COVID-19 is a novel (new) coronavirus, not previously identified in humans, which is responsible for an outbreak of respiratory illness that became a global pandemic in 2020. COVID-19 is different from other coronaviruses that cause mild illness, such as the common cold.
Both COVID-19 and the flu are contagious and are caused by viruses transmitted from person to person through respiratory droplets propelled into the air when an infected person sneezes or coughs. Less commonly, people may catch COVID-19 or the flu by touching a surface or object that has the virus on it, and then touching their mouth, nose, or eyes.
COVID-19 is more contagious and spreads more easily than the flu, and COVID-19 can cause more serious illnesses in some people than the flu. COVID-19 can also take longer before symptoms start, and people can remain contagious with the virus that causes COVID-19 for longer periods of time than with influenza viruses.
The basic reproduction number (R0 /R naught) is a value that indicates how infectious a disease is. The higher the number, the more contagious a virus is.
- The R0 of COVID-19 is 1.4 to 3.9
- The R0 of seasonal influenza is around 0.9 to 2.1
What Are Symptoms of COVID-19 and the Flu?
Symptoms of the influenza (the flu) and COVID-19 that are similar include:
- Muscle or body aches
- Sore throat
- Runny nose
- Stuffy nose
Additional symptoms of COVID-19 may also include:
- Altered sense of taste and/or smell
- Difficulty breathing
- Shortness of breath
- Bluish lips or face
- Persistent pain or pressure in the chest
- New confusion or inability to arouse
- Loss of appetite
- Feeling unwell (malaise)
How Are COVID-19 and the Flu Diagnosed?
All variants of COVID-19 are diagnosed with a medical history which includes any recent known exposure to COVID-19, and a physical exam to check for symptoms of COVID-19.
If COVID-19 is suspected tests used to diagnose the virus include:
- PCR tests (genetic or molecular test)
- Results can take hours to up to one week
- More accurate than an antigen test
- Antigen test
- Results are available in less than one hour
- Less accurate than a PCR test
An antibody test may be used to determine if a person had a past COVID-19 infection, but it is not used to diagnose current infections because it takes up to 3 weeks following infection for the body to produce antibodies to the virus.
The flu (influenza) is diagnosed based upon the patient’s reported symptoms and a physical examination, along with tests including:
- Rapid influenza diagnostic tests (RIDTs)
- Rapid molecular assays
- Other specialized flu tests may be used in hospitals or state public health laboratories
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