In recent weeks, countries such as the United States and Australia have seen a sharp increase in influenza cases, even as COVID-19 activity remains present—albeit at lower levels. This period, marked by the simultaneous threat of multiple respiratory viruses, signals what public health experts are calling a “compound threat season.” It underscores a growing reality: influenza and COVID-19 are coexisting and jointly challenging public health systems worldwide.
Data Shows Flu Is Surging While COVID Holds Steady
In the United States, flu activity remains high. According to CDC surveillance ending February 1, 2025, influenza-like illness (ILI) cases have exceeded baseline levels for 10 consecutive weeks. Outpatient visits for ILI reached 7.8%—well above the 3.0% baseline. Hospitalization rates reached 9.2 per 100,000 people, and among those aged 75 and older, the rate surged to 62.8 per 100,000.
Meanwhile, the World Health Organization (WHO) noted on May 11, 2025, that global COVID-19 test positivity rates rose from 2% to 11%, with Southeast Asia and the Western Pacific showing the most significant increases.
Why Are Multiple Viruses Surging at Once?
Several factors are contributing to the concurrent outbreaks:
- Relaxation of Public Health Measures:
As societies return to pre-pandemic norms, mask usage has declined, and indoor gatherings have become more common—particularly during seasonal transitions and back-to-work or back-to-school periods. These conditions increase the likelihood of transmission. - Waning Population Immunity:
Prolonged periods of reduced exposure to influenza, due to pandemic precautions, have weakened natural immunity—especially among young children and older adults. These groups now face higher risks of infection as viruses re-enter circulation. - Viral Evolution:
Influenza viruses continue to shift between dominant strains, such as H3N2 and H1N1. Meanwhile, COVID-19 has evolved into new subvariants such as NB.1.8.1, which show enhanced transmissibility. This has led to broader and more rapid community spread, particularly in Australia, North America, and parts of Europe.
Australia and the U.S. Enter a Phase of “Multiple Viral Resonance”
Australia has reported a sharp rise in flu cases since early 2025, with flu-related deaths increasing by 65.7%. Winter arrived earlier than usual, bringing with it co-circulation of COVID-19, RSV, and even sporadic H5N1 detections.
In the U.S., several regions are experiencing what experts have dubbed a “quad-demic”: a simultaneous rise in influenza, COVID-19, RSV, and norovirus cases. Pediatric hospitalizations have spiked in some areas.
Combatting seasonal illness is no longer about tackling one virus at a time. A multi-layered, coordinated strategy is required. The era of coexisting viruses is not just on the horizon—it is already underway. The question is: are we ready?
