# Divergent surveillance needs and resource allocation for COVID-19 and influenza: insights from a community-based syndromic surveillance study in Shanghai (2024–2025)

## TL;DR

This study compares how well surveillance systems detect COVID-19 and influenza in Shanghai, showing that each virus requires different monitoring and healthcare planning.

## Contribution

The study provides community-based evidence on divergent surveillance needs and healthcare burdens for concurrent outbreaks of COVID-19 and influenza.

## Key findings

- Standard ILI definitions miss more COVID-19 cases than influenza due to differing symptom profiles.
- Influenza leads to a higher healthcare burden, especially in outpatient visits and post-acute care.
- Symptoms like loss of taste or smell are more common in mild COVID-19 cases compared to influenza.

## Abstract

The rapid evolution and symptom overlap of Coronavirus disease 2019 (COVID-19) and influenza challenge the effectiveness of current surveillance and healthcare resource planning. However, comparative evidence regarding their surveillance sensitivity and healthcare burden remains limited, particularly within concurrent community populations that capture the full spectrum of disease severity.

To address this gap, data were derived from a community-based syndromic surveillance cohort in Shanghai, followed weekly between May 2024 and August 2025. We analyzed symptom profiles and illness duration, assessed the sensitivity of Influenza-Like Illness (ILI) definitions, and evaluated healthcare-seeking behaviors across both acute (0–14 days) and post-acute (>14 days) phases.

From May 2024 to August 2025, 382 COVID-19 and 175 influenza cases were identified. Compared with influenza, COVID-19 cases presented distinctively with upper respiratory symptoms (sore throat: 72.88% vs. 58.29%, runny or stuffy nose: 46.58% vs. 33.71%, loss of taste or smell: 3.84% vs. 0.57%; all p < 0.05), rather than fever (61.64% vs. 74.86%, p = 0.003). Consequently, standard ILI definitions failed to detect a significantly larger proportion of COVID-19 cases compared to influenza (China CDC criteria: 35.89% vs. 50.86%, p = 0.001; WHO criteria: 27.12% vs. 44.00%, p < 0.001). While illness duration was shorter for COVID-19 (6.66 ± 4.35 days vs. 8.25 ± 4.34 days, p < 0.05), influenza imposed a heavier healthcare burden, characterized by a two-fold increase in outpatient visits during the acute phase (OR = 2.12, 95% CI: 1.52–2.95) and sustained demand in the first 90 days of the post-acute phase (HR = 1.29, 95% CI: 1.03–1.61).

COVID-19’s symptom profile limits ILI surveillance sensitivity, whereas influenza imposes a higher burden extending into the post-acute phase. These differences call for adapting surveillance strategies and healthcare resource allocation to these distinct pathogen profiles.