Seasonal influenza - the basics

Although public confusion over flu and its symptoms is rife, presentations to GPs are down.

The annual vaccine is effective in an estimated 70-80 per cent of cases (Photograph: SPL)
The annual vaccine is effective in an estimated 70-80 per cent of cases (Photograph: SPL)

Influenza is a droplet infection. There are three main influenza viruses - A, B and C. Type A is most commonly associated with pandemics.

There are other strains of influenza virus although these are uncommon in western Europe. There is often confusion among patients regarding the difference between a severe common cold and flu. In recent years, the reported outbreaks of 'bird flu' and 'swine flu' worldwide have significantly added to the confusion.

Despite this, there is evidence that the GP consultation rate for flu-like illness has steadily declined over the past decade, perhaps because people are better informed, they receive good advice from the media and pharmacists, and feel more confident to apply self-care in uncomplicated cases.

1. Risk factors and complications

The influenza viruses are spread easily in overcrowded conditions or institutions with large numbers of people.

Seasonal influenza is most common in the UK from October to May each year, particularly between December and March. However, infection can occur all year round.

Complications, such as pneumonia, are more likely to occur if a patient is vulnerable, for example frail and/or elderly, has a chronic respiratory disease, is immunosuppressed or has other significant chronic conditions. These include diabetes, heart disease, malignancy and advanced chronic kidney disease. Pregnant women are also at an increased risk of complications.

Complications, such as pneumonia, can occur in vulnerable patients (Photograph: SPL)

It is difficult to establish the annual influenza mortality rate in the UK and estimations vary between 3,000 to 12,000 deaths. In epidemics or pandemics, this rate can increase and may exceed 50,000 deaths in the UK.

There have been four declared pandemics in the last century, whereas epidemics are expected every two or three years. The threshold to declare an influenza epidemic, which warrants the use of antiviral treatment, was determined by the RCGP sentinel practitioner scheme in 2007 as 30 cases per 100,000.

The main body to oversee the current prevalence of influenza is the Health Protection Agency.

2. Clinical presentation and assessment

The infection has an incubation period from one to seven days and can present with varying severities: some patients will experience symptoms similar to a 'bad cold' for a few days whereas others will be affected by a severe, generalised infection with cough, fever, myalgia, headache, sore throat and other symptoms for a week or longer.

Affected patients are infectious for up to two weeks from the onset of symptoms.

3. Prevention

Prevention is crucial in order to minimise spread into the wider community from one infected patient.

The annual influenza vaccination is based on the presumed main active strains of the influenza viruses that are predominant in the population. The annual vaccination for 2010 includes vaccination for influenza A H1N1.

Although there is a significant amount of accumulated experience going into this procedure, the actual efficacy is estimated at no more than 70-80 per cent.

Therefore, patients need to know that they can still contract influenza after their vaccination, and that they are not protected from the common cold. Patients should also be reassured that vaccination does not increase the risk of contracting influenza.

In recent years, the production and adequate supply of the vaccination has been a concern for primary care providers and some patients choose to receive it privately.

Although the vaccination does not offer full cover against influenza, data suggests it is cost effective and it reduces complications, hospital admissions and death rates.

4. Management

Conservative symptomatic treatment consists of temperature control, analgesia and ensuring the patient is sufficiently hydrated.

Reports into antivirals for the treatment of influenza have increased since the worldwide influenza A H1N1 pandemic in 2009. This has increased expectation and demand, although the evidence of their efficacy remains mixed.

If given within 48 hours of symptom onset, antivirals may improve the overall outcome and recovery to some extent, but should not be used routinely.

Unvaccinated at-risk patients may benefit from taking antivirals in the event of a community outbreak of influenza. Antivirals are contraindicated in pregnancy.

The exaggerated anxiety among the public and healthcare professionals regarding influenza has led to a demand for more antiviral medication use. The NICE guidance regarding antivirals is currently under review.

  • Dr Jacobi is a salaried GP in York
Key points

1. Influenza can be contracted all year, with a higher rate of symptomatic patients between October and March.

2. Vulnerable patients, for example those with chronic conditions, are at higher risk of complications and may benefit from annual vaccination.

3. The influenza vaccination is effective to a degree of 70-80 per cent and consists of inactivated viral material.

4. Antiviral treatments are reserved for specific high-risk patients and special circumstances.

5. Prevention is crucial to minimise the spread of infection.

1. Health Protection agency: luenza

2. NICE. Amantadine, oseltamivir and zanamivir for the treatment of influenza. TA168. London, NICE. 2009

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