Covid-19, colds and flu: discover the differences

Covid-19, colds and flu: discover the differences

With the start of the influenza season, we are now used to fighting and living with influenza symptoms. But in the last two years the challenge is much greater.

Influenza and COVID-19 are both contagious respiratory diseases, but they are caused by different viruses. While we learn more and more every day about COVID-19 and the virus that causes it, there are still aspects, such as post-COVID conditions, that are still being studied. Colds have differences in symptoms and general characteristics that are more pronounced than influenza and COVID-19, so here we are focusing on the differences and similarities of the latter two.

Since some of the symptoms of influenza, COVID-19 and other respiratory diseases are similar, the difference between them cannot be established based on symptoms alone. A specific test is indeed necessary to confirm a diagnosis. People can, however, be infected with both influenza and SARS-CoV-2 at the same time and present symptoms of both. A CDC review explains that although influenza and COVID-19 share many characteristics, there are some key differences between the two.

Covid-19 and Influenza: similarities and differences

What is the cause?

COVID-19: caused by the 2019 coronavirus, known as SARS-CoV-2.

Influenza: caused by the influenza virus. There are two main types of influenza viruses called influenza A and influenza B. Different strains of influenza A and influenza B emerge and circulate every year.

How long until symptoms appear?


For both COVID-19 and influenza, one or more days may elapse between when a person is infected and when he or she starts showing symptoms of the disease.


A person infected with SARS-CoV-2 may take longer to develop symptoms than a person with influenza.

In influenza, a person generally develops symptoms 1 to 4 days after infection.

In COVID-19, a person typically develops symptoms 5 days after infection, although evidence has shown that they may appear as early as 2 days or as late as 14 days after infection, and the time interval may vary.

What are the symptoms?


Both COVID-19 and influenza can present various levels of signs and symptoms, ranging from no symptoms (asymptomatic), paucisymptomatic to severe symptoms. Common symptoms that COVID-19 and influenza share include:

  • Fever or sensation of fever/chills
  • Cough
  • Shortness of breath or difficulty in breathing
  • Fatigue (tiredness)
  • Inflamed throat
  • Runny or stuffed nose
  • Muscle aches or pains
  • Headache
  • Vomiting and diarrhoea, especially in children
  • Change or loss of taste or sense of smell, although this is more common with COVID-19.


The Guardian in its article, “Coronavirus symptoms: how to tell if you have a common cold, influenza or Covid”, elaborates on the topic by specifying that the most common symptoms of COVID-19 are a fever - usually with a temperature above 37.8 °C - a persistent cough (usually dry) and a loss of the sense of taste and/or smell. Patients may also sometimes suffer from fatigue, pain, sore throat, headaches and shortness of breath. Diarrhoea and nasal congestion are rare.

In the common cold, the most common symptoms are sneezing, pain, nasal congestion and sore throat. A slight cough and tiredness may be experienced, but fever and headaches are rare. Colds do not cause diarrhoea.

Finally, influenza is most commonly manifested by fever, fatigue, dry cough, aches and headaches. Patients will sometimes experience nasal congestion or a sore throat. Diarrhoea may sometimes occur in children. Sneezing or shortness of breath do not generally occur.

How long can someone spread the virus?


For both COVID-19 and influenza, it is possible to spread the virus for at least 1 day before manifesting any symptoms.


A person with COVID-19 may be contagious for a longer period of time than someone with influenza.

How long are you contagious with influenza?: Most people with flu are contagious for about 1 day before they show symptoms. Adults with influenza seem to be most contagious during the first 3-4 days of their illness, but many people remain contagious for up to 7 days. Babies and people with weakened immune systems can be infectious for even longer.

How long are people infectious with SARS-CoV-2?: How long someone can spread SARS-CoV-2 is still under investigation. It is possible for people to spread the virus for about 2 days before showing signs or symptoms (or possibly sooner) and to remain infectious for at least 10 days after emergence. If someone is asymptomatic or their symptoms disappear, it is possible to remain infectious for up to 10 days after testing positive for COVID-19. People hospitalised with serious illnesses and people with weakened immune systems can be infectious for 20 days or more.

How do they spread it?


Both COVID-19 and influenza can spread between people who are in close contact with each other. Both are spread mainly by viral particles that are expelled when people with the disease (COVID-19 or influenza) cough, sneeze or talk. These particles can be inhaled by people in the vicinity.

Although most spread is by inhalation, it is possible for a person to become infected by, for example, shaking hands with someone who has contaminated hands, or by touching a contaminated surface or object, and then touching their mouth, nose or eyes.

People infected with the coronavirus or influenza may not realise they are ill for several days and during this time may unknowingly spread the disease to others before they even develop symptoms.


While the virus causing SARS-CoV-2 and influenza viruses are thought to spread in similar ways, the virus causing COVID-19 is generally more contagious than influenza viruses. In addition, it has been observed that SARS-CoV-2 has more super-spreading events than influenza. This means that it can spread quickly and easily to many people and cause a continuous spread among people over time.

Risk of serious illness?


Both COVID-19 and influenza disease can cause serious illness and complications.

Those at highest risk include:

  • The elderly
  • People with comorbidity
  • Pregnant women


Overall, COVID-19 appears to cause more serious illness in some people.

Severe COVID-19 disease leading to hospitalisation and death can also occur in healthy people. Rarer occurrence in cases of influenza.



Both COVID-19 and influenza can cause complications, including:

  • Pneumonia
  • Respiratory failure
  • Acute respiratory distress syndrome (fluid in the lungs)
  • Sepsis (a life-threatening illness caused by the body's extreme response to an infection)
  • Cardiac damage (e.g. heart attacks and strokes)
  • Multi-organ failure (respiratory failure, renal failure, shock)
  • Worsening of chronic medical conditions (involving the lungs, heart or nervous system, or diabetes)
  • Inflammation of the heart, brain or muscle tissue
  • Secondary infections (bacterial or fungal infections that may occur in people who have already been infected with influenza or COVID-19)


Influenza: Most people who contract influenza will recover on their own within a few days or two weeks, but some people will experience serious complications, requiring hospitalisation. Secondary bacterial infections are more common with influenza than with COVID-19.

Diarrhoea is more common in children with influenza.

Additional complications associated with COVID-19 may include:

  • Blood clots
  • Multi-systemic inflammatory syndrome in children (MIS-C) and adults (MIS-A)

Long-COVID is a condition characterised by a range of symptoms that may last for weeks or months after first becoming infected with SARS-CoV-2 or may appear weeks after infection. Long-COVID can occur in anyone who has had COVID-19, even if their illness was mild or they had no symptoms.


In most cases, serious illness and death due to COVID-19 or influenza can be prevented by vaccines. In addition, wearing a mask, washing hands frequently and thoroughly, coughing into the crook of the elbow, staying at home when sick and limiting contact with infected persons are effective safety measures, as is physical distancing that limits the spread of COVID-19 and influenza in communities.


COVID-19: The following vaccines are currently authorised in Italy:

  • Comirnaty by Pfizer-BioNTech - is the first vaccine to be authorised in the European Union: on 21 December 2020 by the European Medicines Agency (EMA) and on 22 December by the Italian Medicines Agency (AIFA).
  • Spikevax vaccine by Moderna - authorised by the EMA on 6 January and on 7 January by AIFA.
  • Vaxzevria by AstraZeneca - authorised by the EMA on 29 January and on 30 January by AIFA.
  • Janssen by Johnson & Johnson - this is the 4th vaccine authorised by the EMA on 11 March and by AIFA on 12 March 2021.

Influenza: a safe and effective vaccine is available to prevent infection and to reduce the severity or duration of influenza. The influenza vaccine is reformulated every year in anticipation of the strains that are expected to circulate. it is therefore very important to receive the influenza vaccine every year.

Global cases

COVID-19: the first cases appeared in China in late 2019. Currently, (as of 30 September 2021) there are 233,136,147 confirmed cases worldwide since the start of the pandemic, of which 4,771,408 have died (Health Emergency Dashboard). In Italy, confirmed cases reached 4,660,314 with 130,697 deaths.

Influenza: the World Health Organisation estimates that the influenza virus affects between 5 - 15% of the adult population (i.e. 350 million to 1 billion people). Estimates warn that death occurs in about 250-500,000 people, especially among fragile population groups such as the elderly and people with chronic diseases. (EpiCentro)

The influenza season in the COVID-19 era: what should I do?

  • Consult your doctor and get vaccinated for COVID-19;
  • Consult your doctor and get your influenza vaccine or contact us to get it at one of our vaccination centres.

Take care of yourself and your family with good nutrition, plenty of rest, proper hydration, regular exercise and stress management. Stay at home if you don't feel well.

Even if you feel tired of following precautions, such as washing your hands often, cleaning and sanitising, wearing a face mask and keeping your physical distance, it is especially important now, especially with the start of the autumn-winter season, to continue to observe all prevention measures and encourage others to do the same.

Where to get tested for Covid-19

If you have symptoms, contact your general practitioner to report the situation and ask what procedure to follow.

If you need to book a swab or a serological test, you can book privately and/or get information about it by going to the following link: BOOKING COVID-19 TESTS.

These services are carried out at Ambimed's partner sample collection points located throughout Italy.

Learn more about influenza in our section ondiseases and vaccinations..

In the table below, we have compared the main symptoms between Covid-19, cold and influenza. Discover the differences but be very careful! The table has an informative value only, not a scientific one. If you have any doubts, it is important to consult your doctor.


CDCJohns Hopkins

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