COVID-19 and the flu are both respiratory illnesses affecting millions globally, with symptoms ranging from mild to severe. Caused by different viruses—SARS-CoV-2 for COVID-19 and influenza viruses for the flu—they share many symptomatic similarities. Understanding their distinctions is important for individual and public health, as many question which illness poses a greater threat.
Similarities and Differences in Symptoms
Both COVID-19 and the flu share symptoms like fever, cough, fatigue, sore throat, runny or stuffy nose, muscle aches, and headaches. Vomiting and diarrhea can also occur, though more often in children with the flu and in any age group with COVID-19. Due to these overlaps, distinguishing between the illnesses based on symptoms alone is challenging, often requiring testing for diagnosis.
Differences include symptom onset and unique manifestations. Flu symptoms often appear suddenly, typically one to four days after infection. COVID-19 symptoms usually have a more gradual onset, appearing around five days after infection, but can range from two to 14 days. A specific symptom more commonly associated with COVID-19 is the loss of taste or smell, which is generally not seen with the flu.
Variations in Disease Severity and Complications
Illness severity varies for both COVID-19 and the flu, from asymptomatic cases to severe outcomes requiring hospitalization. However, severe illness, including hospital stays and death, has been more frequent with COVID-19 than with the flu, especially for individuals aged 18 and older. Hospitalization rates for COVID-19 have been significantly higher across all age groups, with one study showing a 5.6 times higher rate for those 18-49 years old.
Both illnesses can lead to serious complications like pneumonia, respiratory failure, acute respiratory distress syndrome, and sepsis. However, COVID-19 can cause unique complications, including blood clots, post-COVID conditions, and multisystem inflammatory syndrome in children (MIS-C). While secondary bacterial infections are a known flu complication, COVID-19 has also been associated with cardiac injury, multiple-organ failure, and worsening of chronic lung, heart, or nervous system conditions. Even during the Omicron variant period, individuals aged 18 to 49 continued to experience higher in-hospital deaths from COVID-19 compared to the flu.
Contrasting Transmission and Public Health Impact
Both SARS-CoV-2 and influenza viruses primarily spread through respiratory particles in close contact. Transmission is facilitated in poorly ventilated or crowded indoor settings. However, COVID-19 appears contagious for a longer duration and spreads more rapidly than the flu.
The transmissibility of a virus is described by its basic reproduction number (R0), representing the average number of people an infected person will transmit the virus to. While the R0 for influenza typically ranges from 0.9 to 2.1, estimates for SARS-CoV-2 are between 1.5 and 3.5. Asymptomatic transmission and super-spreading events have complicated COVID-19 containment, making its public health impact more extensive than typical seasonal flu epidemics. The combination of a high R0, super-spreading events, and a globally immunologically naive population led to COVID-19 causing the highest number of global deaths in the past two decades.
Long-Term Health Considerations
Both COVID-19 and influenza can lead to long-term health issues or post-viral syndromes, with lingering effects persisting after initial infection. These conditions can manifest with symptoms like persistent fatigue, brain fog, and post-exertional malaise, where symptoms worsen after mental or physical activity. While “long flu” has been recognized historically, “Long COVID” symptoms are more widely documented.
Long COVID, also known as post-COVID-19 syndrome or post-acute sequelae of SARS-CoV-2 (PASC), involves symptoms that can last for months or even years after initial infection. Over 200 symptoms have been linked to long COVID, including extreme tiredness, memory problems, dizziness, and issues with taste or smell. Research suggests that individuals hospitalized with COVID-19 face a higher risk of experiencing post-acute sequelae, such as abnormal breathing, abdominal symptoms, fatigue, and cognitive impairment, compared to those hospitalized with influenza.