As the COVID-19 cases continue to rise, comparisons between influenza (flu) and COVID-19 symptoms have been drawn. Flu and COVID-19 share many characteristics, but there are some key differences between the two. With the flu season fast approaching, have you ever wondered the difference between flu and COVID-19 virus?
Similarities between influenza and COVID-19
Influenza (Flu) and COVID-19 are both contagious respiratory illnesses, but they are caused by different viruses.
COVID-19 is caused by infection with a new coronavirus (called SARS-CoV-2) and flu is caused by infection with influenza viruses.
COVID-19 and influenza viruses have a similar disease presentation, that is, they both cause respiratory disease, which presents as a wide range of illnesses from asymptomatic or mild through to severe disease and death.
Secondly, both COVID-19 and flu can spread from person-to-person, between people who are in close contact with one another (within about 6 feet).
Both are spread mainly by droplets made when people with the illness (COVID-19 or flu) cough, sneeze or talk. These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs.
It may be possible that a person can get infected by physical human contact (e.g. shaking hands) or by touching a surface or object that has virus on it and then touching his or her own mouth, nose, or possibly their eyes.
Both the flu virus and virus that causes COVID-19 may be spread to others by people before they begin showing symptoms, with very mild symptoms or who never developed symptoms (asymptomatic).
Common symptoms that COVID-19 and flu share include:
- Fever or feeling feverish/chills
- Shortness of breath or difficulty breathing
- Fatigue (tiredness)
- Sore throat
- Runny or stuffy nose
- Muscle pain or body aches
- Some people may have vomiting and diarrhea, though this is more common in children than adults
Difference between influenza and COVID-19
While COVID-19 and flu viruses are thought to spread in similar ways, COVID-19 is more contagious among certain populations and age groups than flu.
Also, COVID-19 has been observed to have more super spreading events than the flu. This means the virus that causes COVID-19 can quickly and easily spread to a lot of people and result in continuous spreading among people as time progresses.
1. Speed of transmission
The speed of transmission is an important point of difference between the two viruses.
Influenza has a shorter median incubation period (the time from infection to appearance of symptoms) and a shorter serial interval (the time between successive cases) than COVID-19 virus.
The serial interval for COVID-19 virus is estimated to be 5-6 days, while for the influenza virus, the serial interval is 3 days.
This means that influenza can spread faster than COVID-19.
Further, transmission in the first 3-5 days of illness, or potentially pre-symptomatic transmission (transmission of the virus before the appearance of symptoms) is a major driver of transmission for influenza.
In contrast, this does not appear to be a major driver of transmission for COVID-19, at present.
The reproductive number (number of secondary infections generated from one infected individual) is understood to be between 2 and 2.5 for COVID-19 virus, higher than for influenza.
However, estimates for both COVID-19 and influenza viruses are very context and time-specific, making direct comparisons more difficult.
2. Drivers of virus transmission
Children are important drivers of influenza virus transmission in the community. For COVID-19 virus, initial data indicate that children are less affected than adults and that clinical attack rates in the 0-19 age group are low.
Further preliminary data from household transmission studies in China suggest that children are infected from adults, rather than vice versa.
While the range of symptoms for the two viruses is similar, the fraction with severe disease appears to be different.
For COVID-19, data to date suggest that 80% of infections are mild or asymptomatic, 15% are severe infection, requiring oxygen and 5% are critical infections, requiring ventilation.
These fractions of severe and critical infection would be higher than what is observed for influenza infection.
Who is at most risk?
Those most at risk for severe influenza infection are children, pregnant women, the elderly, people with underlying chronic medical conditions, and those who are immunosuppressed.
For COVID-19, current understanding is that older age and underlying conditions increase the risk for severe infection.
Mortality for COVID-19 appears higher than for influenza, especially seasonal influenza.
While the true mortality of COVID-19 will take some time to fully understand, the data so far indicates that the crude mortality ratio (the number of reported deaths divided by the reported cases) is between 3-4%, the infection mortality rate (the number of reported deaths divided by the number of infections) will be lower.
For seasonal influenza, mortality is usually well below 0.1%.
However, mortality is to a large extent determined by access to and quality of health care.