Flu, a cold or COVID-19: How to tell the difference

Common symptoms of flu, the common cold and COVID-19

All three conditions are respiratory infections. As a result, they are often confused with one another. Typical overlapping symptoms include:

• Cough (usually dry with COVID-19 and flu, more productive with a common cold)
• Fatigue and general exhaustion
• Sore throat and a runny nose
• Headache

What are the key differences?

Although they are similar, there are certain characteristics specific to each condition:

• Common cold: A cold develops gradually over several days. You feel unwell, but are rarely completely exhausted. Fever is quite rare in adults with a cold.
• Flu: Influenza usually begins suddenly. Within a few hours, a high fever, severe aching limbs and a dry cough set in. This is accompanied by pronounced exhaustion and a strong sense of being unwell.
• COVID-19: Symptoms vary significantly depending on the virus variant. Previously, loss of the sense of smell and taste was considered a typical sign; today, COVID-19 often resembles a severe cold or flu. Shortness of breath can be an important warning sign in severe cases.

A comparison of symptoms

Please note: As COVID-19 variants are constantly evolving, it is now almost impossible to distinguish between them based on symptoms alone. If in doubt, only a rapid test or a PCR test can provide certainty.

How can you tell if you have a cold? Symptoms and course

Symptoms and progression of a cold:

• Gradual onset: The first sign is often a dry sensation or a slight tickle in the throat.
• Initial phase (days 1–3): The sore throat gets worse. This is usually accompanied by a general feeling of chilliness, mild fatigue and a runny nose with clear, watery discharge.
• Acute phase (days 4–6): The nose becomes increasingly blocked as the mucous membranes swell. A dry, dry cough sets in, often accompanied by a dull headache and aching limbs.
• Recovery phase (days 7–10): The symptoms subside noticeably. The cough becomes looser and productive (with mucus production), and general energy levels rise again.

It is also important to note that a classic cold usually does not involve a high fever. Often, there is only a slightly elevated temperature whilst the body fights off the viruses.

Unlike the flu, which begins abruptly with a high fever and is accompanied by severe exhaustion, with a cold you can often still ‘drag yourself through the day’. The body usually needs around seven to ten days to recover. Rest, plenty of fluids and keeping the mucous membranes moist are the most effective ways to support this process.

If symptoms do not improve after a week, if the fever rises significantly, or if you experience severe earache or shortness of breath, you should seek medical advice.

How long are you contagious with a cold?

The viruses spread mainly via droplet transmission (coughing, sneezing, speaking) or via contact transmission, for example through shaking hands or touching door handles and light switches. With a cold, you are usually contagious for longer than you feel seriously ill. The risk of infection often begins as early as one day before the first symptoms appear.

Overall, you usually remain infectious for about a week. The risk to others is highest during the first two to three days of illness. During this time, the viral load in the body is at its highest, and a particularly large number of pathogens are spread when you sneeze or cough. As you recover, the viral load decreases steadily. Once the symptoms have almost subsided (usually after seven to ten days), you are generally no longer infectious, even if you still have a slight cough from time to time.

Please note: Children can remain infectious for significantly longer than adults, as their immune systems take longer to fight off the virus. In some cases, the risk of infection can persist for up to two weeks.

How can you tell if you have the flu (influenza)?

Genuine flu (influenza) is often mistaken for a severe cold, but it is a much more serious illness. The most striking feature is the speed at which it sets in.

Typical signs of influenza are:

• The flu does not start gradually with a slight cold, but sets in suddenly. Within a few hours, you go from feeling perfectly healthy to feeling very ill.
• The fever usually rises rapidly to over 39 °C to 40 °C and is often accompanied by severe chills.
• Typical symptoms include severe aches in the limbs, muscles and a headache; often, even the slightest movement is painful.
• Other characteristic symptoms include severe exhaustion and pronounced fatigue. Those affected are often so weakened that they can barely get out of bed.
• Unlike a common cold, the flu is often characterised by a nagging, dry, irritating cough. A runny nose is less common or only develops later.
• Whilst you are usually back on your feet after a week with a common cold, after the flu you often still feel distinctly run down and listless for another two to three weeks.

How long are you contagious?

With genuine flu, you are usually contagious for about a week. As with a common cold, the risk of transmission begins as early as one day before the first symptoms appear. This means you can already spread the virus whilst you still feel healthy.

The risk of transmission is highest in the first three to five days after the onset of the illness. As the viral load is high with influenza, even a brief conversation or a sneeze can lead to transmission. In children or people with a weakened immune system, the period of contagiousness may last longer – in some cases up to two weeks or even longer.

Flu vaccination for prevention

This season, trivalent vaccines are being used again for the first time (they target three virus strains). In previous years, quadrivalent vaccines (targeting four strains) were predominantly used. This change follows a recommendation from the World Health Organisation (WHO), as one of the B virus strains is now virtually non-existent worldwide.

Who is the flu vaccination recommended for?

The National Immunisation Committee (NIG) generally recommends the flu vaccination for everyone, but particularly for the following risk groups:

• People aged 60 and over (preferably with a high-dose vaccine)
• People with chronic underlying conditions (e.g. heart, lung or metabolic diseases, as well as immunodeficiency)
• Pregnant women (to protect both mother and child; ‘nest protection’ for the newborn)
• Children from the age of 6 months
• Medical and nursing staff
• People in close contact with infants or high-risk patients

The ideal time for vaccination is in autumn (October to mid-December). It takes around ten to 14 days for full protection to develop.

The current vaccine offers good protection. Even if a person does fall ill despite being vaccinated, the illness is usually much milder, and the risk of hospitalisation is significantly reduced.

How can you recognise COVID-19?

An overview of the main signs of COVID-19

• The most common symptoms of COVID-19 are a severe cold, a sore throat, a headache and general fatigue.
• A cough does not always occur, but when it does, it is usually a dry, persistent, irritating cough.
• Fever is common, but can range from a slightly elevated temperature to a high fever.
• Loss of smell and taste are significantly less common with the current variants than they were at the start of the pandemic.
• In some cases, nausea, vomiting or diarrhoea may also occur.
• In severe cases, shortness of breath may occur – this is always a warning sign and should be assessed by a doctor.

Please note: As the symptoms largely overlap with those of a flu-like infection, only a test (rapid antigen test or PCR test) can provide certainty.

How long are you contagious?

Contagiousness usually begins one to two days before the first symptoms appear. During this early phase and throughout the first three days of illness, the viral load is at its highest, which is why you are particularly contagious during this period.
As a rule, the risk of transmission decreases significantly after around five to seven days.

In mild cases, you can usually assume that you are no longer infectious after around ten days, provided you feel well again. The most important guideline for returning to everyday life is that you should have been fever-free and without severe symptoms for at least 48 hours, in order to keep the risk to others as low as possible.

COVID-19 vaccination

The National Immunisation Committee (NIG) in Austria generally recommends the COVID-19 vaccination for the 2025/2026 season to everyone aged five and over who wishes to protect themselves. It is particularly recommended for certain groups:

• Anyone aged 60 or over
• People with chronic conditions, particularly of the cardiovascular system, the lungs (e.g. asthma, COPD), the kidneys, or metabolic conditions such as diabetes
• People with congenital or acquired immunodeficiency, as well as those undergoing immunosuppressive therapy
• Pregnant women
• Healthcare and care staff
• Residents of care homes

Ideally, the free vaccination should be taken in the autumn (from mid-October) to provide protection against the winter wave of infections. According to the NIG, the COVID-19 vaccine can be administered at the same time as the flu vaccine (in different arms).

How contagious are these illnesses – and what does that mean in everyday life?

A common cold is moderately contagious, whilst flu and, in particular, COVID-19 are considered highly contagious. Due to the higher viral load, it is advisable to consistently withdraw from social life when you have flu or COVID-19 to prevent further infections.

When is a test advisable, and what types of tests are available?

A test is advisable if clarity is needed for one’s personal circumstances or if targeted treatment (e.g. for influenza) is being considered.

The main tests are:

• COVID rapid test: For use at home; provides a result in around 15 minutes, but may be less reliable if the viral load is low.
• PCR test: The most reliable test (gold standard). It is usually carried out by a doctor or in a pharmacy when a particularly reliable or official result is required.
• Combination tests (3-in-1): Test for SARS-CoV-2, influenza and RSV simultaneously using a single swab.
• Rapid influenza test at the GP’s surgery: Helps the doctor decide on possible antiviral medication.
• Rapid CRP test (blood): Provides an indication as to whether the cause is more likely to be bacterial or viral.

When should you seek medical advice?

A medical assessment is necessary if the following warning signs are present:

• Shortness of breath, chest pain or severe drowsiness
• A fever above 39 °C that lasts for more than three days or rises again after a brief improvement
• Severe earache, purulent sputum or significant difficulty swallowing

Please note: Older people, pregnant women and those with chronic underlying conditions should seek medical advice at the earliest opportunity if they experience any symptoms.

COVID-19, the common cold or flu in children: what’s different?

In children, infections often progress more rapidly and present with different symptoms than in adults:

• Children often develop a fever more quickly and reach higher temperatures. Infections (particularly with flu and COVID-19) frequently manifest as stomach ache, nausea or vomiting, rather than just a cough.
• Children may remain infectious for longer (up to two weeks) and shed viruses over a longer period.
• A visit to the doctor is necessary if the child appears listless, refuses to drink fluids or shows signs of breathing difficulties (e.g. rapid breathing or retractions of the chest).
• The RSV virus can be more dangerous for young children than a COVID-19 infection, as it can severely narrow the small airways (bronchi).

Please note: In the case of infants under three months of age, a doctor should always be consulted immediately if they have a fever.

Flu or COVID-19: Which is more dangerous for children?

For most healthy children, COVID-19 is usually mild, often even without pronounced symptoms. The real flu (influenza), on the other hand, more frequently leads to a high fever and can cause complications such as pneumonia or middle ear infections.

Please note: For infants and young children, the respiratory syncytial virus (RSV) may currently pose a greater risk than influenza or COVID-19, as it can severely affect the lower respiratory tract.

Which home remedies help with flu, the common cold and COVID-19?

The most effective supportive home remedies for all three infections are:

• Drink plenty of fluids: Water, herbal teas (e.g. lime blossom or elderflower) or diluted fruit juices help to thin mucus and compensate for fluid loss caused by a fever.
• Rest and sleep: The most important remedy. The body needs sufficient energy to support the immune system.
• Inhalation: Steam with a saline solution (or thyme) moisturises the mucous membranes and can relieve a cough and a runny nose.
• Calf compresses: In cases of high fever (only if your feet are warm), these can help to gently lower your temperature.
• Gargling: Salt water or sage tea soothe a sore throat and can calm the mucous membranes.
• Chicken soup: Has a mild anti-inflammatory effect and provides fluids and electrolytes.
• Onion juice: A tried-and-tested home remedy for a cough (chop the onion finely, leave to infuse with honey or sugar, then take the resulting juice by the spoonful).

How can you protect yourself?

Here’s how to effectively reduce the risk of all three infectious diseases:

• Vaccination: The most important protection against severe illness, particularly with flu and COVID-19.
• Hand hygiene: Wash your hands thoroughly with soap on a regular basis (especially for common cold viruses).
• Social distancing and face coverings: In crowded places or on public transport, wearing an FFP2 face covering can significantly reduce the inhalation of virus-containing aerosols.
• Ventilation: Regularly airing out indoor spaces reduces the concentration of viruses in the air.
• Coughing and sneezing properly: Cough or sneeze into the crook of your elbow, not into your hand.
• Support your immune system: A balanced diet, sufficient sleep and regular exercise in the fresh air help to boost your overall immunity.
• Reduce contact: If you have symptoms, stay at home where possible to avoid infecting others.