【2025】Influenza + COVID-19 Co-Infection: Real Risks and Symptom Comparison

Co-infection with Influenza and COVID-19, commonly known as “Flurona,” has been confirmed in approximately 1,200 cases nationwide in Japan (as of November) and poses a severity risk incomparable to single-pathogen infections. Latest national data (MHLW, NIID) reveals that Flurona elevates the risk of hospitalization by 4−6 times and the mortality risk by up to 15 times. This article thoroughly compares the real danger of co-infection and the decisive differences in symptoms, based on the predominant strains in 2025. It is crucial to understand the worst-case scenario—the simultaneous onset of “Influenza’s rapid high fever” and “COVID’s breathlessness and persistent cough”—and to take action immediately upon symptom onset.

1. Danger Level of Co-Infection (Measured Data)

Factor	Influenza Monoinfection	COVID-19 Monoinfection	Co-Infection (Flurona)
Hospitalization Risk	1x	1.5−2x	4−6x
ICU Admission Rate	1x	2−3x	8−12x
Mortality Risk (Overall)	1x	3−5x	10−15x
Mortality Risk (Ages 65+)	1x	5−8x	20−30x
Post-Illness Symptoms (Shortness of breath, fatigue)	5−10%	15−25%	40−60%

2025 real data (MHLW, NIID) shows that approximately 1 in 4 co-infected patients progressed to severe pneumonia or ARDS. The case-fatality rate sharply increases, especially in the elderly and those with underlying conditions.

2. Symptom Comparison (Based on 2025 Predominant Strains)

Symptom	Influenza A (2025 Predominant)	COVID-19 (JN.1/XEC Lineages)	Co-Infection Pattern (Worst Case)
Speed of Onset	Ultra-acute (Fever 38∘C–40∘C in hours)	Gradual (Peak in 1−3 days)	Rapid onset fever like the flu, followed by the superimposition of COVID symptoms.
Peak Body Temp.	Typically 39∘C–40∘C	Mostly 37.5∘C–38.5∘C	39.5∘C–41∘C that persists and won’t subside.
Sore Throat	Mild to Strong	Excruciating pain (Like being stabbed with a knife)	Excruciating pain like COVID, lasting 5−7days.
Cough	Deep, hacking (With phlegm)	Dry, constant (No phlegm)	Both mixed → Severe, persistent cough preventing sleep.
Runny/Stuffy Nose	Profuse discharge	Almost none	Profuse discharge like the flu, but protracted.
Headache/Body Aches	Severe body aches (Cannot get out of bed)	Mild to Moderate	Severe body aches like the flu PLUS persist for another two weeks.
Taste/Smell Loss	Almost none	60−80%	50−70% (COVID symptom dominates)
Shortness of Breath(Hypoxia)	Rare (Only in severe cases)	10−20%	30−50% (Worsens rapidly)
Duration of Symptoms	Recovery in 3−5 days	7−14 days	2−4 weeks (Easily transitions into long-term sequelae)

The Worst-Case Flurona Pattern = Simultaneous onset of “Influenza’s rapid high fever + COVID’s breathlessness and persistent cough.”

3. Three Especially Dangerous Patterns (2025 Case Examples)

Elderly + Co-infection: Oxygen saturation dropped sharply to ≤85% on day 3 of onset (twice the speed of monoinfection).
Pregnant Women: Intrauterine hypoxia → Fetal distress risk ≥5 times higher than with COVID monoinfection.
Underlying Conditions (Diabetes, Heart Failure): Myocarditis + Pneumonia double punch resulting in a case-fatality rate ≥30%.

4. Immediate Actions if Co-Infection is Suspected (2025 Protocol)

Keep a “Multiplex Test Kit” (Simultaneous testing for Flu + COVID + RSV) at home.
- Sold at pharmacies for ¥1,800−¥2,500 (Widely available in 2025).
If Positive, Call Immediately:
- Ages 65+, Pregnant women, Underlying conditions → Immediate ambulance or fever clinic (Priority without waiting).
- Healthy adults with “shortness of breath” or “39∘C lasting 3 days” → Seek immediate medical attention.
Treatment Reality (2025): Simultaneous administration of Tamiflu (for Flu) + Paxlovid/Xocova (for COVID) is possible.
- This dual therapy was administered to approximately 80% of co-infected inpatients and demonstrated a 60%reduction in mortality.