Study Finds Influenza Often Undiagnosed in Winter Deaths

Many deaths occur outside hospitals or in older adults with comorbidities, where respiratory symptoms can be nonspecific and testing is uncommon. These gaps complicate disease burden estimates and public health planning. A new study shows how postmortem viral testing reveals frequent, previously unrecognized influenza among winter deaths.

European Society of Clinical Microbiology and Infectious Diseases (ESCMID) details a population-based observational study in Spain that applied postmortem polymerase chain reaction (PCR) testing for a broad respiratory virus panel. Investigators obtained upper airway swabs within 24 hours of death from 857 deceased persons across four influenza seasons, irrespective of the reported cause of death. Sampling coincided with periods when all-cause mortality was about 12% higher than expected, historically linked to seasonal respiratory viruses. The approach was designed to quantify underrecognized infections contributing to winter mortality.

Influenza RNA was detected in 11% of winter deaths. Among those with postmortem influenza, only 41.5% had been hospitalized, and just 17% had a laboratory-confirmed influenza diagnosis within 30 days before death. Only 1.4% of all deaths were recorded as influenza on death certificates, underscoring a large discrepancy between infection detection and official mortality coding. Collectively, the results indicate that routine clinical surveillance underestimates influenza’s contribution to winter deaths.

Other respiratory viruses were also common: at least one virus was identified in 36.4% of all individuals, with rhinovirus (11.4%), coronavirus (7.1%), and respiratory syncytial virus (RSV; 6.9%) frequently detected. Viral prevalence remained consistently high across all four seasons and was particularly elevated among residents of long-term care facilities, even where vaccination coverage is high. While postmortem detection does not prove causation, the authors note that respiratory viruses are thought to precipitate clinical deterioration in older adults with chronic comorbidities. They emphasize the need to complement routine clinical testing with strategies that better capture deaths occurring outside healthcare settings.

The study is published in Clinical Microbiology and Infection and is scheduled for presentation at ESCMID Global 2026. The findings highlight how systematic postmortem PCR testing can refine estimates of respiratory virus–associated mortality. These data point to surveillance approaches that more fully account for infections in high-risk populations.

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