from 2019 to 2023. While RSV epidemics were observed throughout most of the study period, only sporadic outbreaks occurred in 2020 and 2022. Based on our previous observations, the seasonal pattern of RSV epidemics (RSV-A and RSV-B) in Myanmar typically coincides with the rainy season, spanning July to October, with peak activity occurring in September and October⁵. In contrast, almost no RSV outbreak was observed in 2020, coinciding with the onset of the COVID- 19 pandemic²⁷. This suppression of the RSV epidemic in 2020 was caused by the COVID- 19-related public health measures implemented in Myanmar^28,29.After the first COVID- 19 case was reported in March 2020, the government imposed strict public health measures, including mandatory mask-wearing, social distancing, and travel restrictions^28,30,31,32. International borders were closed, domestic travel was restricted, and lockdowns were enforced. Myanmar experienced multiple COVID- 19 waves, with the Delta variant surge in July–August 2021 severely overwhelming the healthcare system. During this time, infectious diseases surveillance was likely limited as hospitals prioritized COVID- 19 patients. Stringent border closures and travel restrictions from 2020 to 2022 minimized RSV and other respiratory virus introductions, while mask mandates, social distancing, and school closures further reduced respiratory pathogen transmission. Widespread RT-PCR testing, contact tracing, and government quarantine programs also contributed to the suppression of respiratory viruses. This trend was similar to that reported in northern hemisphere countries, including the United Kingdom, the United States, and Japan^{13,20,33,34,35}. As a result, the epidemics of respiratory infectious diseases, including RSV and influenza, were suppressed to varying degrees worldwide in 2020^{33,36,42,36,35}. A study by Reicherz et al. indicated that antibody titers in samples collected from infants in 2021 were 15-fold lower than those measured in 2020⁴³. On the other hand, due to the relaxation of non-pharmaceutical measures during the COVID- 19 pandemic, a significant RSV epidemic was observed in 2021 and 2023. This situation has been referred to as “RSV immunity debt,” attributed to a decline in population immunity following a prolonged period of minimal RSV exposure^15,40,41. A similar resurgence was reported in various countries and regions, including Taiwan, the United States, the United Kingdom, Argentina, and Australia^{16,35,44,45,46}. The relaxation of hygiene measures, travel restrictions, and increased respiratory infection screening also contributed to the rise in RSV cases^13,33,47.

We observed that the median age of RSV-infected patients was higher during the COVID- 19 pandemic in 2020 and 2021 compared to the post-pandemic period in 2022 and 2023 (Table 1). In pre-pandemic 2019, the median infection age was approximately 1.0 years, consistent with our previous findings from 2015 to 2018⁵. This suggests that older children were more likely to be infected following the COVID- 19 pandemic, potentially due to the absence of RSV epidemics in 2020, which may have delayed exposure to the virus¹³, resulting in higher infection age.

In terms of gender, the incidence of RSV infection was higher among males than females. Although this difference was insignificant in most years, this result aligns with previous studies conducted in Myanmar and Japan^5,19, and a similar study conducted in China⁴⁸. The most frequent symptoms associated with RSV were cough and rhinorrhea, with percentages of 92.9% and 84.0%, respectively. RSV infection among young children and older adults typically commences with upper respiratory tract illness, including rhinorrhea, nasal congestion, cough, sneezing, and sometimes fever and myalgia⁵. Symptoms may worsen as the infection progresses to the lower respiratory tract, including a more severe cough, increased respiratory rate, dyspnea, and intercostal muscle retractions⁵. Notably, dyspnea was observed in a higher proportion of patients in 2023, all of whom were infected with RSV-B, which was also observed in RSV-B patients in our previous study in Myanmar⁵.

For both RSV-A and RSV-B, the predominant genetic lineages remained largely unchanged even after the pandemic (A.D.1, B.D.4.1.1), except for the appearance of a new lineage for RSV-B in 2023 (B.D.E.1). However, a closer examination revealed that within the same lineage, some groups remained, some groups disappeared, while others were likely introduced from other countries. This suggests the presence of diverse intrinsic groups within each lineage. RSV-A showed that the same lineage persisted within Myanmar throughout the pandemic, leading to a large outbreak post-COVID. In contrast, RSV-B strains appear to have temporarily disappeared during the pandemic, but subsequently, globally circulating strains likely entered Myanmar, resulting in a major outbreak in 2023. This finding suggests that RSV continues to circulate even during periods of very low or no observed incidence in various parts of the world^49,50,,51. We also identified unique amino acid substitutions in the G-ectodomain of certain Myanmar RSV strains: K213 N, Y280H, and T281 K in RSV-A, and Q142L, L284 F, and T300I in RSV-B. These unique amino acids circulating in Myanmar indicated that RSV continued to evolve in this location. Similar observations have been reported in countries such as China, Japan, the United States, and various European nations, but with different amino acid substitutions^20,52,53,,54. Furthermore, the endemic RSV-B strains in 2019 were closely related to strains from China, consistent with findings from our previous study in Myanmar, where the endemic strain from 2018 also showed similarity to Chinese strains⁵. This suggests that the 2019 epidemic may have been a continuation of previous outbreaks. Our previous study from 2015 to 2018 observed that the second HVR genotype of RSV-A in Myanmar was classified as ON1 (A.D lineage), while RSV-B was BA9 (B.D to B.D.4.1.1 lineage). However, due to the change in the target region for genetic analysis in this study, we did not make a direct comparison of genotypes and other country strains⁵.

Our estimated evolutionary rates of the G-gene ectodomain during the pandemic were not significantly different from pre-pandemic rates reported in other countries. A study from China (2016–2019) focusing on the G-ectodomain region reported evolutionary rates consistent with those observed in this study (RSV-A: 1.95 × 10⁻³ substitutions/site/year [95% HPD: 1.45 × 10⁻³–2.48 × 10⁻³], and RSV-B: 3.15 × 10⁻³ substitutions/site/year [95% HPD: 2.45 × 10⁻³–3.95 × 10⁻³])²⁵. Another study from the Taiwan region (2008–2017) also reported similar rates (RSV-A: 2.90 × 10⁻³ substitutions/site/year [95% HPD: 2.26 × 10⁻³–3.59 × 10⁻³], and RSV-B: 5.23 × 10⁻³ substitutions/site/year [95% HPD: 4.14 × 10⁻³–6.37 × 10⁻³])²⁶. This suggests that the evolutionary rate of the G-gene ectodomain was not significantly affected by the interruption of RSV circulation. However, comparable post-pandemic studies have not yet been identified, possibly due to variations in the lengths of G-gene sequences analyzed. Several factors could influence differences in evolutionary rates, including the time frame of analysis, the predominant lineages circulating during the study period, and community herd immunity levels⁵⁵. Therefore, a definitive conclusion regarding these evolutionary rates requires further investigation.

This study has several limitations. First, samples were collected from Yangon, Myanmar, which did not fully represent the entire country. Second, in this study, we only qualitatively tested for RSV viruses and were unable to determine differences in RSV viral load over time or across populations. Finally, our analysis focused solely on the G-ectodomain region rather than the whole genome. Recent studies have increasingly utilized whole genome analysis for RSV genotyping, as it allows for more comprehensive assessments, such as estimating the effectiveness of immunization and analyzing mutations related to antibody-resistant drugs and vaccines^7,51,52. This limitation may impact the comparability of our results with those of other studies.