Influenza World Health Care Professionals

Vaccination is the primary approach for preventing seasonal influenza and reducing its health consequences. Not only does vaccination decrease the risk of influenza infection, it also prevents the spread of infection through the population by providing herd immunity. [1] Influenza prevention strategies generally focus on vaccinating vulnerable, high-risk groups, their caregivers and healthcare professionals, though recommendations for vaccination vary between countries. [2, 3]

Vaccine composition

Three types of inactivated influenza vaccines are currently available, all of which are grown in eggs. [1, 4]

• Whole virion vaccines consisting of complete inactivated viruses.
• Split-product vaccines in which virus particles are disrupted by detergent treatment.
• Purified surface-antigen vaccines whereby surface antigens are formulated into a liposome-based vesicle carrier system.
  

Use of such vaccines is effective in reducing the morbidity and associated complications of influenza. [5, 6] However, influenza vaccines are strain specific and, in order to be effective, must match the circulating strains of influenza virus. Viral mutation means that the influenza virus changes constantly, and so new vaccines must be formulated every year. [1]

The World Health Organization (WHO) Global Influenza Surveillance Network issues recommendations for vaccine content: in February for the northern hemisphere and in September for the southern hemisphere. [3] By surveying the predominant viral strains circulating at any given time, the WHO is able to predict which strains may be expected to cause epidemics, and recommends alterations to the antigenic composition of the vaccine accordingly.

Efficacy

Vaccines containing strains matched to those predominantly circulating are reported to protect against laboratory-confirmed influenza in 70–90% of healthy young adults, [3] though protection may be as low as 23–58% in elderly people due to their impaired immune response. [7, 8] In the elderly living in the community, vaccination reduces hospitalisation for any respiratory condition by 17–39% and due to pneumonia by
19–57%, while reducing mortality by 27–75%. [5]

Side-effects of vaccination

As the majority of commercially available vaccines are inactivated, vaccination cannot cause influenza. The most frequent side-effect observed following influenza vaccination is injection site soreness, which generally lasts around 2 days. [1] Other adverse events include fever, malaise, myalgia and headaches, which usually start 6–12 hours after vaccination and last up to 2 days. [1]

Influenza vaccines are contraindicated in people allergic to egg proteins and should not be given to children under 6 months old or those with an acute illness. [1]

It should be noted that whole virion vaccines can produce febrile reactions and are used infrequently.

Limitations of vaccination

Vaccination can greatly reduce the impact of influenza, though it does not always prevent infection and has limitations.

• Antigenic drift in the wild-type virus may occur after the vaccine has been formulated, rendering the vaccine less protective or even ineffective. [9] [Link to Antigenic drift in About influenza]
• Effectiveness of vaccination programmes can vary widely because of low uptake rates, [10] the age and immunocompetence of the recipient [5] and mismatch between wild-type and vaccine strains. [11]

Vaccines protect against up to three strains of influenza virus in any year. However, should different strains be circulating locally, people will not always be protected.

Despite current recommendations, the uptake rate for influenza vaccination remains relatively low which, together with the fact that vaccination is not always effective, means that outbreaks of influenza still occur every year. Therefore, other effective prophylactic and treatment options are required.

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