COVID-19, caused by the SARS-CoV-2 virus, and influenza, commonly known as the flu, are both highly contagious respiratory illnesses. Both viruses target the respiratory system and cause symptoms that are often indistinguishable without laboratory testing. The co-circulation of these two distinct pathogens raises the question of whether a person can contract both infections simultaneously. This scenario, known as coinfection, has warranted significant scientific investigation since the start of the COVID-19 pandemic.
The Reality of Viral Coinfection
Coinfection, the simultaneous infection by two different pathogens, is biologically possible with SARS-CoV-2 and influenza viruses. The concern about this dual infection arose as the COVID-19 pandemic began to overlap with annual flu seasons. While the overall prevalence of this dual diagnosis is generally low, often estimated to be below 5% among COVID-19 patients, this rate increases significantly during peak respiratory virus seasons when both viruses are widely circulating.
The viruses can coexist because they utilize different cellular entry mechanisms. SARS-CoV-2 primarily binds to the Angiotensin-Converting Enzyme 2 (ACE2) receptor, while influenza viruses attach to sialic acid receptors. This difference means the viruses do not compete for the same cellular pathway for initial entry, allowing for simultaneous infection. Some laboratory studies suggest that influenza infection may increase the expression of ACE2 receptors on lung cells, potentially enhancing SARS-CoV-2’s ability to replicate in the respiratory tissue.
Clinical Presentation of Dual Infection
When a person experiences coinfection, the clinical course often becomes more complicated and potentially more severe than infection with either virus alone. Meta-analyses have indicated that patients with COVID-19 and influenza coinfection face significantly higher odds of severe outcomes compared to those infected only with COVID-19. These severe outcomes include an increased risk of intensive care unit (ICU) admission, a greater need for mechanical ventilation support, and an elevated risk of mortality.
The combined assault of both viruses on the respiratory system can lead to a more intense inflammatory response, sometimes described as an immune overload. This dysregulated immune activity can cause greater damage to lung tissue, increasing the likelihood of complications such as acute respiratory distress syndrome (ARDS). Populations already at higher risk for severe illness from either virus, such as the elderly, individuals who are immunocompromised, or those with underlying conditions like heart disease or diabetes, face the greatest danger from a dual infection.
The Necessity of Accurate Diagnosis
A major challenge for healthcare providers is the clinical overlap in symptoms between COVID-19 and the flu. Both viruses commonly cause fever, cough, fatigue, body aches, and headache, making it nearly impossible to distinguish between the two based on physical signs alone. Without testing, a clinician cannot determine if a patient has the flu, COVID-19, or a coinfection of both.
Accurate diagnosis is paramount because the treatment protocols for each virus are distinct, especially regarding the use of specific antiviral medications. To address this diagnostic challenge, healthcare systems increasingly rely on multiplex molecular assays, which are specialized laboratory tests. These multiplex tests can detect and differentiate between the genetic material of SARS-CoV-2, Influenza A, and Influenza B from a single respiratory specimen, such as a nasopharyngeal swab. This capability ensures that patients receive the correct and timely antiviral treatment, which is most effective when administered early in the course of the illness.
Protecting Against Both Viruses
The most effective strategy to prevent the serious complications associated with viral coinfection is to pursue dual protection against both pathogens. This primarily involves maintaining up-to-date vaccination against both the flu and COVID-19. Annual influenza vaccination is designed to protect against the strains predicted to be most common each season, while remaining current with COVID-19 vaccines and boosters helps maintain strong immunity against circulating SARS-CoV-2 variants.
These vaccinations work by preparing the immune system to fight off both viruses, reducing the risk of severe illness, hospitalization, and death from either infection. Beyond vaccination, simple public health measures offer shared protection against both respiratory illnesses. These actions include frequent and thorough hand washing, wearing a high-quality mask in crowded indoor settings, and staying home and away from others at the first sign of any respiratory symptoms.