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Addressing concerns over the flu vaccine

David Baxter is a lecturer in public health medicine at the Evidence for Population Health Unit at the University of Manchester Medical School

Patients offer many excuses for not wanting a flu vaccination. Dr David Baxter offers convincing arguments in favour

During the current flu vaccine programme the view that the vaccine can cause flu will be one of the reasons given by some patients for refusing vaccination. Others will not want it because they are well; they believe that having the disease is good for the immune system; or are concerned about adverse events or dislike needles.

Influenza vaccine cannot cause flu since the vaccines we use in the UK only contain either the viral surface proteins haemagglutinin and neuraminidase (in the subunit or virosomal preparations like Influvac or Inflexal V), or those same surface proteins together with small amounts of the viral outer membrane (in the split virion vaccines, for example Begrivac). The absence of any viral genetic material means that viral replication cannot occur (see box, page 47).

Nevertheless, the idea that vaccination causes flu is sufficiently common that it must be addressed. The following explanations can be discussed with your patients in order that they can enjoy the undoubted benefits of vaccination.

Intercurrent infection

The flu vaccine programme starts in late September and generally runs through to mid-December. During this period there will be a number of circulating respiratory-borne pathogens: in my own district last year we experienced outbreaks of respiratory syncytial virus (RSV) and parainfluenza, both of which occurred against a background of endemic Streptococcus pneumoniae and Haemophilus influenzae infections. Some people you vaccinate will therefore, by chance, be incubating one of these infections.

Clearly if the vaccine programme extends into December or even January, then the vaccinated individual may be incubating wild influenza since the outbreak in the UK usually starts in mid to late December and runs through to early March (although last year there were some confirmed cases of influenza B infection in April).

Low protective efficacy

 · Age effects The vaccine's ability to prevent flu after exposure to wild viruses is age related. In young children and adolescents the overall protective efficacy is about 90 per cent, but in the elderly it is lower (around 65 per cent in some for B and H1N1 strains and 30 per cent for H3N2 strains). This means that vaccinating the elderly with current vaccines may not prevent a proportion being infected with one of the circulating influenza viruses. However, vaccination also modifies disease severity even if it fails to prevent disease development and data clearly shows it substantially reduces the risk of death or developing complications like pneumonia. The fact that flu can still occur after someone is vaccinated (particularly the elderly) is a point that is often unappreciated by both professionals and the public alike ­ use of the recently licensed virosomal influenza vaccine may be one approach to overcoming this problem.

 · Drift and lack of homology with the vaccine strain Flu vaccines contain the haemagglutinins and neuraminidases from three viruses. Genetic drift among influenza viruses means that vaccine composition has to change each year and this is monitored by the 112 national influenza surveillance centres throughout the world: vaccine composition is based on the circulating types isolated during the previous year. Evidence shows that in nine years out of 10, the vaccine strains and the predicted circulating viruses are very similar so vaccine efficacy is very high: in the one year when the vaccine composition and circulating types are less homologous then efficacy drops and more people (especially the elderly) will develop flu if exposed.

 · Post-vaccination flu The frequency of occurrence of post-vaccination adverse events clearly influences vaccine uptake. Randomised placebo-controlled vaccine trials show that about three-quarters of all vaccinated people experience no side-effects at all: the most common adverse event is pain at the injection site (about 12 per cent): around 5 per cent will report mild body or muscle aches and a similar number weakness, nausea or vomiting: only one in 50 people will develop a fever. Data shows no difference in adverse events frequency in the elderly with underlying co-morbid conditions.

 · Oculo-respiratory syndrome (ORS) ORS was first reported from Canada in 2000 in recipients of a particular influenza vaccine: the cause was not established with certainty but may have resulted from unexpected aggregation of viral particles during manufacturing. Features of ORS include facial oedema, red eyes, wheezing, difficulty breathing and swallowing. Onset is usually within three to five hours of immunisation but a review of clinical features, time onset and skin testing results have shown that it is not an anaphylactic reaction and individuals who experience ORS can be revaccinated with influenza vaccine without risk of anaphylaxis although 5-34 per cent will have another episode of ORS.

Influenza vaccination is a very safe and highly effective intervention programme reducing morbidity and mortality. Flu can occur after influenza immunisation, but it is not due to the vaccine itself, rather intercurrent infection with different pathogens or lower protective efficacy associated with an impaired response in older people are the two most common explanations for this.

Extension of the programme to the

at-risk under-65s on registers will increase the undoubted benefits.

Influenza viruses are divided into A, B and C types. Type C may or may not be a human pathogen but if it is then the resulting illness is only a mild URTI. The A and B types cause the endemic disease seen every winter, which, in the absence of immunisation affects between 10 and 20 per cent of the community ­ such illness is associated, directly or indirectly, with about 20,000 to 30,000 excess deaths.

The vaccination programme targets all people aged 65 years and over, and those under 65 in an at-risk group (chronic lung disease, chronic cardiovascular disease, metabolic disorders, asplenic or hyposplenic patients, the immunosuppressed, institutionalised people and health care workers).

Current vaccines contain antigens from three viruses ­ two A strains (H3N2 and H1N1) and a B strain. Vaccine efficacy (as determined by antibody titres) may vary with strain ­ one study from Italy reported a lower seroconversion rate with H3N2 preparations than B or H1N1 strains.

Vaccine composition is determined from the virus strains isolated during the previous winter by the various influenza reference laboratories throughout the world. The recommended composition in the northern hemisphere for the coming winter is:

 · A/New Caledonia/20/99 [an H1N1 strain]

 · A/Moscow/10/99 [an H3N2 strain]

 · B/Hong Kong/330/2001-like virus

Virosomal influenza vaccines have the neuraminidase and haemagglutinin surface proteins embedded in a 'virus-like' structure made of phospholipid molecules: this formulation is theoretically more immunogenic than the traditional vaccines since the 'virus-like' particle should be endocytosed in antigen presenting cells leading to a cytotoxic T cell response in addition to an antibody response.

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