VIROLOGY
of influenza vaccine for the following northern hemisphere (NH) and southern hemisphere (SH) winter seasons are taken twice a year by a WHO scientific commitee in February and September respectively. Vaccines are formulated to contain the haemagglutinin (HA) protein of circulating strains. The HA is the major viral surface antigen target of the human immune response, alongside the other major virus surface protein, viral neuraminidase (NA). Vaccines primarily induce antibodies to viral HA. These antibodies provide protective immunity against closely related strains. As a result of the rapid evolution of influenza virus, it is necessary to frequently update the strains which are included in seasonal vaccines, so that the antibodies induced by vaccines are well matched to ‘cover’ circulating strains. The necessity for frequent updates to influenza vaccine composition was first recognised in the 1940s when it was realised that the virus was rapidly changing. This led to the creation of an international surveillance network under the auspices of the World Health Organization (WHO) in 1952, now called the Global Influenza Surveillance and Response System (GISRS). Today this comprises of 157 National INfluenza Centres in 138 countries and provides detailed information about the diversity of circulating strains. Seasonal influenza vaccines are
trivalent. The current composition of seasonal influenza vaccines includes three protein antigens, representative of circulating virus strains causing disease in the human population. Currently, the two influenza A virus strains circulating in the human population are influenza A(H1N1) pdm09 and A(H3N2). Until very recently, two different lineages of influenza B (B/ Yamagata and B/Victoria), also circulated (and were included in a quadrivalent vaccine) but B/Yamagata lineage viruses have not been detected since March 2020, and this lineage is now considered extinct, which has caused a shift from quadrivalent to trivalent vaccines. There is an approximately eight-month
WHO Global Influenza Surveillance and Response System (GISRS).
gap between decisions about what strains should be included in the next winter season vaccine and newly manufactured vaccines being delivered to the population. Unfortunately, the time lag between strain selection and vaccine delivery means that, occasionally, new influenza variants arise and dominate, whilst the chosen vaccine strains become outdated, as has occurred this year.
Global surveillance: reaction versus prediction Efforts have intensified over recent decades to improve global surveillance through several different approaches. These include year-round surveillance efforts, even in countries which experience a strongly seasonal pattern of influenza; the application of rapid and high throughput sequencing technologies for the characterisation of more influenza virus genomes; an expansion of viruses which are studied in detail for their phenotypic properties; and the use of advanced analytical tools to develop better predictive models to improve forecasting about the potential behaviour of viral lineages.
K variant viruses were first detected
in June 2025 through surveillance of returning travellers to the United States. They were subsequently seen with increasing frequency in countries around the world until they came to dominate circulating H3N2 viruses by September and October 2025. K virus circulation led to a late and prolonged tail of epidemic activity in some SH countries, such as Australia, and an early influenza season in NH countries such as Japan and the United Kingdom.
Antigenic drift variants The K variant is described as an antigenic drift variant of influenza A. Such variants arise frequently and may be seen on a regular basis, accounting for a small proportion (<10%) of circulating strains. It is impossible, with our current state of knowledge, to know which antigenic variants, seen at a single point in time with low frequency, may emerge to become the next dominant strain. Analysis of growth frequency of diverse viral lineages over short periods of time helps to provide insights into fitness, or competitive advantage that some lineages have over others. Input data for vaccine composition
The successful delivery of influenza vaccine campaigns every year is a major feat of
coordination. Unsurprisingly, this complex operation can occasionally be derailed by unwelcome surprises
updates includes: Epidemiological and clinical data. This data summarises where recent epidemics are occurring, and whether they are unusual in magnitude or disease severity. National epidemiological information ( affected age groups, severity) is reported regionally to WHO; for example by the European Respiratory Virus Surveillance Summary (ERVISS) providing insights as to the clinical
May 2026
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