Frequently asked questions on 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 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.

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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.

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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.

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.

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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 a pandemic plan. This means we were well-placed to react when the pandemic occurred in 2009.  This plan will be modified 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.

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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.

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/ HPA pandemic plans.

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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) on the use of these antivirals

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.

CfI 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. 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.

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.

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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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, for further information on the sources of data used please see Sources of UK Flu Data.

What information does HPA CfI make available?

The weekly Influenza report is produced on Thursdays during the influenza surveillance period and posted onto the HPA web site. Regular information appears in the Health Protection Report throughout the year. 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.

What about international data exchange?

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 clinical and virological data between the HPA and the World Health Organization and the European Centre for Disease Prevention and Control to aid global surveillance of influenza.

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