Influenza

Influenza viruses belong to the family Orthomyxoviridae and have a single-stranded segmented RNA genome. The influenza viruses are classified into types A, B, and C on the basis of their core proteins. Type A viruses are further subdivided according to their envelope glycoproteins with haemagglutinin (HA) or neuraminidase (NA) activity. Characteristic of many RNA genome viruses, influenza virus undergoes high mutation rates and frequent genetic reassortment (combination and rearrangement of genetic material) leading to variability in HA and NA antigens. Minor changes in the protein structure in influenza A strains ("antigenic drift") occur frequently, enabling the virus to cause repetitive influenza outbreaks by evading immune recognition. Major changes in the influenza type A HA antigen ("antigenic shift") are caused by reassortment from different influenza A subtypes, such as between animal and human subtypes, and in rare events, such shifted viruses can result in strains capable of causing large regional or global pandemic outbreaks. Influenza B and C viruses mainly affect humans, whereas influenza A viruses infect a range of mammalian and avian species. Only type A and B cause human disease of any concern.

Influenza virus is transmitted primarily by droplets or respiratory secretions of infected persons. Influenza occurs all over the world, with an annual global attack rate estimated at 5 – 10% in adults and 20 – 30% in children. Influenza is associated with considerable economic burden arising from health-care costs, lost days of work or education, and general social disruption across all age groups. Secondary bacterial pneumonia is a frequent complication of influenza infection, particularly in elderly people and individuals with certain chronic diseases, resulting in a significant level of morbidity and mortality. An influenza pandemic is a rare but recurrent event.

Protection against clinical disease is mainly conferred by serum antibodies, whereas mucosal IgA antibodies contribute to resistance against infection. HA is the major antigenic target of neutralizing antibodies. However, due to antigenic drift and antigenic shift, the protective effect of antibody induced by one strain may be reduced or lost as a function of time, resulting in individuals being relatively or completely unprotected against the new strains in circulation.

Influenza vaccines

The World Health Organization reviews the world epidemiological situation twice annually and if necessary recommends new vaccine strain(s) in accordance with the available evidence. In general, seasonal influenza vaccines are trivalent, containing a mixture of influenza A and B strains thought most likely to circulate in the coming season. However, monovalent vaccines have been produced against candidate pandemic strains. It is now common practice to use reassortant strains for production that give high yields of the appropriate surface antigens. Reassortant strains for vaccine production have the surface glycoproteins (HA and NA) of the circulating epidemic virus but the internal proteins of a standardized production strain, eliminating much of the risks associated with handling pathogenic strains. The virus is grown in chick embryos or cell cultures for the production of vaccines. Due to the need to rapidly manufacture new vaccines in response to the likely strains that are identified, a unique set of regulatory requirements must be applied to the development, testing, and batch release of both seasonal and pandemic response influenza vaccines. Two types of influenza vaccine are available, an inactivated (killed) preparation that is injected and an attenuated influenza vaccine normally delivered nasally.

There are three types of inactivated vaccines, the whole virus vaccines, split virus vaccines, and subunit vaccines. In split virus vaccines, the virus has been disrupted by a detergent. In subunit vaccines, HA and NA have been further purified by removal of other viral components. Some formulations include adjuvants and most multidose vials contain the preservative thiomersal. Live, attenuated influenza vaccines have been based on a temperature-sensitive variant vaccine virus strains that replicate well in the nasopharynx but poorly in the lower respiratory tract.

Influenza Vaccine Standardization

Written Standards

Inactivated vaccines

WHO recommendations for the production and quality control of inactivated influenza vaccines were first developed in 1967 and subsequently revised in 1978, 1990 and 2003. The most recent revision takes into consideration the use of mammalian cells for production, the use of adjuvants, the development of reverse genetics for the generation of vaccine viruses, and increased levels of pandemic planning.

Live attenuated vaccines

WHO recommendations for the production and quality control of live attenuated influenza vaccines were formulated in 1978 to take into account the increased interest in immunization using live attenuated viruses introduced by the natural mode of infection. These recommendations were subsequently revised and adopted by the ECBS at their 60th meeting in 2009.

Pandemic vaccines

Guidance to provide National Regulatory Authorities (NRAs) and vaccine manufacturers with state-of-the-art advice concerning regulatory pathways for pandemic influenza vaccines, regulatory considerations to take into account in evaluating the quality, safety and efficacy of vaccine candidates and requirements for effective post-marketing surveillance of pandemic influenza vaccines were endorsed by ECBS in 2007 and in 2016.

Careful risk assessment and strict biosafety and biosecurity precautions are needed in laboratory and manufacturing environments in order to ensure the safe handling of human pandemic influenza viruses, candidate vaccine viruses (CVVs) and influenza viruses with pandemic potential (IVPP) as the uncontrolled release of such viruses could have a significant impact on public health. In 2007, the WHO biosafety risk assessment and guidelines for the production and quality control of human influenza pandemic vaccines were published in response to the pandemic threat posed by highly pathogenic avian influenza (HPAI) A(H5N1) viruses and the need to begin vaccine development. Since then, experience in the use of both IVPP and pandemic viruses in the development and production of CVVs has increased globally. Moreover, in response to the 2009 pandemic caused by the A(H1N1)pdm09 subtype virus and the emergence of low pathogenic avian influenza (LPAI) A(H7N9) viruses that are able to infect humans and cause severe disease with a high case fatality rate, the 2007 guidance was updated on two occasions by WHO. In addition, several WHO consultations – including the biannual WHO Vaccine Composition Meetings, the Global Action Plan for Influenza Vaccines meetings and “switch” meetings on influenza vaccine response at the start of a pandemic – identified the testing timelines for CVVs as one of the bottlenecks to rapid vaccine responses. In light of these and other developments, requests were made to WHO by industry, regulators and laboratories of the WHO Global Influenza Surveillance and Response System (GISRS) to undertake a revision of the 2007 guidance. Following series consultation process, the updated guidelines were endorsed by ECBS in 2018, published as annex 3 in TRS 1016.

Reference materials

Due to the need for formulations that match the currently circulating influenza strains, candidate-vaccine viruses (production viruses) matching those recommended for inclusion in seasonal and pandemic vaccines are produced and distributed on demand to qualified institutions by the WHO Collaborating Centers for Reference and Research on influenza.

Reference antigens and antiserum reagents for the standardization of vaccines are produced by several laboratories. These are not established as WHO reference materials by the Expert Committee on Biological Standardization (ECBS) due to time constraints.

Generic protocol for the calibration of seasonal/pandemic influenza antigen working reagents by WHO Essential Regulatory Laboratories

This document provides a generic protocol for the calibration of influenza antigen working reagents by the four WHO Essential Regulatory Laboratories (ERLs). It represents the consensus of the ERLs on the process of assigning a potency value to a newly established influenza antigen reagent for use in potency testing of inactivated influenza vaccines. An influenza antigen working (or reference) reagent is a preparation of inactivated whole virus that has been freeze-dried and calibrated as outlined in this document.

The calibration process involves the preparation of a primary liquid standard (PLS) and a large batch of freeze-dried antigen by one of the ERLs. The PLS is distributed to all other ERLs for independent calibration. Samples of the freeze-dried antigen are also distributed to the other ERLs and are calibrated against the PLS using SRD.

International Reference Preparations Catalogue

Prequalified Influenza vaccines

Influenza vaccines, both pandemic and seasonal, are prequalified for procurement by UN organizations:

From March 2016 there is a new system for display of WHO prequalified vaccines

List of Prequalified Vaccines

Find out more about the new listing and how to search and display the list on the link below.

1 March 2016

Web Listing of WHO Prequalified Vaccines