Flu vaccine

Influenza vaccines are prepared using virus strains in line with the WHO recommendations.

• Some inactivated influenza vaccines are trivalent, containing two subtypes of influenza A and one B virus; however, quadrivalent vaccines that contain an additional B virus have now become routinely available (1).

All but one of the influenza vaccines available in the UK are inactivated and cannot cause clinical influenza in those that are vaccinated (1).

• The live attenuated influenza vaccine (LAIV) 'Fluenz Tetra' contains live viruses that have been attenuated (weakened) and adapted to cold so that they can only replicate at the lower temperatures found in the nasal passage.
  • Cannot replicate elsewhere but may cause mild coryzal symptoms.
    
• Administration:
  • the inactivated vaccines are administered by intramuscular injection preferably in the upper arm or anterolateral thigh in infants
  • LAIV is administered by the intranasal route
  • Inactivated influenza vaccines can be given at the same time as other vaccines
    
• Most of the vaccines are prepared from viruses grown in embryonated hens' eggs.
• Influenza vaccine should be offered to:
  • all those aged 65 years or older
  • all those aged six months or older
  • children not in clinical risk groups that are eligible for vaccination
• If a new influenza A subtype were to emerge with epidemic or pandemic potential (as occurred in 2009 with influenza A(H1N1)v), it is unlikely that the influenza vaccine will be well matched to the emerging strain.
  • As occurred during the second wave of the 2009 pandemic, a monovalent vaccine against that strain may be developed and implemented.
    
• After immunisation, protective immune responses may be achieved within 14 days.

A systematic review concerning the efficacy of influenza vaccination concluded that (2):

• influenza vaccines are effective in reducing serologically confirmed cases of influenza
• influenza vaccines are not as effective in reducing cases of clinical influenza and number of working days lost
• universal immunisation of healthy adults is not supported by the results of this review

A systematic review concerning the efficacy of influenza vaccination in patients with chronic obstructive pulmonary disease (COPD) concluded that (3):

• inactivated vaccine reduces exacerbations in COPD patients
• there was no evidence of an effect of intranasal live attenuated virus when this was added to inactivated intramuscular vaccination

With specific reference to use of the influenza vaccine in healthy children (4):

• in healthy children >2 years of age, both live attenuated vaccines and inactivated vaccines can probably reduce influenza and influenza-like illness over a single influenza season

Check uptodate details in the The Green Book before prescribing/administering an influenza vaccination.

Check the Summary of Product Characteristics (SPC) of the influenza vaccination before prescribing/administering an influenza vaccine.

Note: the live attenuated influenza vaccine (Fluenz) should not be given to children or adolescents who are clinically severely immunodeficient due to conditions or immunosuppressive therapy (1).

References:

1. Immunisation Against Infectious Disease - "The Green Book". Chapter 19. Influenza (October 2020)
2. Demicheli V et al. Vaccines for preventing influenza in healthy adults. Cochrane Database Syst Rev 2018;(2):CD001269.
3. Poole PJ et al. Influenza vaccine for patients with chronic obstructive pulmonary disease. Cochrane Database Syst Rev 2018;(6):CD002733
4. Demicheli V et al. Vaccines for preventing influenza in healthy children. Cochrane Database Syst Rev 2018;2(2):CD004879.