COVID‑19, Flu & RSV in Adults: What’s the Difference and How Can You Tell?

Respiratory illnesses caused by viruses such as COVID‑19, the seasonal flu, and Respiratory Syncytial Virus infection (RSV) often look very similar, especially in adults. They share many symptoms, spread in similar ways, and in certain populations all can lead to serious illness. At the same time, they differ in key ways: the viruses themselves, the typical patterns of illness, the risks, and the ways to diagnose and treat them.

This article provides an up‑to‑date overview of similarities and differences between COVID‑19, influenza, and RSV in adults, how to tell which one you might have, and what you should do.

1. Viruses, Epidemiology & Risk

Similarities

All three COVID-19, influenza viruses and RSV are RNA viruses that infect the respiratory tract.
All can cause fever, cough, sore throat, runny/stuffy nose, fatigue, and in some cases shortness of breath or pneumonia, especially in older or high‑risk adults. Cleveland Clinic
All tend to spread via respiratory droplets (coughing/sneezing) and contact with contaminated surfaces or hands.
All have seasonal patterns in many temperate regions, although COVID‑19 has shown less strict seasonality. SpringerLink
Vaccines or preventive measures exist for all three in adults, which influences severity and outcomes. For example, new RSV vaccines for older adults were recently introduced. PubMed

Differences

Virus characteristics: For example, a recent review notes that COVID-19 has a larger genome and broader host‑range than influenza or RSV.
Epidemiologic burden & risk groups:
- COVID‑19 has caused a pandemic, many millions of cases, and remains capable of severe systemic complications (not just respiratory).
- Influenza is long‑known and causes seasonal outbreaks; hospitalisations and deaths are high, especially in older adults and those with comorbidities.
- RSV has historically been under‑recognized in adults (more recognized in infants/children), but newer data show it also causes serious adult illness (especially older adults, immunocompromised). For example: among hospitalized US adults, RSV disease severity was similar to unvaccinated COVID‑19 or influenza patients. PMC
Vaccination and immunity: The vaccination landscape and immunity levels differ. For example, many more adults have had prior flu infections/vaccines than RSV, and COVID‑19 immunity is complex (prior infection + vaccination + variants).
Severe outcome risk: One large study of 27,885 hospitalized adults found that COVID‑19 was associated with higher risk of severe outcomes compared to influenza (adjusted hazard ratio 1.30) while RSV showed no statistically significant difference vs influenza. PubMed
Another found that RSV hospitalizations, though less common, had similar severity to unvaccinated COVID‑19 or influenza, and worse than vaccinated cases of those diseases. PubMed
Unique features: COVID‑19 has been associated with unique complications (e.g., cardiovascular, neurologic, long‑COVID). For example, in a study of children and young adults, myocarditis and bradyarrhythmia were more common in COVID‑19 compared to influenza or RSV. PubMed

2. Symptoms & How They Compare in Adults

The symptoms of these viruses overlap so much, it’s often hard to tell them apart based on symptoms alone but there are subtle clues.

Common overlapping symptoms

Fever or feeling feverish/chills
Cough
Sore throat
Runny or stuffy nose
Muscle/body aches
Fatigue (tiredness)
Headache

Some helpful distinguishing features (though not definitive)

According to credible health‑care institution summaries:

Influenza (flu) tends to have a sudden onset of symptoms (you feel hit‑by‑a‑truck) and high fever is common (103‑104 °F). Vomiting/diarrhea are more common in children but can happen in adults. Cleveland Clinic
COVID‑19 may have a somewhat longer incubation period (2‑14 days) than flu (1‑4 days). A more gradual onset is possible. Loss of taste/smell is more characteristic of COVID‑19 (though not always present). Shortness of breath may appear. Northwestern Medicine
RSV in adults may present more like a bad cold initially with runny nose, sneezing, cough and wheezing or increased shortness of breath may be more prominent in vulnerable adults. Appetite loss might appear. It is less likely than flu or COVID to cause sudden high fever in healthy adults.

3. How to Tell Which One You Have

Consider your risk factors and context
- Have you been exposed to someone with known COVID, flu or RSV?
- What is your vaccination status for COVID‑19 and influenza (and RSV if eligible)?
- Do you have comorbidities (lung disease, heart disease, immunocompromised) or advanced age? These raise risk for severe outcomes from any of these viruses.
- What season/region is it? Many are more common in fall/winter but COVID may circulate year‑round.
Take note of your symptom onset and pattern
- Loss of taste/smell stronger suspicion for COVID.
- High fever + body aches common in flu.
- Runny nose + sneezing + wheezing or increased shortness of breath possible RSV (especially if older or with lung disease).
- Shortness of breath or difficulty breathing  is a red flag for any of them (severe disease).
Get tested
- Because symptoms overlap, if you’re sick and especially if you are high risk, get tested for COVID‑19 (PCR or approved rapid test) and ask about influenza and RSV testing (many clinics/hospitals offer multiplex respiratory virus panels).
- Lab testing is the only way to be sure of the virus type.
Follow treatment/prevention steps
- If it’s flu and you’re within 48 hours of symptom onset, antiviral medications (e.g., oseltamivir) may be helpful (depending on local guidelines and your risk factors).
- If it’s COVID‑19 and you meet criteria, there are antiviral treatments and you may need closer monitoring.
- For RSV in adults, treatment is mostly supportive (rest, fluids, monitor breathing) though newer adult RSV vaccines and prophylaxis are emerging for older/high‑risk adults.
- For all three: stay hydrated, rest, isolate from others as appropriate, monitor for worsening symptoms (e.g., harder breathing, chest pain, confusion). Seek immediate medical care if serious signs appear.
- Vaccination and prevention: make sure you are up to date with flu vaccine annually; COVID‑19 vaccine/boosters as recommended; RSV vaccine if you are in the eligible older adult group or high risk. These reduce risk of severe disease. For example, RSV disease in adults was shown to be more severe relative to vaccinated COVID/flu patients. PubMed
Monitor for complications
- For COVID‑19: long‑COVID (persistent symptoms) remains a possibility.
- For influenza: complications may include pneumonia, myocarditis, worsening of chronic disease.
- For RSV: especially in older adults or those with lung/heart disease, RSV can lead to hospitalization, ICU admission. The JAMA Network Open cohort found that in adults hospitalized with RSV, 12% experienced invasive mechanical ventilation or death similar to COVID/influenza unvaccinated. JAMA Network