Introduction A retrospective study based on sentinel surveillance was conducted in 10 provincial-level administrative divisions (PLADs) in China to enhance the understanding of the epidemiological characteristics of human parainfluenza viruses (HPIVs). Methods From January 2019 to June 2023, respiratory specimens were collected from individuals with acute respiratory infections (ARIs) and screened for four HPIVs serotypes and other common respiratory viruses using multiplex real-time polymerase chain reaction (PCR). This study analyzed the association of HPIVs infections with seasonal patterns, geographical distribution, demographic profiles, clinical features, and co-infection status. Results During the study period, a total of 12,866 ARIs were included. The overall detection rate of HPIVs was 6.15%, varying from 5.04% in 2022 to 9.70% in 2020. The median age of HPIVs-infected patients was 3 years. HPIV2 was more prevalent among individuals aged 5–17 years (42.57%), while HPIV4 was more common in those over 65 years (12.24%). HPIV3 (54.16%) and HPIV1 (27.18%) were the predominant serotypes, and their prevalence exhibited significant seasonal fluctuations post- coronavirus disease 2019 (COVID-19) pandemic. The peak of HPIV3 shifted three months later in 2020 compared to 2019 and returned to a summer peak thereafter. Two peaks of HPIV1 were observed in 2021 following the peak of HPIV3. Additionally, co-infections were frequent in HPIVs cases (overall rate: 22.12%), with human rhinovirus being the most common co-infecting virus. Conclusions The prevalence of HPIVs in China was predominantly due to HPIV3 and HPIV1, and their seasonal patterns were altered by pandemic restrictions. Hence, continuous surveillance of HPIVs is essential.

• Significance of the study: Human adenovirus (HAdV) is an important pathogen causing acute respiratory infections (ARIs). This study investigated its prevalence in China during the period of 2020–2023 to elucidate the epidemiological trends both during and after the coronavirus disease 2019 (COVID-19) pandemic. The findings are intended to provide a basis for future HAdV surveillance and control efforts. • Scientific question: The COVID-19 pandemic led to notable changes in the epidemic patterns of several respiratory viruses. However, data on the prevalence of HAdV in China are limited during and after COVID-19. • Evidence before this study: HAdV is frequently detected in patients with ARIs or pneumonia. Prior to the COVID-19 pandemic, the overall detection rates of HAdV infection in ARIs and pneumonia cases were 5.62 % and 5.69 %, respectively, in 2019, and outbreaks were frequently reported, drawing great public concern. • New findings: This study revealed that the overall HAdV detection rate during the COVID-19 epidemic was lower than in the pre-epidemic period but rebounded in 2023. The HAdV epidemic in China during the 2020–2023 period was likely to be driven primarily by viral infections in the southern region. Additionally, the seasonal pattern of HAdV appeared to be closely related to the targeted control measures implemented against COVID-19 in China.

In a previous outbreak of the human respiratory syncytial virus (HRSV), we identified a variant strain of genotype BA9 with a seven-amino-acid extension (Q-R-L-Q-S-Y-A) at the C-terminus of the attachment protein (G). To assess the impact of this extension on the virulence of HRSV, two full-length infectious clones using the wild strain of genotype BA9 as a backbone, one containing the seven-amino-acid extension (rRSV BA9 WT), and the other deleting this extension (rRSV BA9 Δ7AA), were successfully rescued using a reverse genetics system. The biological properties and virulence of the two rescued viruses were then compared and analyzed in vitro and in vivo. Compared to the rRSV BA9 Δ7AA, the rRSV BA9 WT exhibited a larger plaque size and a more pronounced suppression of the host cell innate immune response in vitro (IFN-β levels: 154.33 pg/mL vs. 11.27 pg/mL). The rRSV BA9 WT demonstrated increased adaptability in mice, with a 10-fold higher lung viral load and a stronger inflammatory response following intranasal exposure. Our study primarily demonstrated that the C-terminal extension of the G protein of the HRSV can enhance viral virulence, underscoring the importance of virological surveillance in the prevention and treatment of severe HRSV-related disease.

The phylogenetic analysis showed that global HPIV2 could be classified into two distinct clusters (I and II) and five lineages (IA to IIE) with at least 0.049 and 0.014 genetic distances between clusters and lineages, respectively. Furthermore, lineages IB and IIE could be further divided into two sublineages (IB1-2 and IIE1-2).

During 2017–2018, nasopharyngeal aspirates (NPAs) from 627 hospitalized patients with severe acute respiratory infection at Luohe Center Hospital were tested by RT-PCR for human parainfluenza virus 4 (HPIV-4). Fourteen (2.2%) of the 627 samples were positive for HPIV-4. The complete HN gene was amplified from nine positive samples and sequenced. Sequence comparisons showed that the HPIV-4 strains circulating in the city of Luohe are closely related to HPIV-4A strains. Our study indicated that there were multiple lineages of HPIV-4 circulating in Henan Province in China during the study period. This will improve our understanding of the epidemiological and clinical characteristics of HPIV-4.