the influenza FAQ

cold and fluWhat is influenza?

Influenza is caused by a virus, which was first identified in 1933. There are two main types that cause infection, influenza A and influenza B (Influenza C is an uncommon type that infrequently causes infection). Influenza A is usually a more severe infection than influenza B. Influenza A subtypes exist which are named according to the haemagglutinin and neuraminidase antigens on the viral surface.

A respiratory illness can be associated with infection by the influenza virus. Symptoms include headache, fever, cough, sore throat, aching muscles and joints. There is a wide spectrum of illness ranging from minor symptoms through to pneumonia and death.  Several articles review the symptoms most likely to be associated with influenza e.g. Monto AS, Gravenstein S, Elliott M, et al. Clinical signs and symptoms predicting influenza infection. Arch Intern Med 2000; 160: 3243-7. Zambon M, Hays J, Webster A, et al. Diagnosis of influenza in the community: relationship of clinical diagnosis to confirmed virological, serological, or molecular detection of influenza. Arch Intern Med 2001; 161: 2116-22.

What does genetic drift and shift of influenza mean?

The influenza virus is antigenically unstable and new strains and variants are constantly emerging. Each year one or two subtypes of influenza A may be in circulation and one type of influenza B.

Antigenic drift Minor changes to the amino acid sequence of the haemagglutinin (HA) molecules in the virus envelope take place all the time and cause genetic drift. Haemagglutinin is the main antigen associated with immunity. Neuraminidase (NA), the second main antigen in the virus plays a minor role in immunity. The drifted strains of influenza may infect partially immune people who have been exposed in previous winters. Influenza A drifts more than influenza B.

Antigenic shift Genetic shift occurs when major changes in the HA or NA take place and a virus emerges which contains a haemagglutinin different from those of previously circulating viruses. When this happens it gives rise to major epidemics or pandemics in populations throughout the world that have no immunity to the new strains, e.g. Influenza H1N1 (2009), Spanish flu (1918), Asian flu (1957) and Hong Kong flu (1968/69). Influenza virus strains are classified according to the place and year in which they were first isolated, e.g. A/California/7/2009 (H1N1) is the name of the 2009 pandemic virus strain which was used in the vaccine as it was isolated in California in 2009. Many epidemics of influenza originate in South East Asia, although the pandemic virus in 2009 is thought to have originated in Mexico, and are thought to be due to rare recombination events between human, avian and other animal strains of influenza following coinfection in a susceptible host.

How serious is influenza infection?

Influenza makes people feel worse than an ordinary cold. For most people influenza infection is just a nasty experience, but for some it can lead to illnesses that are more serious. The most common complications of influenza are bronchitis and secondary bacterial pneumonia. These illnesses may require treatment in hospital and can be life threatening especially in the elderly, asthmatics and those in poor health. The influenza virus does not necessarily cause high mortality, but for old sick people it may speed up their death. During a pandemic, though, influenza can cause serious illness in young healthy individuals.

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. Most people confuse influenza with a heavy cold, however influenza is usually a more severe illness than the common cold, which is caused by other respiratory viruses such as rhinovirus.

What are the symptoms of the common cold?

Cold symptoms are limited to the upper respiratory tract with runny nose, sneezing, watery eyes and throat irritation. The symptoms usually occur gradually and do not cause a fever or body aches.

When does influenza occur?

Influenza occurs most often in the winter months and usually peaks between December and March in the northern hemisphere. Illnesses resembling influenza may occur in the summer months but they are usually due to other viruses.  The timing of influenza activity is unpreditable, which is best demonstrated by the activity caused by influenza A H1N1 (2009) in the spring and summer of 2009.

Why do people get infected with influenza during winter?

In temperate climates influenza tends to strikes from late autumn through to spring, although technically influenza is not bound by seasons, and can occur all year round in tropical climates. A possible explanation for the high influenza virus activity in the wintertime is that people congregating indoors during winter facilitate the transmission of the virus or that more humid air indoors may help the viruses survive longer.

How is influenza diagnosed?

Usually, a doctor will diagnose a case of the flu based on typical symptoms of fever, chills, headache, cough and body aches. Specific lab tests to confirm the flu are costly and time consuming and are usually limited to outbreak or disease surveillance efforts. Recent articles document the relationship between clinical diagnosis and laboratory diagnosis e.g. Zambon M, Hays J, Webster A, et al. Diagnosis of influenza in the community: relationship of clinical diagnosis to confirmed virological, serological, or molecular detection of influenza. Arch Intern Med 2001; 161: 2116-22.

How is influenza spread?

The flu virus is highly contagious and is easily passed from person-to-person when an infected person coughs or sneezes. Transmission can also occur by touching a surface contaminated with respiratory secretions and then putting the fingers in the mouth or nose or near the eyes. The flu virus can live on a hard surface for up to 24 hours and a soft surface for around 20 minutes.

To reduce transmission, it is vital that if someone has a respiratory infection that they cover their nose and mouth when they cough and sneeze, preferably with a tissue, and wash their hands afterwards.  Tissues need to be bagged and disposed of appropriately if they are used outside the home; otherwise they can be disposed of in normal household waste.

Normal household products can be used to clean the room of someone who has had flu as the virus can easily be destroyed. Open the windows, wash bedlinen but make sure you wash your hands afterwards, pay particular attention to hard surfaces and allow as much contact time with the cleaning product before wiping it clean.

The incubation period – the period between infection and the appearance of symptoms – is about two to three days. Although virus has been detected before symptoms appear, adults are usually considered infectious once symptoms appear and for 3-5 days afterwards. This period is longer in children.

What should you do if you get flu?

Rest, drink plenty of fluids and take analgesics (paracetamol for all ages, aspirin may be taken by adults).

Most influenza-like 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. Medical advice should be sought if symptoms become severe or last more than about a week. Those with chronic or long-standing illness may need medical attention earlier.

Who is most at risk from the complications of influenza?

The young have a greater risk of being infected because they have not developed immunity to the virus.

The elderly have a greater risk of the severe complications of infection such as pneumonia, because they often have underlying diseases, which reduce their resistance to infection. The immune response may also be less effective in elderly persons.

The high-risk groups include individuals whose respiratory, cardiac or immune systems make them more vulnerable to flu and more likely to suffer severe illness.

What precautions should people take?

Routine vaccination offers the best protection and people who are at high risk of infection should be vaccinated. It is difficult to avoid infection if there is an epidemic. Keeping away from crowded places can reduce the risk of becoming infected and spreading it to others. A previous flu infection or vaccination will not necessarily provide protection against further infections because the virus is continually changing genetically and different subtypes circulate each winter.

Eidemics and pandemics

What is an epidemic?

An epidemic is the occurrence in a community or region of cases of an illness in excess of what might normally be expected.

What is a pandemic?

A pandemic is an epidemic occurring over a very wide area and usually affecting a large portion of the population.  Pandemics of influenza are triggered by spontaneous and unpredictable major changes in protein antigens found on the surface of the influenza virus particle. These changes occur at irregular intervals and lead to the development of new subtypes of the influenza virus. As most people have not had the chance to become immune to these new strains, widespread infection may occur.  The 2009 pandemic was the first in the 21st century; in the 20th century, pandemics occurred in 1918 (‘Spanish Flu’), 1957 (‘Asian Flu’), 1968 (‘Hong Kong Flu’).

Are we expecting another pandemic or a severe epidemic this year?

Serious epidemics within the UK do not occur on a cyclical basis, and so it is not possible to predict likely levels of infection in a given year. We have no reason to suppose that a serious epidemic is more likely this winter than in any other year.

The level of infection within the population depends on how much the virus “mutates” in any given year, and the consequent levels of immunity within the population. The bigger the change, the greater the likelihood of a larger outbreak.

We do not know if influenza H1N1 (2009) virus will cause more illness in the coming years or if it will mutate sufficiently so that people’s immune systems do not recognise it.

The 2009 pandemic was the first for forty years, however, this does not preclude the emergence of another new influenza virus with pandemic potential although we have no reason to suppose that this will happen.  There remains concern over the continued occurrence of Avian Influenza in humans.

The UK has pandemic plan called the UK Influenza Pandemic Preparedness Strategy 2011.  The plan updates the clinical, health and scientific advice and information contained in the previous pandemic plan and is based on what has been learnt in 2009/10.  Preparing for another pandemic is important, although the actual risk of a pandemic is very low.

What are the key elements of pandemic plans?

The key elements of pandemic planning incorporate:

  • Monitoring the pandemic
  • Advice and information
  • Emergency services provision
  • Hospitalisation provision
  • Laboratory diagnosis strategy
  • Treatment policy
  • Vaccination policy

Further information is available from DH [external link]/ HPA pandemic plans.

What control measures are envisaged?

  • Vaccine preparation and vaccination of identified high risk groups
  • Use of anti-influenza drugs
  • Antibiotic usage to prevent secondary complications such as bacterial pneumonia
  • Appropriate advice about the use of healthcare services

Further information is available from DH [external link]/ HPA pandemic plans.


Who needs a seasonal flu vaccination?

Prior to the flu season each year, a letter from the Chief Medical Officer (CMO), Chief Nursing Officer(CNO) and the Chief Pharmaceutical Officer (CPO) is sent to all doctors in England reminding them of the need for patients in certain risk groups to be offered annual influenza vaccination.
The Welsh CMO sends out a similar letter to doctors in Wales and a similar letter is sent from the Scottish Office to doctors in Scotland.

The CMO’s advice is that you need a seasonal flu vaccination if:

  • You are aged 65 years or over
  • If you are aged 6 months or over and fall into the following risk group :

1. Chronic respiratory disease, including asthma that requires continuous or repeated use of inhaled or systemic steroids or with previous exacerbations requiring hospital admission

2. Chronic heart disease

3. Chronic renal disease

4. Chronic liver disease

5. Chronic neurological disease*

6. Diabetes requiring insulin or oral hypoglycaemic drugs

7. Immunosuppression

8. Pregnant

* GPs should consider on an individual basis the clincial needs of their patients including individuals with multiple sclerosis and related conditions, or hereditary and degenerative disease of the central nervous system.

  • Vaccination is also recommended for those living in long-stay residential homes or other long-stay facilities where rapid spread is likely to follow introduction of infection and cause high morbidity and mortality (this does not include prisons, young offender institutions, university halls of residence etc).
  • Vaccination is also recommended for carer’s defined as those who are in receipt of a carer’s allowance, or those who are the main carer for an elderly or disabled person whose welfare may be at risk if the carer falls ill. This should be given on an individual basis at the GP’s discretion in the context of other clinical risk groups in their practice
  • In addition, it is recommended that immunisation be offered to all health care workers involved in the delivery of care and/or support to patients. Social service employers have also been asked to consider offering immunisation to all staff involved in the delivery of care and/or support to clients.

Was there a pandemic vaccine?

Soon after the isolation of the new influenza virus in 2009 work began on developing a vaccine.  The seasonal flu vaccines that were already available did not protect against the new H1N1 virus so a new vaccine was developed and became available in the UK from Autumn 2009. There were two different brands of the pandemic vaccine called Pandemrix and Celvapan.  People in clinical risk groups and health care workers were recommended to receive this vaccine, and later all children under 5 years of age were added to the list.The seasonal influenza vaccine for 2012/13 contains three flu strains one of which is the H1N1 (2009) virus.

Does the CMO’s advice mean we are expecting a severe epidemic this year?

This advice is issued annually, irrespective of the predicted levels of influenza activity.The recommendations are issued to protect those in the population who are most vulnerable to the complications of influenza.

Who makes the decisions about the vaccine and vaccination?

The WHO advises on the components of the vaccine depending on the strains expected to circulate in the coming season.

Advice on who should actually be offered the vaccine is given each year by the CMO who is advised by an expert statutory group, the Joint Committee on Vaccination and Immunisation (JCVI). Research has shown that certain ‘at risk’ groups (in particular those with existing chronic risk conditions) benefit most from vaccination.

Why do we get flu every year and why won’t the vaccine provide long term protection?

The virus is constantly changing – genetic shift and drift – with new strains and variants constantly emerging. People develop immunity either as a result of becoming infected, or through vaccination. As immunity is specific to individual strains of the virus, the emergence of a new strain will mean that people will contract the disease and a new vaccine will need to be developed. Each year one or two subtypes of influenza A may be in circulation and one type of influenza B.

Wouldn’t it be better to vaccinate everybody who wants it?

For the majority of people flu is not life-threatening, however unpleasant it may be. A bout of flu offers long term protection against the same and closely related strains of influenza. It is the ‘at risk’ groups who benefit most from vaccination.

Calculated numbers of vaccine doses are available to supply the high-risk groups – the vaccine should therefore be targeted at those most in need and for whom it will be most effective.

How is the vaccine made?

The viruses for the vaccine are grown in eggs, then killed and purified before being made into the vaccine. Because the flu virus is continually changing, and different subtypes circulate each winter, a new influenza vaccine has to be produced each year. This will normally contain three components, two subtypes of influenza A and one of influenza B. The decision as to which strains to include in the vaccine is made each February by WHO in Geneva, on the basis of analysing several thousand influenza viruses at the WHO influenza laboratories of London, Atlanta, Melbourne and Tokyo. These laboratories assess which strain has been dominant over the previous winter and look for evidence of new strains that have the potential to spread, and against which current vaccines offer poor protection. Production of the vaccine starts in March each year and continues throughout the spring and summer for the Northern Hemisphere. GPs are encouraged to order the vaccine in the summer in preparation for distribution in the autumn.

Much interest surrounds research into influenza vaccination. Current areas of development include intranasal vaccine, live-attenuated vaccine and cell culture vaccine.

What are the current recommendations for vaccine composition?

See the page on Influenza Vaccine Composition for details of the current and recent recommendations for northern and southern hemisphere influenza vaccines.

How are vaccination uptake rates monitored?

The HPA monitors uptake of influenza vaccine on behalf of the Department of Health.

How does the vaccine work?

About seven to ten days after vaccination, your body makes antibodies that help to protect you against any similar viruses that may infect you. This protection lasts about a year.

How effective is the vaccine?

Flu vaccinations are 70-80% effective in healthy adults, in years when there is a good match between the vaccine and the strains of flu in circulation. In recent years we have been getting better at predicting the strains which are likely to circulate, and in most years there is now a good match between the vaccine and the circulating strains:

  • Most people who have been vaccinated don’t get the kinds of flu from which the vaccine was made
  • If you do catch flu it is likely to be milder than if you had not been vaccinated.

Does the vaccine have any side effects?

Flu vaccines are very safe. They may cause some soreness where you were injected and, less often, a slight temperature and aching muscles for a couple of days.

Can the vaccine cause flu?

No. The vaccine cannot cause flu because it doesn’t contain live virus.

I am allergic to eggs – can I have the flu vaccine?

People with an egg allergy may be at increased risk of reaction to influenza vaccines. In recent years, influenza vaccines that are egg-free or have very low ovalbumin (egg protein) content (< 0.12 μg/ml) have become available. Influenza vaccines with an ovalbumin content < 0.12 μg/ml have been shown to be safe in patients with egg allergy (Gagnon et al, 2010).

Patients who have a serious allergy (confirmed anaphylaxis) to egg or egg allergy with uncontrolled asthma, can be immunised with an egg-free influenza vaccine (if available) as a single dose (two doses in the case of children aged under 13 years that have not been previously vaccinated) in primary care. If no egg-free vaccine is available, patients should be referred to specialists for vaccination in hospital using vaccine with an ovalbumin content less than 0.12 μg/ ml (i.e. containing less than 0.06μg per 0.5ml dose).

All other egg allergic individuals can be given egg-free vaccine or influenza vaccine with an ovalbumin content less than 0.12 μg/ml (i.e. containing less than 0.06μg per 0.5ml dose) as a single dose (two doses in the case of children aged under 13 years that have not been previously vaccinated) in primary care. Facilities should be available and staff trained to recognise and treat anaphylaxis .

Vaccines with ovalbumin content (more than 0.12 μg/ml i.e. containing more than 0.06μg per 0.5ml dose) or where content is not stated should not be used in egg-allergic individuals.

When is the best time to be vaccinated?

The best time to be vaccinated is between late September and early November, ready for the winter. You shouldn’t wait until there is a flu epidemic.

How do I go about getting vaccinated?

If you think you need a flu vaccination, check with your doctor or the practice nurse – or if a nurse visits you regularly, ask them. Try to do so as early in the autumn as possible. Most doctors organise special vaccination sessions in the autumn and will arrange an appointment for you then.

Does past infection with influenza make a person immune?

To a certain extent. The viruses that cause flu, however, frequently change, so people who have been infected or given a flu vaccination in previous years may become infected with a new strain. Because of this, and because any immunity produced by the flu shot will possibly decrease in the year after vaccination, people in high-risk groups should be vaccinated every year.

If there are anti-influenza drugs already available, why aren’t they used – either in conjunction with vaccination, or in preference to vaccination?

The purpose of vaccination is to help prevent infection, and so vaccination should take place before the “flu season” begins to ensure maximum protection (pre-season).

Some drugs are designed to be used during an epidemic to provide short-term protection (in-season). The best way to provide protection that will last throughout the flu season is therefore to vaccinate the groups who are at highest risk. Depending on the circumstances, anti-flu drugs may then be appropriate to use later on in the season during periods of increased flu activity as a complement to the flu vaccine.

Which antiviral drugs are licensed for use in the UK?

There are currently two drugs recommended for the treatment of influenza, in the UK. In a normal influenza season, Oseltamivir and zanamivir are only recommended as treatment for influenza in those considered to be ” at risk” of developing more serious complications from flu infection, such as the elderly or those with underlying conditions like asthma or heart disease. It is recommended for “at risk” patients who present and who can start treatment within 48 hours of the onset of symptoms of influenza-like illness .These drugs are only recommended for use in this way during the period when flu is known to be circulating.

The table below summarises the current guidance from National Institute for Health and Clinical Excellence (NICE) [external link] on the use of these antivirals

Suitable for the treatment of CHILDREN
“at risk”

Suitable for the treatment of ADULTS “at risk”

Suitable for short- term protection of those “at risk” who have been exposed to influenza.

YES (>5 years old)


YES (>5 years old)


YES (>1 year old*)


YES (>1 year old*)

*in exceptional circumstances oseltamivir can be used for the treatment or post exposure prophylaxis of influenza in children under one year of age

Zanamivir and oseltamivir belong to a family of drugs that attacks the flu virus and prevents it spreading within the body; they are important tools to manage flu infection. For most people, flu is unpleasant but is not dangerous; it is generally only in certain specific risk groups that more dangerous complications can occur.

The HPA  has an important role to play in helping doctors prescribe zanamivir and oseltamivir. NICE has recommended that these drugs should be made available only when flu is circulating to ensure that they are used appropriately. The HPA in collaboration with the Royal College of General Practitioners’ Research and Surveillance Centre [external link], can provide the surveillance information which will show when flu is circulating and therefore when the drugs can be most appropriately used.

What changes were there to antiviral drug use during the pandemic in 2009?

As part of UK pandemic preparedness the government stockpiled supplies of Oseltamivir (Tamiflu).  From the beginning of the pandemic in April 2009 up to the beginning of July 2009 antiviral drugs were offered to all virologically confirmed cases of influenza H1N1 (2009) and their close contacts as treatment or prophylaxis respectively, in an attempt to slow the spread of the disease in the community.  From July onwards the ‘Containment’ phase moved on to the ‘Treatment only’ or ‘Mitigation’ phase in which antiviral drugs were offered to all suspected cases (those with an influenza-like illness).  In England, this was largely managed through a telephone and internet service called the National Pandemic Flu Service.  People who were unwell contacted this service and were taken through a series of questions to ascertain whether they were eligible for antiviral drugs.  If they were, they were issued a unique code to enable them or a friend to collect their antiviral drugs from designated collection points.

This approach of offering antiviral drugs to everyone with an influenza-like illness was different to in a normal influenza season.  This is because there was considerable uncertainty around the new virus and the disease it could cause and there were cases of severe disease and deaths in young previously healthy people.

What should people do if they want to get antivirals?

In a normal influenza season, antivirals are only recommended for adults who are at increased risk of the complications of flu. Details on the risk groups for flu can be found above in the section on “Vaccination”. They are not recommended for otherwise healthy adults.

If you are in a risk group or are aged 65 years or more, the most important way of preventing the serious complications of flu is still the flu vaccine. You should take up the opportunity to be vaccinated: this will help prevent you from catching flu, and in the unlikely event that you do still contract the infection it is likely to be less severe if you have been vaccinated. However, if you are in a risk group and have contracted the flu, you should seek advice early from your doctor or pharmacist who can advise you on whether you need to take antivirals.

Can influenza viruses develop resistance to antiviral drugs?

As the flu virus is constantly changing and evolving, random mutations in the genetic code can create a virus which is resistant to a drug.  This occurred in the winter of 2007/08 in seasonal H1N1 influenza viruses and viruses that were resistant to Oseltamivir (Tamiflu) were soon detected across the globe.  It was discovered that all the resistant viruses had the same mutation.  This same mutation was discovered in some influenza H1N1 (2009) viruses as well, causing them to be resistant to Oseltamivir.  Most of these cases were sporadic and many are assumed to have occurred spontaneously in a patient who has been treated with Oseltamivir. This is most likely to occur in a patient who is treated for a long time with the drug but whose immune system has not been able to clear the virus from the body (e.g. a person who is immunocompromised due to an illness such as HIV infection or treatment such as drugs given after an organ transplant).  An influenza infection can be severe in such a patient so it is important to be aware of possible resistance to inform treatment decisions.


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