Coronavirus disease 2019 (COVID-19) is an infectious disease caused by the SARS-CoV-2 virus. The most common symptoms of COVID-19 are fever, cough, fatigue, and shortness of breath. However, the symptom that is most strongly associated with COVID-19 is loss of smell and taste.
Loss of Smell and Taste
Loss of smell (anosmia) and taste (ageusia) has emerged as one of the most specific symptoms of COVID-19. Multiple studies have found that loss of smell and taste is highly predictive of a COVID-19 diagnosis:
| Study | Findings on Loss of Smell/Taste |
|---|---|
| Study of 202 patients in Italy | 65% of patients experienced loss of smell or taste |
| Study of 417 patients in Spain | 70% of patients had smell impairment, compared to only 24% with fever |
| Literature review of 24 studies | Loss of smell was reported in 52% of COVID-19 patients overall |
Several factors make loss of smell and taste a hallmark symptom of COVID-19:
- Loss of smell often occurs early in the disease course, even in mild cases
- The prevalence is higher compared to other viral infections like the flu
- Smell loss may persist after other symptoms resolve
For these reasons, the Centers for Disease Control and Prevention (CDC) added loss of smell and taste as a symptom of COVID-19 in April 2020. Doctors now routinely ask patients about changes in their sense of smell and taste when screening for possible coronavirus infection.
Mechanism Behind Loss of Smell
Researchers believe smell loss in COVID-19 is likely due to damage to the olfactory epithelium – the specialized tissue high up in the nasal cavity that contains receptors that detect odors.
The ACE2 receptor that SARS-CoV-2 uses to enter human cells is highly expressed on the olfactory epithelium. This allows the virus to directly invade and damage the olfactory neurons, leading to rapid reduction in the ability to smell.
Inflammation triggered by the virus may also temporarily disrupt the olfactory system. Fortunately, olfactory neurons have the ability to regenerate every few weeks. This allows smell function to recover once the infection resolves in most people.
Other Causes of Loss of Smell
While COVID-19 is a common reason for loss of smell, other causes include:
- Common cold
- Flu
- Allergies
- Nasal polyps
- Head trauma
- Alzheimer’s disease
- Parkinson’s disease
However, loss of smell due to COVID-19 tends to be sudden and severe compared to more gradual smell decline with other conditions. The timing of symptom onset and exposure history can help distinguish COVID-19 from other causes.
Other Common Symptoms
While loss of smell and taste is most strongly linked to COVID-19, other symptoms are also associated with the disease:
Fever
Fever is a hallmark symptom of COVID-19, though not everyone with the disease has a fever. In a study of over 55,000 people with COVID-19:
– 70% had a fever on admission to the hospital
– Only 44% had a documented fever during their illness
This suggests fever may be more common in severe cases requiring hospitalization. However, mild or moderate cases can occur without fever.
Cough
Cough is another classic symptom of COVID-19, though it also occurs ubiquitously with many other respiratory conditions. One study found cough in 67.7% of people with COVID-19, compared to 3.4% of people without the disease.
Cough due to COVID-19 is typically dry rather than productive. It may persist for several weeks after other symptoms improve.
Fatigue
Fatigue related to COVID-19 can range from mild lethargy to an extreme exhaustion that prevents activities of daily living. Studies report fatigue frequencies between 38-70% among hospitalized COVID-19 patients.
Severe fatigue may be linked to underlying medical complications in severe cases, like low oxygen levels or endothelial dysfunction. However, fatigue can also occur in mild cases as an early symptom.
Shortness of Breath
Shortness of breath, or dyspnea, occurs commonly in COVID-19 as the infection progresses to affect the lower respiratory tract. Shortness of breath was found in:
– 53.1% of hospitalized patients in one study
– 17.6% of all COVID-19 cases in another study
Shortness of breath may be the result of pneumonia, low oxygen levels, or lung damage associated with severe COVID-19. Mild cases may experience shortness of breath with exertion.
Other Less Common Symptoms
In addition to the most prevalent symptoms above, COVID-19 can cause:
- Headache
- Sore throat
- Muscle aches and pains
- Chills
- Nausea and vomiting
- Diarrhea
- Runny nose
These symptoms are relatively nonspecific and can occur with many other viral infections. However, they may be seen in mild COVID-19, especially early in the course of disease.
Some people with COVID-19 remain completely asymptomatic but can still transmit the virus. Exact estimates vary, but one study found about 17.9% of COVID-19 cases overall are asymptomatic.
Symptom Comparison by Severity
The prevalence of COVID-19 symptoms can vary based on disease severity. This table compares symptom frequency across mild, moderate, and severe cases:
| Symptom | Mild Cases | Moderate Cases | Severe Cases |
|---|---|---|---|
| Fever | Uncommon | Common | Very common |
| Cough | Common | Very common | Very common |
| Fatigue | Sometimes | Common | Very common |
| Smell/Taste Loss | Very common | Very common | Common |
| Shortness of Breath | Rare | Common | Very common |
This demonstrates how shortness of breath and high fever occur more often in severe disease, while smell loss remains prevalent across all severities.
Symptom Timeline
Symptoms of COVID-19 generally appear 2-14 days after exposure to the virus. The average incubation period is about 5 days.
This is a typical COVID-19 symptom progression timeline:
| Time from Exposure | Signs and Symptoms |
|---|---|
| Days 1-3 | Asymptomatic |
| Days 3-5 | Fever, cough, fatigue emerge early |
| Days 5-7 | Shortness of breath possible; loss of smell common |
| Days 7-14 | Moderate to severe symptoms peak |
| Week 2+ | Gradual symptom improvement in mild cases |
However, not all patients follow this exact course. Some may rapidly deteriorate, while others have prolonged symptoms lasting weeks or months after onset.
Severity and Prognosis Based on Symptoms
Certain symptoms provide insight into likely COVID-19 severity and prognosis:
- Mild cases are more likely with symptoms like loss of smell, headache, sore throat, or muscle pains.
- Severe disease is more likely with shortness of breath at rest, high fever, or low oxygen levels.
- Critical cases may experience respiratory failure, septic shock, or multiple organ dysfunction.
However, there are exceptions. Some patients with mild symptoms may worsen quickly. Others with severe symptoms recover without intensive interventions.
Symptoms that tend to predict poorer outcomes include:
- Shortness of breath within 1 week of symptom onset
- Confusion, headache, or rhinorrhea
- Chest pain or pressure
- High fever lasting over 1 week
Advanced age, obesity, and other chronic diseases also increase the risk of severe or fatal COVID-19.
When to Seek Medical Care
The CDC recommends seeking emergency medical care for any of the following warning signs:
- Trouble breathing
- Persistent chest pain or pressure
- New confusion
- Inability to wake up or stay awake
- Pale, gray, or blue-colored skin, lips, or nail beds
Milder symptoms can typically be managed with over-the-counter medications and home isolation. However, contact a doctor for worsening symptoms or high-risk conditions like age over 65 or chronic illness.
Getting an accurate COVID-19 diagnosis is critical, as early treatment with antivirals may improve outcomes in vulnerable groups.
Conclusion
While many symptoms can occur with COVID-19, loss of smell and taste is the most specific sign of infection. Fever, cough, fatigue, and shortness of breath are also common. Symptom prevalence and severity varies across mild, moderate, and severe disease courses.
Rapidly progressing shortness of breath, high fever, or confusion increase chances of poor outcomes. Seeking prompt medical attention for worsening symptoms can enable early treatment and prevent complications in those at risk. Being aware of the symptom timeline and progression of COVID-19 helps identify cases and guide appropriate interventions.