1 Introduction

Acute respiratory tract infections (ARTIs) remain one of the most common major public health threats1. According to the data of the main causes of death in the world released by the World Health Organization, Lower Respiratory Tract Infections (LRTIs) ranks fourth in the world and ranks first in low-income countries2. Viral infections are the most frequent cause of ARTIs3. Since HMPV was first discovered in 2001 in the Netherland and has been detected in other countries, it has been determined to be one of the main pathogens of ARTIs in children, immunocompromised people and the elderly4,5. As a member of the Metapneumovirus genus of the Pneumoviridae family6, HMPV genome is a negative-sense single-stranded RNA molecule, 13Kb long, composed of eight genes encoding nine proteins: 3′-N, P , M, F, M2-1/M2-2, SH, G, L-5′7. Based upon the sequence variability of the attachment (G) and fusion (F) surface glycoproteins, HMPV can be divided into four subtypes (A1, A2, B1 and B2). The A2 subgroup is the most genetically heterogeneous of the four subgroups and some studies have suggested its further sub-division into A2a, A2b1 and A2b2(A2c) sub-lineages based on sequence data7-9. It is well known that these 2 main subgroups are prevalent around the world and that major HMPV subtypes switch every year10-12. However, it is questionable whether there is an association between genotype and disease severity.
Since late 2019, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was discovered in Wuhan, China, it has caused a huge economic and health burden worldwide. To prevent the spread of the virus, large-scale non-pharmaceutical interventions have been implemented in China, including working from home, online teaching, mandatory wearing of masks in public places, social distancing, and hand hygiene. Reductions in influenza and respiratory syncytial virus cases under these interventions have been reported in many countries, yet few studies have examined changes in HMPV cases during outbreaks13-17. The purpose of this study was to evaluate the epidemiological, clinical, and molecular characteristics of HMPV infections occurring among hospitalized children with ARTIs in China from April 2018–March 2019. In addition, this study explored the impact of the COVID-19 pandemic on HMPV infection.