INFLUENZA cases were missed in 83% of winter deaths with positive postmortem tests across four flu seasons in Spain, a new population-based study presented at ESCMID Global 2026 has found.
The respiratory virus was detected in 11% of winter deaths but only 1.4% were recorded accordingly on death certificates, highlighting a substantial gap between detection and mortality records.
High Rates of Undetected Infection
Researchers analysed 857 deceased persons during periods when deaths were around 12% higher than expected, likely due to seasonal respiratory viruses.
They obtained postmortem swabs within 24 hours of death and tested by PCR for an extensive panel of respiratory viruses.
Of the 94 people who tested positive for influenza, only 41.5% had been hospitalised and a mere 17% had received a diagnosis within 30 days prior to death.
Lucía Argente-Colás, lead author, University Hospital of Navarre, Spain, said: “These findings were surprising.
“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.”
Reasons Behind Underreporting
In older populations and those with underlying conditions, symptoms can be less pronounced or masked by existing illness, researchers highlighted.
Mortality records often prioritise pre-existing chronic conditions when a respiratory infection contributes to death.
Hence, the viral trigger may go unsuspected and untested, Argente-Colás explained.
She added: “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.
“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.”
New Surveillance Approaches
Findings suggested that influenza may play a more significant role in winter mortality than routine surveillance currently captures.
Researchers urged a review of surveillance strategies, particularly for high-risk populations.
Whilst authors acknowledged that routine testing remains essential, they called for new approaches that integrate deaths occurring outside healthcare systems.
Reference
Trobajo-Sanmartín C 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.
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