A newly published population-based study has found that influenza was detected in 11% of winter deaths, yet only 17% of these infections were diagnosed before death.
The study, published in Clinical Microbiology and Infection and due to be presented next week at ESCMID Global 2026, tested 857 deceased persons across four influenza seasons in Spain, regardless of the reported cause of death.
The research was conducted during periods when deaths were around 12% higher than expected, a gap long associated with seasonal respiratory viruses. Postmortem swabs were obtained within 24 hours of death and tested by PCR for a broad panel of respiratory viruses.
Of the 94 individuals who tested positive for influenza in postmortem testing, only 41.5% had been hospitalised and just 17% had received a confirmed influenza diagnosis within 30 days before death.
Only 1.4% of deaths in the study population were recorded as influenza on death certificates, highlighting a substantial gap between detection and official mortality records. Together, these findings suggest influenza may play a larger role in winter mortality than routine surveillance captures.
"The findings were surprising," said Lucía Argente-Colás, lead author of the study. "The high proportion of infections not detected before death or recorded on death certificates underlines how much we are missing. Many people who die from infectious diseases do so at home, where testing is less likely to take place. Respiratory viral infections can also develop suddenly and in some cases, death may occur at an early stage of illness, before medical attention is sought."
The researchers identified several reasons why respiratory viral infections often go unrecognised at the time of death. In older people and those with underlying conditions, symptoms can be less pronounced or masked by existing illness. Where a respiratory infection contributes to death, the pre-existing chronic condition usually takes precedence as the recorded cause.
"Clinicians managing patients with chronic conditions typically focus on the underlying disease and a viral trigger may go unsuspected and untested," said Argente-Colás. "Respiratory viral infections rarely cause death in healthy people, but they can trigger deaths in those with severe chronic conditions and advanced age, and it is these conditions that are usually registered as the cause of death."
Influenza was not the only virus found to be underrepresented in mortality records. At least one respiratory virus was detected in over a third (36.4%) of all participants, with rhinovirus (11.4%), coronavirus (7.1%) and respiratory syncytial virus (6.9%) also commonly identified. The prevalence of any respiratory virus remained consistently high across all four seasons studied and was particularly elevated among residents of long-term care facilities, highlighting the vulnerability of this population, even in settings with high vaccination coverage.
"While detecting a virus postmortem does not automatically imply causation, respiratory viral infections are thought to trigger deterioration and death in older people with chronic comorbidities," added Argente-Colás. "In this study, older adults with underlying conditions comprised the large majority of the study population. In these patients, a viral infection may have played a role in death even where the underlying chronic condition appears on the death certificate."
Looking ahead, the researchers emphasise the need to rethink surveillance strategies, particularly for high-risk populations such as older adults. While routine testing remains essential in clinical settings, additional methods may be needed to capture the full burden of disease.
“Healthcare systems are missing an important proportion of deaths associated with respiratory viruses,” Argente-Colás concluded. “To better understand and respond to these infections, we need to complement existing surveillance with new approaches that include deaths occurring outside the healthcare system.”
Lead study author Lucía Argente-Colás is a final-year microbiology resident in the University Hospital of Navarre. She graduated in Biotechnology from the University of Valencia in 2020. Currently, she is also pursuing a PhD focused on dermatophyte fungi.
- Trobajo-Sanmartín C, Navascués A, Olazábal-Arruiz M, et al. Prevalence of influenza and other respiratory viral infections in deceased persons: a population-based observational study over four influenza seasons. Clin Microbiol Infect. 2026;32(3):474-481. doi:10.1016/j.cmi.2025.11.019