4 Discussion
Since its discovery in 2001, HMPV has been considered an important cause
of ARTIs. Some studies have shown that the prevalence of HMPV is
different in different regions and years. The detection rate of HMPV in
Chinese hospitalized children with ARTIs is about
2%-18.2%18,24,25, and the detection rate of HMPV in
ARTIs inpatients in other countries is
1.2%-20.3%12,26,27. In this study, we mainly
explored the molecular epidemiology and clinical characteristics of HMPV
infection in hospitalized children with ARTIs in Beijing from April 2018
to March 2019 and September 2020 to August 2021, and the impact of the
COVID-19 epidemic on HMPV infection. The detection rate of HMPV from
April 2018 to March 2019 was 7.9%, and the detection rate of HMPV from
September 2020 to August 2021 was 1.7%. A previous study in our
laboratory showed that the detection rate of HMPV in hospitalized
children from April 2017 to March 2018 was 4.1%25,
which indicated that the detection rate of HMPV in hospitalized children
with ARTIs in Beijing from April 2018 to March 2019 was higher than the
previous year (p =0.000), while the detection rate of HMPV
decreased significantly during the period of the COVID-19 epidemic. It
shows that the detection rate of HMPV in hospitalized children with
ARTIs has been significantly reduced under the strict prevention and
control of the COVID-19 epidemic in China(p =0.001). Mandy
Jongbloed et al. reported that despite a 324% increase in HMPV testing
during the COVID-19 outbreak in Europe, there was no increase in HMPV
incidence28. Since the outbreak of the COVID-19,
various countries have adopted different prevention and control
strategies. A series of measures such as wearing masks, frequent hand
washing and disinfection, delaying the start of school, and maintaining
social distance have cut off the transmission of respiratory viruses and
reduced HMPV infection. During the COVID-19 outbreak, the number of
hospitalized children with ARTIs decreased significantly, and the number
of samples collected from September 2020 to August 2021 decreased
accordingly. Due to the small number of HMPV-positive samples, the virus
mixed infection and clinical characteristics have not been counted yet.
From April 2018 to March 2019, 78.2% of HMPV infections occurred in
hospitalized children aged ≤5 years, and 9.1% (16/176) occurred in
children aged 3-4 years; There was no significant difference in HMPV
detection rate between male and female children(p =0.185), which
was consistent with the studies in China and
Argentina25,27 . HMPV infection had typical seasonal
distribution characteristics, with the detection rate in winter and
spring significantly higher than that in summer and autumn
(P =0.000), and the prevalence period of HMPV was concentrated
from December to May of the following year, which was consistent with
the results of a study in Guangzhou, China23. Research
shows that HMPV epidemics are typical in early spring, while small peaks
in summer appear to be related to local weather
conditions22,27. In ARTIs cases, HMPV is often
co-infected with other respiratory viruses, such as ADV, RSV, HRV and
HPIV25,29,30. In this study, from April 2018 to March
2019, the proportion of HMPV-positive samples mixed with other viruses
was 37.1%, of which HPIV3 had the highest mixed infection rate of
32.6%, which was similar to the findings of Fathima
S27, which may related to the prevalence of HPIV3 in
that year. Some studies have concluded that co-infection of HMPV with
other respiratory viruses can aggravate clinical symptoms and prolong
hospitalization18,32. However, in this study, there
were no statistically significant differences in viral load, length of
hospital stay, cough, fever, nasal obstruction, rhinorrhoea and other
clinical symptoms between children with HMPV single infection and mixed
infection, which is consistent with the results of previous research in
our laboratory25. In addition, the relationship
between HMPV viral load and clinical symptoms remains
unclear25,33,34, and no association between clinical
symptoms and HMPV viral load was found in the study.
There are five gene subtypes in HMPV, so it is of great significance to
explore the relationship between genetic diversity of HMPV and clinical
characteristics. In this study, for HMPV samples from April 2018 to
March 2019, 78 NPAs samples were successfully amplified by nested PCR
amplification of the conserved fragment of the F gene. The phylogenetic
analysis showed that 82.1% (64/78) of the prevalent strains belonged to
A2b subtype, 11.5% belonged to B1 subtype and 6.4% belonged to B2
subtype, and no A1 and A2a subtypes were found. We found that the viral
load of HMPV A2b genotype samples was significantly higher than that of
B samples (p =0.009). In addition, there was a statistically
significant difference between A2b subtype and B subtype in the number
of children with hospitalization time > 7 days, and the
hospitalization time of A2b subtype was longer than that of B1 subtype
(p =0.030), which suggests that we should pay more attention to
A2b subtype in pediatric clinic. From September 2020 to August 2021,
only one HMPV case was identified by phylogenetic analysis as subtype B1
infection. From April 2017 to March 2018, the detection rate of B1
subtype (54.5%) were highest, followed by the A2b subtype (40.9%) in
the previous study of our laboratory, and no HMPV A1 and A2a subtypes
were found25. However, in this study, the A2b subtype
was the predominant (82.1%), indicating that the main circulating
strain of HMPV infection in Beijing in 2017-2019 switched from B1
subtype to A2b subtype. The study by Matsuzaki Y showed that laryngitis
is more common in children with HMPV subtype B1 infection, while
wheezing is more common in children infected with HMPV subtypes B1 and
B2 than in children with subtype A2 infection35. Many
data suggest that HMPV genotypes are associated with disease severity or
clinical manifestations 36,37, but some studies have
inconsistent conclusions 24,38, so it is still
controversial whether HMPV genotypes are associated with clinical
features.
This study was limited by the large decrease in hospitalized children
with ARTIs during the COVID-19 outbreak. From September 2020 to August
2021, there were only 4 HMPV positive children of 232 hospitalized
children with ARTIs, therefore a detailed epidemiological analysis of
HMPV cases during this period was not possible. We will continue to
collect respiratory samples from hospitalized children with ARTIs in the
subsequent studies, and make a detailed analysis of the impact of
SARS-CoV-2 on the prevalence of HMPV.