VIROLOGY
Influenza vaccines: the intricacies of tracking a moving target
Influenza vaccine campaigns take place twice each year to protect against this rapidly changing virus. Here, Maria Zambon describes the worldwide process of preparing and coordinating such vaccines, as well as how new strains can have an unwelcome impact.
Close to a billion doses of seasonal influenza vaccine are used throughout the world every year. Most of these vaccines are given within a very compressed time frame of 6-12 weeks, to ensure maximum vaccine coverage in the most clinically vulnerable, ahead of winter season when influenza circulates. The successful delivery of annual influenza vaccine campaigns is a major feat of coordination. Unsurprisingly, this complex operation can occasionally be derailed by unwelcome
surprises. We have recently seen an example during the northern hemisphere (NH) winter of 2025/26 where a newly recognised viral strain, known as the K variant, became the dominant circulating influenza A(H3N2) virus. Unfortunately, the influenza vaccines in use were matched to earlier strains that had been superceded by the K variant. This situation gave rise to lurid
headlines about a new ‘super flu’ and media speculation about the likely
disastrous health service impact of this situation, which has mostly turned out to be misleading overstatement. How on earth is this situation normally avoided, and what has been the impact of a so called ‘vaccine mismatch’ this winter? The translation of information about what is circulating globally into the decisions about what is included in manufactured vaccines is the subject of this article.
Vaccine preparation Manufacturing influenza vaccine is an extraordinarily complex process undertaken largely by multi-national pharmaceutical companies; though in recent years, the capacity for government sector vaccine manufacture has expanded considerably in parts of the world, eg India. The production process of all influenza vaccine manufacturers needs to be strongly linked to public sector surveillance programmes, to ensure that the most appropriate strains are included in vaccine preparations, and there is consistency in what is included in vaccines in different parts of the world. National and regional surveillance programmes, publicly funded, track influenza strains to obtain the data about what is circulating and rapid virus evolution. Data from multiple sources are
WHO influenza vaccine strain composition commitee, London, February 2025. 32
WWW.PATHOLOGYINPRACTICE.COM May 2026
consolidated at a global level by the World Health Organization (WHO), to make recommendations about what should be included in the next vaccine formulation. Decisions about the strain composition
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