3 Results

3.1 HMPV epidemiology

From April 2018 to March 2019, there were 900 males and 672 females among the 1572 samples (sex ratio: 1.34:1), and the age range was from 1 day to 14 years of age with a median age of 3 years. The detection rate of HMPV was 7.9% (124/1572), of which 78 were male and 46 were female, with no significant gender difference (p =0.185). As shown in Table 1, HMPV was detected among hospitalized children (< 14 years) with ARTIs at Beijing Friendship Hospital between April 2018 and March 2019, and 78.2% (97/124) of children under 5 years old. The seasonal distribution of HMPV cases in hospitalized children from April 2018 to March 2019 is shown in Figure. 1, and HMPV was detected throughout the year. 34.7%(43/124)cases of HMPV infection were detected in spring, 13.4%(17/124)in summer, 13.4%(17/124)in autumn and 37.9%(47/124)in winter, and there were significant differences in HMPV detection rates in different seasons (p = 0.000). From September 2020 to August 2021, A total of 232 hospitalized children with ARTIs were enrolled in this study, among which 134 (57.8%) were male and 98(42.2%) were female (sex ratio: 1.34:1). The detection rate of HMPV was 1.7% (4/232), which was significantly lower than that of the previous year, and the 3 positive cases were children under 5 years old.

3.2 Detection of viral co‑infection in HMPV‑positive specimens

A total of 124 HMPV-positive samples were detected during the period from April 2018–March 2019, of which 62.9% (78/124) of HMPV single-infected samples and 37.1% (46/124) of HMPV co-infected samples with other respiratory viruses. Among the 46 co-infected cases, there were 32 males and 14 females (sex ratio:2.29:1), with no statistical significance for gender (p = 0.238). As shown in Table 2, the most common mixed infection virus was HPIV3 (32.6%,15/46), and the mixed infection rates with HRV, ADV, FLV-A, and RSV were 19.6% (9/46), 17.4% (8/46), 17.4% (8/46), and 10.9% (5/46), respectively. The viral load of the 124 HMPV positive cases ranged from 14 copies/mL to 4.6×106 copies/mL NPA, with no statistical difference in viral load between HMPV mono-infection and coinfection. From September 2020 to August 2021, among the NPAs samples from 232 hospitalized children with ARTIs, 4 were HMPV-infected, with viral loads ranging from 19 copies/mL to 3.16×103 copies/mL NPA. 2 cases were combined with other respiratory virus infections, which were mixed with HBOV and HRV respectively.

3.3 Clinical characteristics of HMPV infections

From April 2018 to March 2019, among the 124 children with HMPV infection, 115 (92.7%) were diagnosed with pneumonia, 8 (6.5%) with bronchitis, and 1 with upper respiratory tract infection (0.8%). The main clinical symptoms of children with HMPV infection were cough (95.2%, 118/124) and fever (91.9%, 114/124), and other symptoms included rhinorrhoea (53.2%, 66/124), nasal obstruction (29.8%, 37/124) 124), sneeze (15.3%, 19/124) and Shiver (8.1%, 10/124). There was no statistical difference in the above clinical symptoms between children with HMPV single infection and mixed infection, as shown in Table 3. The hospitalization time of HMPV-infected children was 2-14 days, and the average hospitalization time was 6.35 days. A total of 90 children were discharged within 7 days (72.6%, 90/124). Statistics on the number of children who were hospitalized for more than 7 days showed no significant difference between HMPV combined with other viral infections and HMPV single infection (p =0.320).

3.4 HMPV genotyping and phylogenetic analysis

Nested PCR was used to amplify the conserved sequence (610bp) of the F gene in 128 HMPV-positive samples, and the target fragment was amplified from 78 (63.0%, 78/124) HMPV-positive samples from April 2018 to March 2019. Phylogenetic analysis showed that 64 strains (82.1%, 64/78) belonged to A2b subtype, 9 strains (11.5%, 9/78) B1 subtype and 5 strains (6.4%, 5/78) B2 subtype, A1 and A2a subtype was not found in Figure2. As shown in Figure3, the viral load of A2b subtype samples was significantly higher than that of B type samples (p =0.009), and there was a statistically significant difference in the hospitalization time >7 days between the two subtypes (p =0.031). From September 2020 to August 2021, among the 4 HMPV-infected children, only 1 case was positive for F gene amplification, and was identified as B1 subtype by phylogenetic analysis.