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Common Cold vs. Flu: How to Tell the Difference?


There are many common ailments that have signs and symptoms that overlap. Just like there are differences in people who get the same illness, there is a variety of presentations even in the same individual when they get sick again. A perfect example is COVID-19 and how the same person reacts to its different strains or variants.

Upper respiratory tract infections (URIs) are the most common patient presentation in the outpatient setting. URIs can be a mild, self-limited, runny nose (catarrhal) syndrome with a sore throat, or they can be a life-threatening illness such as epiglottitis, a serious inflammation of a part of the throat that can block the airway.

The flu usually means an infection caused by Influenza virus and there are three types: A, B, and C. Influenza represents one of the most common infectious diseases. It is highly contagious.

Influenza or flu occurs in seasonal epidemics and manifests as an acute febrile illness with variable degrees of systemic symptoms, ranging from mild fatigue to respiratory failure and death. Influenza causes significant loss of workdays, human suffering, and mortality.

Because of the variability of both cold and flu, they are often difficult to distinguish apart.

What is the common cold?

The most common cause of the common cold is the rhinovirus. This virus typically causes upper respiratory tract infections (URIs), but they can also affect the lower respiratory tract and cause pneumonia.

Potential complications from the common cold can include ear infections (otitis media), sinusitis, bronchitis, and reactive airway disease or asthma. Rhinovirus infections occur year-round, but the incidence is higher in the fall and spring.

Other viruses can cause the common cold including enterovirus, adenovirus, respiratory syncytial virus, and yes, the coronavirus. The coronavirus is one of the more common causes, but these refer to strains of the virus that do not cause the profound consequences of COVID-19. COVID-19 is an entirely different organism and disease process.

What are the signs and symptoms of the common cold?

For the purposes of this discussion, we will focus on the rhinovirus infection since it is the most common.

Manifestations of rhinovirus infection typically appear after an incubation period of 12 to 72 hours and last seven to 11 days but may persist for longer.

Signs and symptoms include the following:

  • Nasal dryness or irritation - May be first symptom
  • Sore throat or throat irritation – Common and bothersome initial symptom
  • Nasal discharge, nasal congestion, and sneezing – Intensify over 2-3 days
  • Headache
  • Facial and ear pressure
  • Loss of sense of smell and taste
  • Cough (30% of infected individuals)
  • Hoarseness (20%)
  • Post-tussive (after coughing) vomiting
  • Irritability or restlessness
  • Fever (unusual; when present, typically low grade)

Age-related differences in presentation are as follows:

  • Infants and preschoolers – Fever more likely, often 38 to 39°C
  • Infants and toddlers – May display only nasal discharge
  • School-aged children - Usually complain of nasal congestion, cough, and runny nose

Physical examination findings:

  • Typically, less severe than the symptoms reported by the patients
  • Fever uncommon, though temperatures of 38 to 39°C are possible in younger children
  • A red nose with a profuse, dripping nasal discharge may be present
  • Nasal discharge can be clear and watery or mucopurulent (yellow or green)
  • The nasal mucous membranes have a glistening, glassy appearance, usually without obvious erythema or edema
  • The pharynx typically appears normal, without any erythema, exudate, or ulceration
  • Mildly enlarged, nontender cervical lymph nodes are present
  • Auscultation of the chest may reveal rhonchi (crackling sounds)

How does a patient with influenza (flu) present?

The presentation of influenza virus infection varies. However, it usually includes many of the symptoms described below. Patients with influenza who have existing immunity or who have received the annual vaccine may have milder symptoms.

Onset of illness can occur suddenly over the course of a day, or it can progress more slowly over the course of several days. Typical signs and symptoms include the following (not necessarily in order of prevalence):

  • Cough and other respiratory symptoms
  • Fever
  • Sore throat
  • Myalgias (muscle aches)
  • Headache
  • Nasal discharge
  • Weakness and severe fatigue
  • Tachycardia
  • Red, watery eyes

Cough and other respiratory symptoms initially may be minimal but frequently progress as the infection evolves. Patients may report nonproductive cough, cough-related pleuritic chest pain, and dyspnea. In children, diarrhea may be a feature.

Fever may vary widely among patients, with some having low fevers (in the 100°F range) and others developing fevers as high as 104°F. Some patients report feeling feverish and feeling chills.

Sore throat may be severe and may last three to five days. The sore throat may be a significant reason patients seek medical attention.

Weakness and severe fatigue may prevent patients from performing their normal activities or work. Patients report needing additional sleep. In some cases, patients with influenza may be bedridden.

The incubation period of influenza averages two days but may range from one to four days in length

How does a healthcare provider tell the common cold and flu apart?

Fortunately, there are tests that can help rather than relying solely on the clinical presentation.

Most cases of the common cold are self-limited and last only a few days.

The gold standard for confirming influenza virus infection is reverse transcription-polymerase chain reaction (RT-PCR) or viral culture of nasopharyngeal or throat secretions. Rapid diagnostic tests for influenza are available and are becoming more widely used. These tests have high specificity but only moderate sensitivity.

Findings of standard laboratory studies, such as a complete blood count (CBC) and electrolyte levels, are nonspecific but helpful in the workup of influenza and the common cold.

Leukopenia (low white blood cell counts) and relative lymphopenia are typical findings. Thrombocytopenia (low platelet counts) may be present.

In severe cases of influenza, the patient is likely to have hypoxemia (low oxygen) and poor respiration. Some patients with physical examination findings may have signs of meningitis and lumbar punctures should be performed to rule out that potentially serious complication.

What are the common tests for influenza?

The accuracy of these tests depends in part on the collection technique and skill of the person performing the test.

Nasal swabs must be deeply inserted and then swirled to attach the influenza virus. This is the technique borrowed for testing for COVID-19 as well.

The following three rapid diagnostic tests are considered of low complexity and may be used in physicians’ offices:

  • Quick Vue Influenza A+B test (Quidel)
  • ZstatFlu (ZymeTx)
  • Quick Vue Influenza test (Quidel)

Conclusion

Prevention is the most effective management strategy for influenza. To prevent seasonal flu, the Advisory Committee on Immunization Practices (ACIP) of the US Centers for Disease Control and Prevention (CDC) and the American Academy of Pediatrics (AAP) recommend routine annual influenza vaccination for all persons aged six months or older, preferably before the onset of influenza activity in the community.

Key takeaways

Rhinovirus or common cold infections are predominantly mild and self-limited.

Rhinovirus or common cold infections treatment is generally focused on symptomatic relief and prevention of person-to-person spread and complications.

Influenza is a potentially more serious illness.

Attention should be given to the appropriate use of personal protective equipment (PPE) by the pre-hospital providers, and advance notification should be given to the hospital regarding the potential need for patient respiratory isolation.

References:

Mayo Clinic. Common Cold: Symptoms and Causes.

Johns Hopkins Medicine. Common Cold.

Mayo Clinic. Flu: Symptoms and Causes.

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