[Epidemiological characteristics of test-negative severe acute respiratory infections during the 2024-2025 surveillance years in Beijing].

Objective: To analyze the epidemiological characteristics of cases with severe acute respiratory infection (SARI) in Beijing who tested negative for 22 common respiratory pathogens by nucleic acid testing, and to explore the potential pathogen spectrum using metagenomic next-generation sequencing (mNGS). Methods: Data were obtained from the Beijing Acute Respiratory Infectious Disease Surveillance Network. Hospitalized SARI cases from week 40 of 2024 to week 39 of 2025 were included. All cases were tested for 22 common respiratory pathogens using nucleic acid assays. Among those test-negative results, 50 specimens were randomly selected for mNGS analysis. Multivariable logistic regression was performed to identify factors associated with test-negative results. Results: A total of 7 202 SARI cases were included, of whom 4 212 (58.5%) tested negative for all 22 common respiratory pathogens. The proportion of negative results increased with age, with 32.9% (322/978) in children aged 0-5 years, 69.1% (972/1 407) in adults aged 18-59 years, and 65.0% (2 506/3 856) in those aged ≥60 years, the difference was statistically significant (all P<0.001). Multivariable analysis showed that age was independently associated with negative results (18-59 years: aOR=4.62, 95%CI:3.85-5.55; ≥60 years: aOR=4.08, 95%CI:3.49-4.78). Upper respiratory samples were more likely to test negative. Among 48 valid mNGS samples, 32 pathogens were identified. At least one pathogen was detected in 44 cases (93.6%), and multiple infections were common (37 cases, 84.1%). Human herpesvirus 7 (20 cases) was most frequently detected, followed by Stenotrophomonas maltophilia (16 cases), Human herpesvirus (15 cases), and Streptococcus pneumoniae (12 cases). Conclusions: A high proportion of SARI cases in Beijing tested negative for common respiratory pathogens, and age played an important role. mNGS identified predominantly opportunistic pathogens and herpesviruses, and did not detect novel pathogens with clear respiratory significance. These findings indicate that the current SARI surveillance covers the most common respiratory pathogens.