Questions and answers on seasonal influenza

What is influenza?

Influenza is an infectious respiratory illness caused by infection with an influenza virus. Not everyone who becomes infected gets sick but for those that do, common symptoms include headache, fever, cough, sore throat, aching muscles and joints and generally feeling of malaise. Influenza is usually more severe than a simple cold although the illness can also be mild. The severity of the illness ranges from minor symptoms to severe pneumonia, encephalitis and a general whole-body infection and any of the latter three conditions can be life-threatening. These severe illnesses may be due to the influenza virus itself or other bacterial/viral infections that occur after flu has weakened the body’s defences. More severe illness is more common in the elderly, the very young and those with other chronic medical conditions, such as diabetes or heart disease. However, severe infections can also occur in people of any age group who are fit and well. The best way to prevent influenza is by getting an influenza vaccination each year.

What are the symptoms of influenza?

The most common symptoms of influenza are an abrupt onset of fever, shivering, headache, muscle or body ache, runny nose, fatigue (tiredness) and dry cough. Some people may experience vomiting and diarrhoea, although this is more common in children than adults. People can confuse influenza with a heavy cold. However, influenza is usually a more severe illness than the common cold.

For most people influenza infection is just an unpleasant illness, but for some it can become severe. The most common complications of influenza are bronchitis and pneumonia due to bacterial infections following an infection with the influenza virus.

Rarer but more severe complications are encephalitis (inflammation of the brain) and generalised infections. These complications often require treatment in hospital and can be life-threatening, especially for the very young, the elderly, and those in poor health.

What are the symptoms of the common cold and how do they differ from simple (uncomplicated) Influenza?

Cold symptoms are limited to the nose and throat (e.g. runny nose, sneezing, watery eyes, throat irritation and headache). The symptoms usually occur gradually and only rarely cause a high fever or shivers/aches in the body. For those with chronic respiratory conditions (for example, people with asthma) the illness may be worse for a few days.


Common cold

Influenza (with symptoms)


Uncommon and then low (under 38°)

Common and often high fever (over 38°)

Aching muscles – body



General malaise and lack of energy






Runny nose

Almost always

Common, but a minor feature


Almost always

Common, but a minor feature

Watery eyes


A minor feature

Throat irritation

Almost always

A minor feature




What type of virus cause influenza?

Influenza viruses are RNA viruses from the Orthomyxoviridae family. There are four main types of influenza virus known as A, B, C and D. Only influenza types A and B are important in human disease and these cause seasonal epidemics of disease almost every winter in the northern hemisphere. Influenza A viruses are the only influenza viruses known to cause influenza pandemics. A pandemic can occur if an antigenically new and different influenza A virus emerges that both infects people and has the ability to spread efficiently. Influenza B usually produces less severe illness than influenza A.

Two proteins on the surface of the influenza virus are particularly important, haemagglutinin (H) and neuraminidase (N). The main classification of the sub-types of influenza virus is based on these two glycoproteins. Haemagglutinin is the main antigen associated with immunity (the human body’s protective reaction). Neuraminidase plays a minor role in immunity.

During some influenza seasons, different influenza A or B sub-types can co-circulate at the same time in the population.

How do human influenza viruses change?

A feature of influenza viruses is that they are unstable in their genetic make-up and so are prone to change. The changes mainly take place through antigenic drift or antigenic shift.

Antigenic drifts occur as a result of minor changes (mutations) to the surface proteins of the virus, haemagglutinin and neuraminidase. These ‘drifted strains’ of influenza may not be recognised by the body’s immune system, making people susceptible to influenza, even though they have been exposed or immunised in previous years. Antigenic drift is the main reason for reviewing and updating the composition of flu vaccines for use in the northern and southern hemispheres annually, as necessary, to keep up with evolving flu viruses.

Antigenic shifts occur as a result of major changes in genetics, causing a virus to emerge which contains a haemagglutinin considerably different to those of previously circulating viruses. Major epidemics or pandemics can occur when this happens, as humans may have little or no immunity to the new strains.

Occasionally antigenic shifts occur through an animal virus changing and adapting spontaneously to infect humans or through genetic exchange between animal or human viruses. This is what is thought to have happened in the 1918–19 ‘Spanish Flu’ pandemic.

How infectious is influenza and how is it spread?

Influenza is a moderately infectious virus and, on average an infectious person will infect another one or two people who are not immune. The virus can be transmitted in one of two ways. Firstly, by breathing in larger droplets from infectious people who cough or sneeze without covering their mouth and nose people (causing those within a range of one metre to be infected). This is why it is so important that a person covers their mouth and nose (preferably with a disposable tissue) when they cough or sneeze. Smaller droplets and ‘aerosols’ seem to be less liable to carry the virus and therefore, since only the larger droplets are infectious, it is generally only people close to an infectious person who are at risk.

Influenza infection can also be spread by direct contact with mucous membranes from the nose, mouth and throat - for example on the hands of infectious people who have rubbed their noses. That is why hand hygiene and proper use of disposable handkerchiefs (respiratory hygiene) are so important.

In hospital some medically invasive procedures such as intubation (inserting a tube down the throat of an unconscious patient to help them breathe) can produce infectious aerosols and fine droplets which may carry the virus. This is one reason why doctors and nurses take more protective precautions and wear special masks in hospitals.

If a person gets infected how quickly does it show?

The incubation period– the time between infection and the appearance of symptoms – is usually about two to three days. However, longer or shorter intervals are also sometimes seen and the range can from one to seven days.

When is a person with influenza infectious to others?

The infectious period varies from person to person but an adult with flu is able to pass on the infection from the day before their symptoms appear and will remain potentially infectious for approximately five to seven days after onset of illness. People with influenza are most contagious in the first three or four days after their illness begins, however young children, immunocompromised persons of any age, and those who are critically ill with influenza can shed the virus in the respiratory tract for prolonged periods.

The risk of infecting others is not constant throughout the illness. Infectiousness increases considerably when a person starts feeling unwell and that is when they are most infectious to others. The risk of catching influenza from someone just before they become ill or after the first three days of illness is quite low. This is why it is very important that, as soon as a person starts feeling ill they stay home for a few days, especially during the influenza season.

If someone thinks they have influenza, what should they do? 

There are two principles: 1. Look after yourself and 2. Do not infect others.

In some countries, people expect to see a doctor when they start having symptoms, while in other countries it is not always believed to be necessary to see a doctor unless the person feels very ill or is in a ‘risk group’. However, people with chronic or long-term illnesses may need to seek medical attention earlier and more quickly.

The best advice is for the person to stay at home, use handkerchiefs properly (preferably disposable ones – and dispose of them safely), wash their hands frequently, not to share towels, rest, drink plenty of fluids and take medicines that will reduce the temperature and alleviate the aching (paracetamol for all ages, aspirin may be taken by adults, but not children). If it is necessary for people to be around the patient at home, he/she might consider wearing a simple disposable mask.

Most influenza illnesses are self-limiting and may be caused either by influenza or other viruses/pathogens. It is best to treat the infection at home until the person is well enough to return to normal activities. However, advice from healthcare professionals must be sought quickly if symptoms become more severe or last more than about a week.

Are there carriers of influenza?

No, not in the sense of people going around for days unwittingly spreading influenza. Some people may become infected with an influenza virus without experiencing any symptoms or only having mild symptoms for a short period. This may be how some people catch influenza without recalling having been in contact with anyone with the disease. However, it is thought that those without symptoms, or with minor symptoms, are generally less infectious than those with stronger symptoms.

Who is at greatest risk from influenza? Which risk groups need vaccination?

Children under 18 years are more than twice as likely to become infected with influenza because they have not developed immunity to the virus. For this reason, some countries recommend routine immunisation of young children. However, with the exception of the very young, healthy children generally do not become severely ill with influenza.

The immune response in the elderly is less effective than in young people and therefore if infected, they have on average a greater risk of developing severe complications from influenza (e.g. pneumonia). They also more often have underlying diseases than younger adults, which increases their risk of becoming severely ill.

Other high-risk groups are people of any age with chronic medical conditions (such as asthma, diabetes, or heart disease), pregnant women, and children under five years. These people are more likely to suffer from severe illness than those who are otherwise healthy.

As a result, most countries recommend annual influenza vaccination of the elderly and of those with chronic illnesses. The specific policies vary from country to country. Some countries offer vaccination against influenza to everyone over  the age of 65 years, while other countries offer to everyone aged over 60, or even 55 years.

How do I protect myself and my family against influenza?

The risk of contracting influenza or transmitting it to others can be reduced by taking the measures set out below.

  • Frequent hand-washing which reduces the risk of contracting influenza during the influenza season. It also protects against a number of other infections.
  • Proper respiratory hygiene - covering the mouth and nose and using a disposable handkerchief when coughing or sneezing.
  • Staying at home if unwell with a fever. People who develop a fever and respiratory infection should be advised to stay at home. Those infected with influenza are most infectious to others during the early stages of the illness.
  • Routine annual vaccination where recommended. The influenza vaccine offers good protection and people who are at high risk of infection should be vaccinated. Specific vaccination policies for influenza may vary from country to country. In some countries the elderly and others in risk groups are also offered vaccination against bacterial pneumonia (pneumococcal pneumonia).

How does hand-washing prevent influenza? Can you use alcohol gels for hand-washing?

ECDC recommends washing hands to reduce the risk of influenza transmission and alcohol gels also work against influenza. Hand hygiene works by interrupting certain types of transmission – for example, a person has influenza, they wipe their nose or mouth, they forget to wash their hands, they shake hands with someone who then touches his/her nose or mouth and contracts influenza.

There is positive evidence that hand-washing helps against acute respiratory viral infections.

Does the seasonal flu vaccine contain the H1N1 component?

Yes, along with components against the A(H3N2) and B strains.

Should people wear masks to protect against influenza?

The recommendations for mask wearing have changed substantially during the COVID19 pandemic, as this has been one of the most important measures for containing and reducing ongoing community transmission. Considerable evidence has emerged since the beginning of the pandemic regarding mask effectiveness in reducing the spread of SARS-CoV-2. Since influenza follows the same transmission route as SARS-CoV-2, ECDC recommends wearing a mask in confined public spaces, such as shops, supermarkets, transportation hubs and when using public transport. Wearing a face mask should be considered in crowded outdoor settings where physical distancing is not possible. Moreover, face masks should be considered during the period when influenza and SARS-CoV-2 are co-circulating, especially for vulnerable people such as the elderly or those with underlying medical conditions.

Mask use is best undertaken as part of a package of personal protective measures, including hand hygiene and avoiding close contact. Correct and consistent wearing of masks can help prevent exposure and reduce the risk of infection. Wearing a mask can also raise awareness of the infection risk and the importance of additional preventive behaviour, such as more frequent handwashing and avoiding physical contact or crowded public places. A face mask may also reduce transmission through contact by preventing people from touching their mouth or nose with their hands or other objects potentially contaminated with the virus. However, it is important to point out that wearing a mask for too long may make it wet and soggy and this is more dangerous than using disposable handkerchiefs. Therefore, masks should be changed regularly and at least twice a day.

If I would like to wear a mask, when should I do it and which type of mask should I wear?

There are certain times when it makes more sense to wear a mask. If there is no influenza circulating, there is little value in wearing a mask. A mask can be worn in confined public spaces, such as shops, supermarkets, transportation hubs and when using public transport, as well as in crowded outdoor settings where physical distancing is not possible. For most people the risk of catching influenza is low unless someone close to them becomes ill. The following considerations apply for different face masks:

  • regular face masks may be labelled as surgical, dental or medical procedure masks. If worn properly, a face mask is meant to help block large-particle droplets, splashes, spray or splatter that may contain germs (viruses and bacteria) from reaching your mouth and nose. While a face mask may be effective in blocking splashes and large-particle droplets, it does not filter or block very small particles in the air (aerosols) that may be transmitted by coughing, sneezing or certain medical procedures. Face masks are cleared for use in the EU as medical devices. They should be used once and then disposed of properly in a bin.
  • A respirator is a personal protective device that is worn on the face, covers the nose and mouth, and is used to reduce the wearer’s risk of inhaling hazardous airborne particles including infectious agents, gases or vapours. One of the most commonly used respirators is a filtering facepiece respirator (often referred to as an FFP-2/3). Studies have shown that respirators are more effective than medical face masks, both in limiting the release of infectious respiratory droplets when worn by an infected person and in limiting exposure when worn by the exposed person.

Are there any rules about wearing masks?

Yes – if you chose to wear a mask then you need to change it around twice a day and more often if it gets soggy. Additionally, it is very important to wash your hands with soap and water or hand sanitizer before and after touching the face mask. Single-use masks should not be re-used.

Should masks be worn in the home?

There is not generally much need to wear masks at home, especially if there is no influenza virus in circulation. An exception would be if there is someone in the house with suspected flu needing close care. To prevent transmission from an individual with influenza symptoms to other family members, face masks should be used by all family members as soon as possible (within 24 hours) after the onset of the symptoms.

Page last updated 12 Apr 2022