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 Unfortunately, the influenza vaccines in use were matched to earlier strains that had been superceded by the K variant.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
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