Immunity following influenza disease and administration of influenza vaccines

For infants the first encounter with influenza viruses commonly occurs in their first or second winter season. Subsequently, everyone acquires multiple influenza infections throughout life. It is expected that up to ~15% of a European population in a temperate climate is infected with influenza in any winter season with higher percentages in children and lower in older people [1].

Whether individuals fall ill when they are infected, is dependent on a number of factors. These include previous exposure to a similar influenza virus that has induced a complete or partial protective immunity to the now circulating virus or exposure through vaccination with an updated matching influenza vaccine strain.

Commonly, the youngest children and older adult individuals are most affected by severe seasonal influenza infections each year. Paradoxically older adults are less likely to be infected than children but, when they are infected, these older adults are more likely to suffr from severe disease [2].

Evaluation of immune response to influenza vaccines in the EU

EU guidelines on observational studies evaluating product-specific effectiveness provide recommendations and scientific considerations related to characterisation of the immune response and immunogenicity issues, pre-authorisation clinical studies of protective efficacy and/or post-authorisation studies of vaccine effectiveness, pre- and post-authorisation safety studies and pharmacovigilance plans [3].

The 2014 Interim guidance on enhanced safety surveillance for seasonal influenza vaccines additionally sets out specific criteria for annual enhanced safety surveillance to rapidly detect any increased local and systemic reactogenicity, or other unexpected adverse immune response that may arise during the influenza vaccine product life-cycle [4].


  1. WHO: Joint statement - Influenza season epidemic kicks off early in Europe as concerns over RSV rise and COVID-19 is still a threat (,2022.
  2. Hayward AC, Fragaszy EB, Bermingham A, Wang L, Copas A, Edmunds WJ, Ferguson N, Goonetilleke N, Harvey G, Kovar J, Lim MS, McMichael A, Millett ER, Nguyen-Van-Tam JS, Nazareth I, Pebody R, Tabassum F, Watson JM, Wurie FB, Johnson AM, Zambon M; Flu Watch Group. Comparative community burden and severity of seasonal and pandemic influenza: results of the Flu Watch cohort study. Lancet Respir Med. 2014 Jun;2(6):445-54. doi: 10.1016/S2213-2600(14)70034-7.
  3. EMA: Guideline on Influenza Vaccines Non-clinical and Clinical Module, 2016.
  4. EMA: Interim guidance on enhanced safety surveillance for seasonal influenza vaccines in the EU, 2014.
Page last updated 9 Mar 2023