Human Metapneumovirus (HMPV) Outbreak: An Urgent Call for Enhanced Surveillance and Public Health Action

Human metapneumovirus (HMPV) circulation and other recent trends in acute respiratory infections (ARIs) in the Northern Hemisphere are consistent with seasonal patterns that are typical for this time of year. Numerous countries' surveillance data show increased activity of respiratory pathogens, including influenza, respiratory syncytial virus (RSV), and HMPV. In places like China, influenza is the most often found infection. Although reports of an increase in cases, especially in northern China, have drawn attention to HMPV, the total burden is still within normal seasonal expectations, and healthcare facilities are not overburdened according to China CDC report. A recent outbreak of human metapneumovirus (HMPV), a respiratory disease that poses serious risks to children, the elderly, and immunocompromised people, has occurred in several countries, including China, Malaysia, India, the United Kingdom, and the United States. Identified in 2001, HMPV is a leading cause of acute respiratory infections, often mimicking symptoms of respiratory syncytial virus (RSV) and influenza [1]. The present peak in HMPV infections, which also coincides with seasonal increases in other respiratory viruses, emphasizes how healthcare systems may be put under more stress if effective surveillance and preventive measures are not put in place. HMPV generally has a low fatality rate in healthy individuals but poses a higher risk to vulnerable groups such as young children and immunocompromised individuals, in whom it can lead to severe complications like pneumonia. With an estimated 1% of acute lower respiratory infection-related deaths in children under five attributed to HMPV, it is less deadly than COVID-19 but still poses a significant threat in specific populations. Even though there have been reports of severe lower respiratory tract infections, especially in young children, the elderly, and people with underlying medical conditions, the situation highlights the necessity of improved genomic surveillance to ascertain whether the HMPV strains that are currently in circulation have changed transmission patterns or are more virulent. Preliminary data suggest efficient person-to-person transmission, likely exacerbated by crowded environments such as schools and nursing homes. It is crucial to prioritize real-time genomic sequencing of circulating strains, strengthen public health messaging, and guarantee sufficient resource allocation for early detection and management of severe cases to reduce the risk of overwhelming healthcare systems. The clinical manifestations of HMPV infections include fever, cough, nasal congestion, and breathing difficulties, with complications such as pneumonia and bronchiolitis often resulting in prolonged hospital stays. Concurrent circulation of influenza and RSV has exacerbated healthcare strain, increasing ICU admissions and ventilator usage [2]. Disruptions to routine medical services and amplified health inequities in underserved communities further compound the outbreak's public health impact. Co-infections with other respiratory viruses add complexity to patient management, necessitating integrated diagnostic and therapeutic strategies [1]. Diagnostic efforts have relied on sensitive real-time polymerase chain reaction (RT-PCR) assays, while genomic sequencing has revealed mutations in the fusion (F) and attachment (G) glycoproteins, potentially enhancing transmissibility or immune evasion [3]. Co-infections with RSV and influenza raise concerns about synergistic effects on disease severity. Multiplex RT-PCR assays and complementary sequencing methods such as Sanger and MinION are advancing respiratory virus detection and elucidating genetic diversity [4]. HMPV is a negative-sense, single-stranded RNA virus with a genome of around 13 kb. It encodes nine proteins, including the F and G glycoproteins, which are essential for immune evasion and viral entry. Human metapneumovirus (HMPV) primarily enters human cells using cellular receptors such as αvβ1 integrin, which facilitates attachment and internalization, and heparan sulfate proteoglycans (HSPGs), which act as initial attachment factors. Additionally, annexin II has been implicated in HMPV entry, particularly in epithelial cells, though further research is ongoing to fully understand the mechanisms and potential co-receptors involved. The virus has six lineages, A1, A2a, A2b, A2c, B1, and B2, and is split into two genetic groups (A, B). The A2b1 subtype has recently gained prominence in China, characterized by a 111-nucleotide duplication in the G gene linked to increased transmission and pathogenicity [5]. In Beijing, 92.59% of sequenced genomes contain this duplication, while southern China continues to see endemic A2c lineage activity [5]. Significant variation in the L and G genes is shown by genomic research. The severity of the disease is higher in emerging lineages, particularly those with G gene duplications. Low dN/dS ratios support viral stability by indicating little positive selection. The evolutionary rate of the G gene is estimated by Bayesian phylogenetic analysis to be 3.654 × 10⁻³ substitutions/site/year, highlighting fast adaption. The stable F protein, essential for viral entry, offers a promising vaccine target [5]. HMPV causes 5%–7% of pediatric hospitalized respiratory tract infections (RTIs) worldwide, with regional variations in seasonal peaks. Group A strains, especially A2b, were the most common kind of HMPV found in 10.2% of nasopharyngeal aspirates, according to a Chongqing study conducted between 2008 and 2011. A Henan study (2017–2023) reported an HMPV positivity rate of 7.78%, with seasonal peaks in spring and a subtype switching pattern highlighting the virus's evolving nature [6]. Genetic variability complicates immune responses, with emerging lineages linked to enhanced transmission and severity. HMPV's immune evasion mechanisms challenge vaccine development. Viral proteins G and SH suppress innate immunity by disrupting interferon pathways, while upregulated Bcl-2 inhibits apoptosis and cytokine dysregulation contributes to pathogenesis. Despite genetic variability, the F protein remains highly conserved, supporting cross-lineage vaccine development [7]. As of now, there are no approved vaccines or specific antiviral drugs available for Human Metapneumovirus (HMPV). However, Ribavirin, a medication commonly used to treat respiratory syncytial virus (RSV), has shown effectiveness against HMPV in animal models. Meanwhile, Moderna's candidate modRNA vaccine for metapneumovirus, which successfully completed Phase I clinical trials in October 2019 and demonstrated good tolerability and an immune response that boosts neutralizing antibody production, is still undergoing further clinical evaluation. Treatment for HMPV infections is primarily supportive, focusing on managing symptoms such as fever, cough, and respiratory distress. Surveillance studies underscore HMPV's global spread, fueled by genetic diversity, international travel, and limited diagnostics. The burden in children under five rivals that of influenza and RSV [8]. Improved diagnostic tools, targeted vaccine development, and public health measures are critical to mitigating HMPV's impact. Genomic surveillance reveals rising prevalence of genotypes with G gene duplications, particularly within the A2.2.2 lineage, since 2014 [9]. Seasonal patterns—winter peaks in temperate regions and spring-summer prevalence in subtropical areas—reflect HMPV's adaptability. COVID-19 restrictions temporarily reduced circulation, offering insights into transmission dynamics. To effectively address the ongoing HMPV outbreak, immediate action is needed to integrate enhanced surveillance, molecular diagnostics, and genomic insights into public health strategies. In China, for instance, there has been a notable rise in HMPV cases, with the virus linked to 6.2% of positive respiratory illness tests and 5.4% of respiratory illness hospitalizations, surpassing the rates for COVID-19, rhinovirus, or adenovirus. Reports indicate a significant rise in infections, particularly among vulnerable populations, prompting health authorities to implement new monitoring measures and manage pneumonia cases of unknown origin. Prioritizing global collaboration and health education is essential to tackle HMPV's evolving burden, as it poses serious risks similar to those of other respiratory pathogens. Advancing vaccine development and therapeutic research will be crucial in mitigating the impact of HMPV and ensuring public safety during peak respiratory illness seasons. We acknowledge the efforts of healthcare workers and researchers around the world who are working tirelessly to address the HMPV outbreak. The authors declare no conflicts of interest. The authors have nothing to report.