Frequently asked questions on influenza

A respiratory illness 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.

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. Three subtypes of influenza A are currently in circulation: A (H1N1), A (H1N2) and A (H3N2). 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.

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. 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. Many epidemics of influenza originate in South East Asia 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.

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.

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.

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.

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.

In temperate climates influenza 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.

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.

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.

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.

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.

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.

An epidemic is the occurrence in a community or region of cases of an illness in excess of what might normally be expected. The community or region, and the period in which the cases occur are specified precisely.

A pandemic is an epidemic occurring over a very wide area and usually affecting a large portion of the population. In the 20th century, pandemics occurred in 1918 ('Spanish Flu'), 1957 ('Asian Flu'), 1968 ('Hong Kong Flu'). 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.

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.

There has not been a pandemic for over thirty years and we have no reason to suppose there will be a pandemic of flu this year. However, in light of the recent Avian Influenza outbreak there has been some concern regarding the creation of a new human influenza virus to which the population would have little immunity. These concerns have led to heightened influenza surveillance both internationally and throughout the U.K.

The UK has a pandemic plan. This means we are well-placed to react if a pandemic occurs in any year. This sort of preparation is important, although the actual risk of a pandemic is very low.

The UK has developed a plan for dealing with a pandemic. The Department of Health (DH) and the HPA have prepared plans for a graduated series of public health measures to reduce the impact of and to help control a pandemic. The HPA pandemic plan was published at the beginning of 2005, and is available on the HPA web site. The DH plan is available on the Department of Health website and the WHO pandemic plan can be found on the WHO web site.

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/ HPA pandemic plans.

• 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/ HPA pandemic plans.

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

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.

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.

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.

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.

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.

See the Influenza Vaccine Composition composition for details.

See the web page on vaccine uptake monitoring for details.

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.

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.

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.

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

You should not be vaccinated if you have a serious allergy to hens' eggs.

There is no evidence that influenza vaccine prepared from inactivated virus causes damage to the foetus. However, it should not be given during pregnancy unless there is a specific indication.

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.

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.

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.

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.

There are currently two drugs recommended for the treatment of influenza, in the UK. 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) on the use of these antivirals

Zanamivir and oseltamivir belong to a new family of drugs that attacks the flu virus in a way that has not been used before and prevents it spreading within the body. Their introduction is therefore an important addition to the tools we have 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.

CfI will have 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. CfI in collaboration with the Royal College of General Practitioners' Research Unit in Birmingham, can provide the surveillance information which will show when flu is circulating and therefore when the drugs can be most appropriately used.

Recommendations and further information regarding the safe use of antivirals can be found on the NICE website.

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.

The annual enhanced surveillance period starts at week 40 (the beginning of October), and ends at week 20 (May the following year). Within the HPA, the surveillance is carried out nationally by the HPA Surveillance of Influenza Group - Epidemiologists from the Respiratory Diseases Department of the HPA Centre for Infections and Virologists from the Influenza Section of the Virus, Reference Department of the HPA Centre for Infections.

The Centre for Infections (CfI) conducts surveillance of flu activity, carries out laboratory tests to see which strains of flu are in circulation, and communicates information about flu to other health professionals and to the public. Information is gathered from GPs on community cases of flu, from HPA and NHS laboratories on hospitalised cases, and on notifications of deaths from respiratory disease. This builds up a picture of flu activity.

The weekly Influenza Report is produced on Wednesdays during the influenza surveillance period and posted onto the HPA web site. Regular information appears in the Health Protection Report during the flu season. The HPA also has an active research programme on influenza, which focuses particularly on the development of new diagnostic tests, methods of detecting influenza, improving existing vaccines and ways to improve surveillance. See HPA publications Publications.

Data are released on a weekly basis to the WHO, CDC Atlanta, World Influenza Centre, Mill Hill, England and are based on the most recently isolated influenza viruses. There is also electronic exchange of data between the HPA and the WHO Global Health Atlas database and the European EISS database.

Influenza activity is monitored through a variety of sources, one of which is the RCGP scheme.   

Data provided by the RCGP scheme comes from a network of 73 GP surgeries covering a population of around 700 000 patients. They report the number of weekly consultations for new episodes of "influenza and influenza-like illness".

Human metapneumovirus (hMPV) is a respiratory pathogen closely related to Respiratory Syncytial Virus (RSV). It is associated with a range of illnesses from mild infection to severe bronchiolitis and pneumonia. Symptoms may include a runny nose, cough, temperature, sore throat, and wheezing. Like RSV, hMPV is thought to be a seasonal virus occurring mostly during the winter months. However, the number of people which suffer from hMPV each year is still to be determined.

HMPV infection occurs in infants and young children with studies suggesting that nearly everyone has had hMPV infection by the age of 5 years old. However, hMPV has also been found in older children and adults suggesting re-infection may occur later on in life.

HMPV was first discovered in the Netherlands 2001 followed by Australia, North America and the UK indicating this virus is found in the population world wide. Despite only recently being identified studies carried out in the Netherlands on old blood samples suggest this virus has been in the human population for at least the past 50 years. HMPV however, remained unidentified as techniques routinely used for virus growth and identification did not allow for the slow growth of this virus or the inability of antibodies used to identify related viruses to recognise hMPV.