COVID-19, caused by the SARS-CoV-2 virus, and influenza, caused by various influenza viruses, are both respiratory illnesses that have significantly impacted global health. While both illnesses present similarities, understanding their distinct characteristics is helpful for comprehending their differing effects on individuals and communities. This comparison explores various aspects of these infections.
Comparing Symptoms and Onset
Both COVID-19 and influenza share many common symptoms, making differentiation challenging. Individuals with either illness may experience fever, cough, fatigue, sore throat, runny or stuffy nose, muscle pain, body aches, and headaches. Vomiting and diarrhea can also occur, though these gastrointestinal symptoms are more frequently observed in children with influenza.
A distinguishing symptom with early COVID-19 variants was the new loss of taste or smell, which was less common with influenza. This symptom was reported by a notable percentage of COVID-19 patients, ranging from approximately 38% to 55% for taste loss and around 40% for smell loss. While influenza symptoms typically appear within one to four days after exposure, COVID-19 symptoms often develop more gradually, usually around five days after infection, but this can range from two to fourteen days.
Transmission and Contagiousness
Both viruses spread via respiratory droplets and aerosols released when an infected person coughs, sneezes, speaks, or breathes. These particles can be inhaled or land on mucous membranes. Both viruses can spread more easily in crowded or poorly ventilated indoor settings.
The basic reproduction number (R0), a measure of contagiousness, illustrates a difference. For seasonal influenza, the R0 typically falls between 1 and 2, meaning one infected person might infect one to two others. In contrast, the original SARS-CoV-2 variant had an R0 estimated between 2 and 3, indicating greater contagiousness. Later variants, such as Delta, were even more transmissible, with an R0 estimated between 5 and 7, while Omicron was approximately three times more infectious than Delta.
Presymptomatic and asymptomatic transmission has been a more significant factor in COVID-19’s spread. People with COVID-19 can also be contagious for a longer duration, sometimes ten days or more after symptoms appear. For influenza, contagiousness typically lasts about seven days from symptom onset.
Severity and Risk of Complications
Severity of illness and risk of complications vary considerably between COVID-19 and influenza. While most cases for both are mild, COVID-19 has generally shown a higher risk of severe outcomes. Initial data from the World Health Organization suggested approximately 15% of COVID-19 cases were severe, with 5% critical. COVID-19 patients were significantly more likely to require mechanical ventilation and had higher mortality rates than influenza patients in some hospital settings.
Specific severe complications associated with COVID-19 include acute respiratory distress syndrome (ARDS), systemic inflammation, and blood clots, such as venous thromboembolism. COVID-19 can also lead to multi-organ damage, including injury to the heart, kidneys, and liver. In contrast, the most common severe complication of influenza is often secondary bacterial pneumonia. Research indicates that COVID-19 can cause more hospital stays and deaths, especially for adults aged 18 and older, even those without other health challenges.
Long-Term Health Consequences
A notable difference lies in their potential for long-term health consequences. Post-Acute Sequelae of COVID-19 (PASC), known as Long COVID, refers to health problems that persist weeks, months, or even years after the initial COVID-19 infection. These persistent symptoms can be debilitating and include chronic fatigue, cognitive dysfunction often described as “brain fog,” shortness of breath, and postural orthostatic tachycardia syndrome (POTS).
Long COVID can occur even in individuals who experienced mild initial infections or were not hospitalized. Symptoms are highly varied, impacting multiple organ systems, and can fluctuate or worsen after mental or physical exertion, a phenomenon called post-exertional malaise. While post-viral syndromes are recognized after influenza and other infections, they are generally considered less common and less systemically complex than the spectrum of symptoms observed with Long COVID.
Impact on Different Populations
Both COVID-19 and influenza pose risks to various demographic groups. Older adults are particularly vulnerable to severe outcomes from both viruses, including hospitalization and death. The risk of severe illness from both increases with age, affecting both the very young and the very old.
For children, COVID-19 led to the emergence of a specific and severe condition called Multisystem Inflammatory Syndrome in Children (MIS-C). This rare but serious condition involves inflammation of different body parts, including the heart, lungs, kidneys, and brain, typically appearing two to six weeks after SARS-CoV-2 infection, often in children who had mild or no initial COVID-19 symptoms. Immunocompromised individuals and pregnant people also face heightened risks from both infections, with pregnant women having a slightly increased risk of severe illness if they contract COVID-19.