How to tell the difference between COVID-19 and flu

With both flu and COVID-19 posing a risk to our health this winter, it's more important than ever to be able to tell the difference between these viruses. Knowing which illness you're dealing with can help you to protect others and ensure your own recovery.

Influenza (flu) and COVID-19 are contagious respiratory illnesses, each caused by different viruses. COVID-19 is a result of a coronavirus infection and flu a result of infection with influenza viruses. There are other differences between them, such as how they are spread. However, their similarities in symptoms can be confusing, so it's important not to make a judgement on symptoms alone and to seek a professional diagnosis.

Dr Sarah Jarvis explains what to do if you think you have either COVID-19 or flu: "Regardless of which infection is to blame for your symptoms, it's really important to stay home if you think you have either, to avoid infecting others. In addition, it's essential to arrange a PCR COVID-19 test as soon as possible, and to self-isolate until you have the results.

"Even if your test is clear, do stay away from others until your symptoms have settled, to avoid passing on the virus. And do seek medical advice if:

  • You're finding it hard to breathe.
  • You cough up blood or rust-coloured sputum.
  • You have a long-term medical condition (eg, diabetes, heart, lung, liver or nervous system condition).
  • Your immune system is weakened by illness (such as blood cancer or HIV) or treatment such as chemotherapy or medicine to suppress your immune system.
  • You're pregnant.
  • Your symptoms are getting worse rather than better."

Similarities between COVID-19 and flu

Symptoms shared by COVID-19 and flu viruses

Both COVID-19 and flu can be asymptomatic and a person can have either illness without showing any signs. However, they also both share a range of moderate to severe symptoms including:

  • Fever.
  • Having chills.
  • Coughing.
  • Difficulty breathing.
  • Fatigue.
  • Sore throat.
  • Runny or blocked nose.
  • Muscle pain or aching body.
  • Headaches.
  • Vomiting.
  • Diarrhoea.
  • Change in or loss of taste or smell, although this is more frequent with COVID-19.

How do these viruses spread?

Both COVID-19 and flu are spread through particles containing the virus when someone with either illness coughs, sneezes or talks. These particles can travel to the mouths or noses of nearby people, which is especially likely when people are within six feet of each other.

Particles containing the viruses can also spread further in environments with poor ventilation.

Both COVID-19 and flu are mostly spread by inhalation, but infections can also happen through touch - this includes physical contact with an infected person (eg shaking hands) or touching a contaminated surface before touching their own mouth, nose or eyes.

Both viruses can be passed on regardless of whether someone is showing severe symptoms or not. Flu can spread for at least one day before any symptoms begin showing and COVID-19 for at least two days.

Who is at high risk?

Those most at risk of becoming seriously ill or dying as a result of COVID-19 and flu are:

  • Older adults.
  • Pregnant people.
  • Those with underlying health issues.

These groups are more likely to develop complications from the viruses and require hospital care. People in these groups are also more likely to die from a COVID-19 or flu infection.

What complications can develop from having COVID-19 or flu?

Severe forms of COVID-19 and flu can lead to other health complications such as:

  • Pneumonia.
  • Respiratory failure.
  • Fluid in the lungs (acute respiratory distress syndrome).
  • Sepsis.
  • Heart attack and stroke.
  • Organ failure.
  • Worsening of existing chronic medical conditions involving the lungs, heart or nervous system, or diabetes.
  • Inflammation of the heart, brain, or muscle tissues.
  • Secondary bacterial or fungal infections.

Treating COVID-19 or flu

Treatment required for the viruses depends on their severity. Many people with mild to moderate symptoms of COVID-19 or flu are able to relieve their symptoms at home using over-the-counter medications, drinking plenty of fluids and resting. However, those who fall into high-risk categories or have been hospitalised as a result of COVID-19 or flu should receive professional medical care. If you are worried about your health or your condition is worsening, contact your GP or NHS 111. In an emergency, call 999.

Differences between COVID and flu

Differences in symptoms of COVID-19 and flu

COVID-19 and flu have different incubation periods (the number of days it can take between becoming infected and starting to see symptoms). Those infected with COVID-19 can start showing symptoms 2-14 days after becoming infected, whereas flu has an incubation period of 1-4 days.

Though they share many symptoms, some symptoms are more common in one virus than another. For example, body aches, headaches and chills are common flu symptoms that only appear sometimes in someone with COVID-19.

On the other hand, shortness of breath and respiratory issues are common signs of COVID-19, but are less common in someone with flu.

Flu symptoms also appear abruptly, whereas COVID-19 symptoms may develop more gradually.

Is one virus contagious for longer?

COVID-19 and flu viruses spread in very similar ways, but COVID-19 is generally more contagious. COVID-19 can spread more quickly to infect a large number of people.

COVID-19 is also contagious for longer than the flu. It is not yet known exactly how long COVID-19 remains contagious in an infected person but a person with COVID-19 can be capable of passing on the virus for at least 10 days after symptoms appear. Even if someone is asymptomatic or has recovered, it can still be passed on.

Most people with flu are contagious for one day before they show symptoms, but many remain contagious for about one week. Those with weakened immune systems can be contagious for longer.

People who are hospitalised with COVID-19 or flu can be contagious for 20 days or more.

Which virus is more dangerous?

Older people, those with underlying health conditions and pregnant people are at a higher risk of becoming seriously ill or dying from COVID-19 or flu.

As with flu, complications following COVID-19 may lead to hospitalisation or death, even in healthy people with no pre-existing health conditions. However, the pandemic has seen this happening at a higher rate among people who contract COVID-19 than those who contract flu.

Some people who have had COVID-19 can go on to develop ongoing conditions which can last for weeks or even months. Common smptoms include profound tiredness, difficulty breathing, heart palpitations, coughing, sleep problems and changes in smell and taste. This has been called long Covid.

Multisystem inflammatory syndrome (MIS) is also a possibility. This causes different body organs to become inflamed, including the heart, lungs, kidneys, brain, skin, eyes, or gastrointestinal organs. This very rare complication largely affects young children.

What about complications?

Most people recover from flu with a few days or a couple of weeks. Most people can manage their symptoms at home without hospital treatment. However, some complications may require hospitalisation if someone has become seriously unwell.

Pneumonia is a serious complication which can result from either flu infection alone or from co-infection of flu virus and bacteria. Other possible serious complications following the flu can include inflammation of the heart, brain, or muscle tissues, and multi-organ failure.

Flu also can worsen existing medical problems. For example, people with asthma may have more asthma attacks while suffering from the flu.

Secondary bacterial or fungal infections are more common with flu than with COVID-19.

With COVID-19, further health complications could include blood clots, and some may suffer from long COVID, in which symptoms last for many weeks or months following infection. Long COVID can affect your mental as well as your physical health. Anyone can be affected by long COVID, even someone who didn't have any symptoms when they had COVID-19.

How are flu and COVID-19 treated?

Most people can manage flu or COVID-19 at home. Sometimes GPs will prescribe oral antiviral medications to shorten the duration of flu and reduce the severity of symptoms. However, these are not recommended on the NHS unless the number of circulating cases is high. Neither flu nor COVID-19 is treatable with antibiotics as they are caused by viruses.

Medications and other treatment options are still being explored for COVID-19 to see if they can effectively address its symptoms. Medication is not currently available to those with COVID-19 outside of hospitals, though many people find that painkillers and over-the-counter remedies commonly used for coughs, colds and sore throats can offer relief from symptoms.

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