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 awful. It is usually more severe than a simple cold though the illness can be mild. There is a wide range of severity of illness from minor symptoms through to severe pneumonia, encephalitis and a general whole-body infection any of which can be life threatening. These severe illnesses may be due to the influenza virus or because of other bacterial or viral infections that occur after flu has lowered the body’s defences. The more severe illnesses are more common in the elderly, very young and those with other chronic medical conditions like diabetes or heart disease. However, severe infections can occur in people who are fit and well of any age group.

What are the symptoms of influenza?

The most common symptoms of influenza are an abrupt onset of fever, shivering, headache, muscle ache and dry cough. People can confuse influenza with a heavy cold. Unfortunately some people call even a simple cold ‘a touch of flu’. 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 lead to illnesses that are more serious. More common complications of influenza are bronchitis and pneumonia due to bacterial infections on top of the infection with the influenza virus.

Rarer but more severe complications are encephalitis (brain infections) and generalized infections. These complications often require treatment in hospital and can be life threatening especially in 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 with runny nose, sneezing, watery eyes, throat irritation and headache. The symptoms usually occur gradually and only rarely cause a high fever or body aches. In those with chronic respiratory conditions e.g. people with asthma they can make those conditions worse for a few days.


Common Cold

Influenza with symptoms


Uncommon and then low (under 38°)

Common and often a high fever (over 38°)

Aching muscles – body



General malaise and lack of energy






Running 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 viruses cause influenza?

Influenza viruses are RNA viruses from the Orthomyxoviridae family. There are three main types of Influenza virus known as A, B and C. Only influenza types A and B are important in human disease. Influenza B usually produces less severe illness than influenza A.

The viral envelope which surrounds the RNA core has two glycoproteins, 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.

At any one time there is a mix of influenza viruses circulating among humans.

How do the 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 take place mostly through either antigenic drift and antigenic shift.

Antigenic drifts occur when minor changes occur in the haemagglutinin. These ‘drifted strains’ of influenza can infect people who have been exposed or immunized in previous years and are only partially immune to the drifted virus.

Antigenic shifts occur when major changes in the genetics take place and a virus emerges which contains a haemagglutinin considerably different from 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 happen through an animal virus changing and adapting to infect humans through spontaneous change 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?

Among viruses that infect humans influenza is moderately infectious. On average an infectious person will infect another one to two non-immune people. The viruses are mostly passed on in one of two ways. Firstly by people breathing in the larger droplets coming from infectious people coughing or sneezing. If the infected person doesn’t cover his or her mouth and nose people within a range of three meters can be infected. That is why it’s so important that a person covers his or her mouth and nose (preferably with a disposable tissue) when he or she sneezes. Smaller droplets and ‘aerosols’ seem to be less liable to carry the virus. Because 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 of the mucous membranes of the nose, mouth and throat with virus, for example from the hands of infectious people who have rubbed their noses. That is why hand-washing and proper use of disposable handkerchiefs (respiratory hygiene) are important.

In hospital some medical procedures like ‘intubation’ (inserting a tube down a throat of an unconscious patient to help them breathe) can produce infectious aerosols and fine droplets which may have the virus. That 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. Longer or shorter intervals are also sometimes seen and the range is said to be one to seven days).

When is a person with influenza infectious to others?

The infectious period varies a bit from person to person but it is considered that an adult with flu starts being able to pass on the infection from during the day before their symptoms appear and then they remain potentially infectious for three to five days. The start is probably the same for children but it is often stated that it runs on for up to seven days after onset.

However, the risk to others is not constant throughout. Infectiousness rises steeply when the person starts feeling unwell and that is when he or she is most dangerous to others. The risk of catching influenza from someone even just before they become ill and after the end of the first three days is quite low. This is why it is so important, especially when influenza is around, that as soon as someone starts feeling ill they go off work and stay home for a few days.

What should a person do who thinks they have influenza?

There are two principles: 1. Look after yourself and 2. Don’t infect others.

The customs vary with the country. In some countries most people expect to see a doctor while in other countries it is not always thought necessary to actually see a doctor unless the person seems very ill or is in a ‘risk group’. People with chronic or long-standing illness may need medical attention earlier and quickly.

The best advice is for the person to go home, use handkerchiefs properly (preferably disposable ones – and dispose of them safely), wash his or her hands frequently, do not share towels, rest, drink plenty of fluids and take medicines that will reduce the temperature and the aching (paracetamol for all ages, aspirin may be taken by adults but not by children). If it is necessary to have people about him or her at home he or 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 the person’s doctor or nurse 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. Unfortunately there are people who seem to have the illness without any symptoms or only with symptoms for a short period. That may be how some people seem to catch influenza without being in contact with anyone with the disease. However it is thought that these people without symptoms or with minor symptoms are generally less infectious than people with the high temperatures and feeling unwell.

Who is at higher risk from influenza? Who are the risk groups that need immunizing?

The young have a greater risk of being infected because they have not developed immunity to the virus and a few countries recommend routine immunization of young children. However, apart from the very young, fit healthy children generally don’t get as sick with influenza as often as the elderly.

The elderly, when they are infected, on average have a greater risk of developing severe complications of infection such as pneumonia. They also more often have underlying diseases than younger adults, which reduce their resistance to infection. The immune response may also be less effective in elderly persons.

Other high-risk groups are people with long term heart or chest disease or any other chronic medical condition. These conditions make them more likely to suffer severe illness than people who are basically healthy.

Because of these facts most countries recommend annual influenza vaccination of the elderly and those with chronic illnesses. The exact policies vary from country to country. For example some countries immunize everyone over 65, others everyone over 60 or even 55 years.

How do I protect myself and my family against influenza?

You can reduce the risk of getting influenza or transmitting it to others by doing the following:

  • Frequent hand-washing is considered to reduce the risk of people catching influenza in the influenza season. It also protects against a number of other infections.
  • Practice good respiratory hygiene Making sure that people who are coughing and sneezing cover their mouth and nose and use a disposable handkerchief.
  • If people are unwell with a fever they should go home Also making sure that people who develop a fever and respiratory infection go home immediately. People are most dangerous to others when they are in this early stage of influenza
  • Routine annual vaccination of those for whom it is recommended offers good protection and people who are at high risk of infection should be vaccinated. Exact vaccination policies in Europe 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 work to prevent influenza? Can you use alcohol gels for hand-washing?

ECDC recommends using hand-washing to reduce influenza transmission and the alcohol gels do work against the influenza as well. The way it works is by interrupting the following kind of transmission – A person has influenza, he wipes his nose or mouth, he forgets to wash his hands, he shakes hands with someone who then touches his nose or mouth and catches influenza.

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

So really it is good general hygiene, hand-washing, and using and disposing of tissues that is important. Of course these are also important to prevent other infections.

Does the seasonal flu vaccine contain the H1N1 vaccine?

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

Should people wear masks when influenza is about? Will it protect us from influenza?

There is no good answer to these straightforward questions and ECDC is neutral on general mask-wearing because there is hardly any evidence either way. This is because there has been little research to see whether wearing masks helps protect people or not. At present we simply do not know if mask-wearing by the general public helps or not.

In some societies, e.g. in the Far East, masks are worn a lot in the influenza season, but they still get plenty of influenza.

Some people suggest wearing masks because they see it as an extension of covering up your mouth and nose with a tissue when you cough or sneeze. However, others argue that wearing masks may give a false sense of security and encourage people to ignore other more important behaviour like hand-washing or going home to bed when they are sick. They also suggest that wearing a mask for too long may allow it to get wet and soggy and that is more dangerous than using disposable handkerchiefs properly.  

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

There are certain times when it makes more sense to wear a mask. To begin with first check that there is influenza about. If there is no influenza there is little value in wearing a mask. Then consider the risk – there are certain places and people where there is more risk – crowded public transport, or workers who have to have close face to face contact with a lot of people. For most people the risk of catching influenza is small until someone close to you becomes sick.

The issue of the type of mask is rather up to you. There are some expensive masks which give more protection if they are used properly. But they are really meant for people working with very dangerous infections or in high risk settings. Given the lack of evidence that mask-wearing helps there seems little reason for spending money on buying anything more expensive than the simplest ‘surgical’ masks. 

Are there any rules about wearing masks?

Yes – If you chose to wear a mask then you need to change it about twice a day and more often if it gets soggy, rather like a disposable tissue.  

Should masks be worn in the home?

There would generally not be much purpose, especially if there is no flu about. An exception would be if there is someone in the house with suspected flu and they need a lot of close care.