How Each Year's Flu Vaccine Strains Are Chosen and Why the Match Varies

Every year, the flu shot is a forecast. Roughly six months before a season starts, the World Health Organization examines global surveillance data and names the exact virus strains that manufacturers will grow, test, and bottle. Because influenza's surface proteins keep mutating during those months, the viruses that actually spread can drift away from the ones in the vaccine, which is why a well-matched season can prevent a large share of illness while a mismatched season offers thinner protection.

A decision made twice a year, months in advance

Strain selection runs through the Global Influenza Surveillance and Response System, a network of national laboratories and WHO Collaborating Centres that samples circulating viruses year round. Twice annually the WHO convenes experts to recommend vaccine composition: in February for the Northern Hemisphere and in September for the Southern Hemisphere, since the two halves of the world have opposite winters.

The lead time is dictated by manufacturing. Most influenza vaccine is still grown in chicken eggs, a process that takes months to scale, so the recommendation has to be finalized long before the first case of the coming season. For 2026-2027, the WHO issued its Northern Hemisphere recommendation on 27 February 2026, naming an A/Missouri/11/2025 (H1N1)pdm09-like virus, an A/Darwin/1454/2025 (H3N2)-like virus, and a B/Tokyo/EIS13-175/2025 (B/Victoria lineage)-like virus. Most seasonal vaccines are now trivalent, covering two influenza A subtypes and one B lineage, after the B/Yamagata lineage stopped circulating and was dropped.

What the ferrets tell us

Naming a strain is an exercise in measurement, not guesswork. To decide whether last year's vaccine virus still resembles what is spreading now, laboratories perform antigenic characterization: they test how well antibodies raised against the vaccine strain recognize newly collected viruses. The reference standard for that comparison is post-infection ferret antiserum. Ferrets are used because their respiratory response to human influenza mirrors ours closely, and their antisera are highly sensitive to small changes on the virus surface.

The workhorse assay is hemagglutination inhibition, which measures how strongly those antibodies block the virus from binding red blood cells. When circulating viruses react poorly with antisera against the current vaccine strain, that signals antigenic distance. By long-standing convention, a drop of roughly fourfold or greater in that reaction is treated as antigenically meaningful, a threshold meant to avoid swapping strains over differences too small to matter clinically. Genetic sequencing of the hemagglutinin gene runs alongside the antibody work, so evolutionary and antigenic signals are read together.

Why the match varies: antigenic drift

Influenza's two surface proteins, hemagglutinin and neuraminidase, tolerate a steady stream of point mutations. Antibodies from prior infection and vaccination apply selective pressure, and variants that partly escape that immunity gain an edge and spread. This gradual reshaping is antigenic drift, and it is the core reason the vaccine must be updated almost every year.

The trouble is timing. A drifted variant can emerge or take over after the composition is already locked and the eggs are already inoculated. Writing in the New England Journal of Medicine during the severe 2003-2004 season, John Treanor described exactly this trap: an antigenically variant H3N2 virus, A/Fujian/411/2002, rose to prominence after that year's vaccine strain had been chosen, producing a notable mismatch. The 2025-2026 season followed the same script. An H3N2 subclade, informally called subclade K, spread widely while being substantially drifted from the H3N2 virus in the vaccine, and ferret antisera raised against the season's vaccine strains showed reduced reactivity to it. Interim estimates put protection against H3N2 illness in the modest range for adults that season, and higher in children.

How to read a low-effectiveness season

A mismatch lowers effectiveness; it rarely erases it. The US Centers for Disease Control and Prevention notes that the more closely the vaccine resembles circulating viruses, the more robust the protection tends to be, and that even when the viruses are quite different, vaccination can still cut risk by as much as 30 percent. Two other points help interpret the headline numbers. First, the outcome being measured matters enormously: a vaccine can look far more effective against laboratory-confirmed influenza than against "all respiratory illness," because most winter respiratory illness is not influenza at all. Second, protection against severe outcomes such as hospitalization often holds up better than protection against milder infection, even in a drifted season.

So a season labeled "low effectiveness" usually means reduced protection against getting sick, not the absence of benefit, and the benefit is generally largest where it matters most. This is also why public health authorities continue to recommend annual vaccination even when a strong match cannot be guaranteed, since the downside of a drifted season is usually a smaller benefit rather than none. That distinction is easy to lose when a single percentage circulates in the news.

This article is educational and is not medical advice; decisions about vaccination belong with a qualified clinician who knows your history.