The emergence of SARS-CoV-2 caused significant morbidity and mortality worldwide, leading governments to prioritize non-pharmaceutical interventions (NPIs) to slow the spread of the infection and alleviate the burden on healthcare systems. Since NPIs aim to reduce the transmission of respiratory viruses, notable disruptions occurred in the usual circulation of common respiratory viruses [1] - [4] . This resulted in a reduction in viral activity, particularly of influenza and respiratory syncytial virus (RSV), thus impacting the prevalence of viral respiratory infections [1] - [4] . These changes, which vary by virus type, are multifactorial, influenced in part by the collective implementation of NPIs and by interactions among viruses, including viral interference [5] .

Acute respiratory infections (ARIs) are highly contagious, transmitted through direct contact via respiratory secretions or indirectly via contaminated objects or surfaces [6] . They exhibit similar clinical manifestations, ranging from a common cold to severe acute respiratory distress syndrome [6] [7] . The viruses responsible for ARIs belong to different families, exclusively targeting the respiratory system and following a seasonal pattern [6] . These infections can manifest in epidemic or pandemic form, representing one of the leading causes of morbidity and mortality worldwide, particularly among children under five years of age [7] [8] .

Approximately 97% of severe ARIs are reported in low-income countries, where around 70% of cases originate from South Asia and sub-Saharan Africa [6] - [8] . In these regions, the causes of respiratory infections are often poorly understood due to the diversity of respiratory pathogens, lack of diagnostic resources, similarity in ARI symptoms, and confusion with tropical febrile illnesses, making their management particularly challenging, especially in children [7] - [10] .

In the Democratic Republic of the Congo (DRC), a surveillance system for severe acute respiratory infections and influenza-like illnesses has been in place since 2006, has provided valuable insights into the epidemiology and extent of influenza [11] . This system, along with the resources of respiratory virus laboratories, has also been used to monitor, study, and respond to SARS-CoV-2 infections.

This study aims to document the impact of the COVID-19 pandemic on the circulation of influenza viruses and the viral etiology of ARIs during this period in Kinshasa, to inform future public health responses.

The objective of our study was to document the impact of COVID-19 on the circulation of respiratory viruses and the etiology of acute respiratory infections (ARIs) in Kinshasa during the pandemic.

In our study, a higher proportion of samples was collected from outpatients (66.3%) compared to hospitalized patients (33.7%), particularly among adults. This trend may reflect increased concern among adults who are worried about COVID-19 and seek testing quickly at the onset of symptoms.

The overall prevalence of respiratory viruses in our study was 45.7%. Several respiratory viruses were co-circulated during the pandemic in Kinshasa, which aligns with findings from other studies [2] - [4] . This prevalence is higher than that observed in Southeast China (32.5% [12] ) and Madagascar (19% [13] ), but lower than the rate reported in Taiwan region (75.3% [14] ). The differences in prevalence may be attributed to variations in susceptibility to respiratory viruses across populations, influenced by factors such as behavior, culture, demographics, and environmental conditions [15] .

We observed a higher prevalence of SARS-CoV-2 at 19.5% compared to other non-SARS-CoV-2 respiratory viruses. Indeed, SARS-CoV-2 was the predominant virus during the 2020 pandemic, affecting anywhere from 0 to over 40% of the global population, which was naive to SARS-CoV-2 [16] . Furthermore, the prevalence of other respiratory viruses, excluding SARS-CoV and influenza viruses, was also 19.5%.

Severe cases of ARI associated with SARS-CoV-2 were less common (25%) compared to mild cases (75%). Although SARS-CoV-2 was the predominant virus during the pandemic, infecting a significant portion of the global population [16] , its severity is often associated with factors such as comorbidities and older age [17] . Our study revealed that children under 5 years old were significantly impacted, particularly after the emergence of the Delta variant, in contrast to the early stages of the pandemic when severe cases in this age group were rare [18] . These findings could be attributed to the circulating variant, along with factors such as comorbidities, limited healthcare access, and challenging socio-economic conditions.

For other non-SARS-CoV-2 respiratory viruses, the overall prevalence was 19.5%, with RSV (5.7%) and RV (5.3%) being the most common. Many studies have reported the impact of COVID-19 on the prevalence of respiratory viruses [1] - [3] [19] [20] . The absence of pre-pandemic data limits our ability to assess these effects precisely, but low rates of respiratory viruses were observed during the pandemic [21] [22] . Preventive measures such as non-pharmaceutical interventions (NPIs), lockdowns, restrictions, and COVID-19 vaccination efforts helped protect against other respiratory viruses.

Moreover, our study found that 45.8% of ARI cases caused by respiratory viruses other than SARS-CoV-2 and influenza were associated with severe illness (SARI). This finding highlights the significant burden of non-SARS-CoV-2 and non-influenza respiratory viruses in causing severe respiratory infections during the pandemic. While SARS-CoV-2 was the primary focus, other respiratory pathogens continued to contribute to severe cases, underscoring the need for comprehensive surveillance and management strategies that account for a broad range of viral agents. These results suggest that the co-circulation of multiple respiratory viruses can exacerbate the severity of infections, potentially overwhelming healthcare systems, particularly in resource-limited settings like Kinshasa.

ARIs due to respiratory viruses, particularly RSV, were more frequent among hospitalized patients (SARI), especially in children under 5 years (p<0.001) and the elderly. These findings are consistent with other studies indicating that RSV is a major pathogen of lower respiratory tract infections, particularly among young children and the elderly [13] [14] [18] [23] . Children and older adults are more vulnerable to ARIs due to immune immaturity or the presence of chronic illnesses [24] [25] . Other modifiable factors such as prematurity, malnutrition, and indoor air pollution also play a role [26] .

We also observed an increased susceptibility of men to severe forms of acute respiratory infections (ARI), while women, although more frequently infected, tend to develop less severe cases. This sexual dimorphism could be attributed to hormonal, chromosomal, and behavioral factors, as well as comorbidities [27] .

The extent of influenza during the pandemic (6.8%) was lower than before the pandemic [11] [12] , which could be explained by a disruption in influenza virus circulation. No B/Yamagata lineage was detected during this period, supporting the idea of an interruption in the transmission of this lineage, as reported elsewhere [28] . Social distancing measures and movement restrictions may have reduced the transmission of the B/Yamagata lineage due to its slower growth phase and shorter transmission chains.

The COVID-19 pandemic led to the implementation of various control measures, including non-pharmaceutical interventions, which significantly impacted the transmission of other respiratory viral infections, particularly influenza and RSV [2] [4] [11] . The seasonal dynamics of influenza viruses, well known in our region, were disrupted during the pandemic. Detection of influenza viruses dropped after SARS-CoV-2 was detected on March 10, 2020, only resuming at the end of 2020. In 2021, influenza virus circulation was weak and sporadic, as observed in other studies [12] [29] . Changes in behavior, such as social distancing, mask-wearing, hygiene measures, and travel restrictions, were the main factors contributing to the reduction in influenza incidence. Additionally, viral interference between SARS-CoV-2 and influenza viruses may have limited co-infections [5] .

This study has certain limitations, including The absence of data on the prevalence of respiratory viruses before the pandemic limits the ability to compare changes and understand the specific impacts of COVID-19 on virus circulation; The concentration of samples at sentinel surveillance sites in Kinshasa may introduce bias, as these sites may not be representative of the entire population or rural areas.; Despite The two-year study period, although covering a significant phase of the pandemic, may not capture all seasonal variations or long-term trends in respiratory viruses. The relatively low frequency of detected co-infections may not fully reflect the possible interactions between SARS-CoV-2 and other respiratory viruses, due to limitations in detection methods or the reduced number of co-infections.

Despite the limitations presented, this study has several strengths the Comprehensive Analysis of Viral Circulation who offers a detailed assessment of the prevalence and circulation of various respiratory viruses during the COVID-19 pandemic, including influenza, non-influenza viruses, and SARS-CoV-2.; the Impact of COVID-19 show how the pandemic altered the dynamics of respiratory viruses, with a notable disruption in the circulation of influenza viruses and a decrease in acute respiratory infections unrelated to SARS-CoV-2. By focusing on Kinshasa, the study provides valuable data on the sociodemographic characteristics of patients and prevalence variations by age group and gender, which is crucial for local public health policies. and The use of the sentinel surveillance system established since 2006 demonstrates the effectiveness of this system in adapting surveillance to new epidemiological challenges, such as the COVID-19 pandemic.

Our study revealed the impact of COVID-19 on the circulation of influenza viruses and clarified the role of other respiratory viruses in ARIs. These findings underscore the crucial importance of maintaining an integrated disease surveillance system to better understand and monitor respiratory viruses of public health interest.

This study was funded by the Kinshasa School of Public Health with support from CDC Atlanta, which provided specific reagents for diagnosis. The authors wish to thank the National Institute of Biomedical Research (INRB), which hosts the Respiratory Virus Laboratory, and the Microbiology Department of the Faculty of Medicine at UNIKIN. The authors also acknowledge the Ministry of Public Health of the DRC, through the Epidemiological Surveillance Directorate (DSE), for providing sentinel sites, as well as the Influenza Sentinel Surveillance Network and the influenza sentinel sites in Kinshasa, without which this study would not have been possible.