Influenza (Flu)

What is influenza?
Influenza is an acute respiratory illness caused by infection with an influenza virus.

There are three types in all with influenza A and influenza B causing the majority of infections. A third type, influenza C, is rarely reported as a cause of human illness.

What are the symptoms?
Influenza is characterised by sudden onset of symptoms with patients often recalling the exact hour the fever commenced. The symptoms include a temperature of 38C or more with a dry cough, headache, sore muscles and sore throat. Cough is often severe and protracted, but otherwise the disease is self-limiting and recovery is in 2-7 days. Long-term sequelae that can occur include depression and fatigue (asthenia) that can last weeks.

What are the complications from influenza?
The most frequent complication is pneumonia, most commonly secondary bacterial pneumonia. Primary influenza viral pneumonia is an uncommon complication but is associated with a high death rate. Other complications include worsening of pre-existing chronic medical conditions such as chronic bronchitis or chronic heart failure. Reye's Syndrome is a particular syndrome that almost exclusively occurs in children, primarily in association with influenza B infection and presents with severe vomiting, confusion and coma.

Death is reported in 0.5 - 1 per 1000 cases of influenza. The majority of deaths occur in those over the age of 65.(1) Even in winters when the incidence of influenza is low, 3,000-4,000 excess deaths may be attributable to influenza in the United Kingdom. (2) The current best Irish national estimate of the number of deaths annually from influenza and its complications is 300-400 deaths per year and is based on extrapolation of studies done in the UK and the US.

How does influenza spread?
The virus multiplies in the nose and airway passages and usually spreads by aerosol droplet spray. It is highly infectious and can survive on worktops especially in low temperatures and in low humidity. The incubation period (delay between infection and appearance of symptoms) is short typically 1-3 days. A person can spread the virus by sneezing or coughing from 1-2 days before the onset of symptoms and continue to be infective for a further 3-5 days. This however may be prolonged to a week in children.

What precautions can I take?
Annual vaccination remains the best protection against influenza, especially in people who are at high risk of complications from influenza. As the virus can spread through sneezing, coughing, contaminated hands or surfaces, ensuring good hygiene practices will help such as washing hands.

Who is at risk of influenza?
Influenza can affect all ages, however it has more severe consequences in the elderly or people defined as being high risk. High-risk groups include people with chronic heart conditions, chronic respiratory disease, diabetes mellitus and those who are immunosuppressed. These groups of people are targeted for influenza vaccination.

Is it flu or the common cold?
It can be difficult at times to distinguish between the common cold and flu. The main difference is that the symptoms of influenza come on rapidly and are typically accompanied by muscle aches and a fever. The common cold has a more insidious onset and is associated with a runny nose, sneezing, and blocked nasal passages. Please see table below.

How is influenza diagnosed?
Confirmation of influenza infection can be obtained either from throat or nasopharyngeal swabs or by performing a blood test.

What can I take for influenza?
If influenza or influenza-like illness has been diagnosed, often the best treatment is to stay indoors, keep warm and drink plenty of liquids. Simple painkillers such as paracetamol may help relieve headache or muscle pains. Note: Aspirin should NOT be given to children under the age of 16 years.  

Antibiotics are only required if a person develops influenza related complications such as a bacterial pneumonia.

Is there a vaccine?
Yes. Each year flu vaccine is made available from September onwards. The best time to be vaccinated is from mid September to October, i.e. before influenza season commences. This is because it takes two weeks for the vaccine to produce the required antibodies to protect against the infection.

Who should get the vaccine?
There are guidelines set out by the Royal College of Physicians Immunisation Advisory Committee.(3) Two groups are targeted:
1) Any individual over the age of six months of age who is at risk of influenza related complications
2) Those at increased risk of transmitting influenza to a person who is at high risk of influenza related complications.

These categories include:

• All persons over the age of 65
• Persons with chronic illness such as chronic heart disease, chronic lung disease, diabetes mellitus
• Persons who are immunosuppressed due to disease or treatment, including asplenia or splenic dysfunction.
• Children and teenagers on long-term aspirin therapy
• Residents of nursing homes, old people's homes and other long stay facilities where rapid spread is likely to follow introduction of infection.

In addition to these groups, vaccination should also be considered for health care workers both for their own protection, as these are a group likely to come into contact with influenza during outbreaks, and for the protection of their patients.

How often do I need to be vaccinated?
If you fall within the above categories then vaccination is required annually.

Why do I need to be vaccinated annually?
The influenza virus is subject to minor changes in its structure, and this gives rise to slightly different variants circulating each season. In order to produce the most effective vaccine, information is gathered on the current circulating strains and recommendations are made on the best combinations to produce the vaccine.

How effective is the vaccine?
The vaccine is 70-90% effective. In residents of nursing homes, the vaccine is effective in preventing severe complications and deaths. Studies have shown that hospitalisation rates, cases of pneumonia and respiratory illness, and death rates were reduced by over 50% in elderly residential population that were vaccinated.(4 - 8)

Is the vaccine safe?
Yes. There are contraindications to the vaccine, which are as follows:

• A history of known anaphylaxis to egg protein
• A history of a severe reaction to a previous influenza vaccination.

Are there any side effects to the vaccine?
One third of recipients may develop soreness and redness at the injection site. More generalised reactions such as fever, sore muscles and feeling unwell can occur following vaccination and most often affect persons who have had no previous exposure to the flu virus antigens in the vaccine for example young children. The reaction usually begins about 6-12 hours after vaccination and lasts up to 48 hours.

Less common side effects that can occur after vaccination include allergic reactions and Guillain-Barr syndrome (GBS), a severe paralytic illness. Life-threatening allergic reactions are very rare, but can happen in people who have a severe allergy to any vaccine component, most commonly allergy to eggs.

In 1976, swine flu vaccine was associated with an increased number of cases of GBS. Influenza vaccines since then have not been clearly linked to GBS. However, if there is a risk of GBS from current influenza vaccines, it is estimated at 1 or 2 cases per million persons vaccinated - much less than the risk of severe influenza, which can be prevented.(9)

Can I still get influenza despite having the vaccine?
Yes. Depending on the match between the vaccine received and the strain that has caused the infection. In most circumstances, the illness is milder if you have been vaccinated.

Does the vaccine cause influenza?
No, the vaccine is not a live vaccine and cannot give you influenza.

What are antiviral drugs?
There are currently four antiviral drugs that can shorten the course of infection if given early in the disease and provide short-term protection against influenza: amantadine, rimantadine, zanamivir and oseltamavir. Of these only zanamivir (trade name Relenza) and oseltamivir (trade name Tamiflu) are currently licensed in Ireland.

The UK National Institute for Clinical Excellence (NICE) do not recommend the use of amantidine for the treatment of influenza A. Although amantidine (Symmetrel) is licensed in Ireland, it is not licensed for the treatment of influenza. Rimantadine is not licensed in Ireland.  

Zanamivir inhibits the neuraminidase enzyme, thereby preventing release of virus from infected cells. It is indicated for treatment of both influenza A and B in adults and children aged 12 and over who present with symptoms typical of influenza when influenza is circulating in the community. In order to be effective the drug needs to be administered within 48 hours of the onset of symptoms.(10,11) For detailed information on indication, contraindication, dosing and side effects, consult the package insert.

What is an epidemic?
An epidemic is the occurrence of more cases of disease than expected in a given area or among a specific group of people over a particular period of time. Epidemics of influenza can occur annually, during the winter months and last on average between 6-8 weeks.

What is a Pandemic?
Influenza virus undergoes minor changes on its surface regularly as it multiplies. This is known as antigenic drift and is the reason why a person needs to be vaccinated annually.

When more major changes occur in the structure of the virus such that a new virus subtype is produced, this is known as antigenic shift. This is of major importance, as the general population will not have any protection against this new virus. The new subtype can cause a pandemic if it has the ability to spread rapidly from person to person and if it is virulent. As a result large numbers of people all over the world are affected over a relatively short space of time and some cases can prove fatal.

References:

1) Influenza In: Atkinson W, Humiston S, Wolfe C, Nelson R, eds. Epidemiology and Prevention of Vaccine- Preventable Diseases. Department of Health and Human Services, USA, 2000: 231-248
2) Immunisation against infectious diseases. HMSO 1996: 113.
3) National Immunisation Committee Royal College of Physicians of Ireland. Immunisation Guidelines for Ireland. 1999 Edition.
4) Prevention and Control of influenza: Recommendations of the Advisory Committee on Immunization Practices. MMWR July 29, 2005; 54(RR08): 1-40. http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5408a1.htm
5) Effectiveness Matters. Influenza Vaccination and Older people. NHS Centre for Reviews and Dissemination, University of York. Vol 2, Issue 1, October 1996.
6) Gross PA, Hermogenes AW, Sacks HS, Lau J, Levandowski RA. The efficacy of influenza vaccine in elderly persons: A meta-analysis and review of the literature. Ann Intern Med 1995; 123: 519-527
7) Nichol KL, Margolis KL, Wuorenma J, Sternberg von T. The efficacy and cost effectiveness of vaccination against influenza among elderly persons living in the community. N Engl J Med 1994; 22: 778-784
8) Mullooly JP, Bennett MD, Hornbrook MC, Barker WH, Williams WW, Patriarca PA, Rhodes PH. Influenza vaccination programs for elderly persons: Cost-effectiveness in a Health Maintenance Organization. Ann Intern Med 1994; 121: 947-952
9) Vaccine Information. Centers for Disease Control and Prevention.
Available at http://www.cdc.gov/ncidod/diseases/flu/prevent.htm
10) Hayden FG, Osterhaus ADME, Treanor JJ, Fleming DM, Aoki FY, Nicholson KG, et al. Efficacy and safety of the neuraminidase inhibitor zanamivir in treatment of influenza virus infections. N Engl J Med 1997;337:874-80
11) The MIST (Management of Influenza in the Southern Hemisphere Trialists) Study Group. Randomised trial of efficacy and safety of inhaled zanamivir in treatment of influenza A and B virus infections. Lancet 1998; 352:1877-81

Last reviewed: February 5, 2008.