What real-world vaccine effectiveness estimates are emerging against subclade K from test-negative studies in England, Japan, Canada, and the United States?

1. England: test‑negative signal — strong protection in children, modest in adults

The clearest test‑negative evidence comes from the UK Health Security Agency’s early analysis: using emergency‑department and hospital attendance endpoints during a period dominated by H3N2 subclade K, vaccine effectiveness against medically attended illness and hospital attendance was reported at roughly 70–75% in children aged 2–17 and about 30–40% in adults — figures that the UKHSA authors and multiple outlets have repeatedly cited as comparable to typical end‑of‑season H3N2 VE in recent years ^{[1] [2] [4]}.

2. Why England’s numbers matter — antigenic mismatch but preserved clinical protection

Laboratory antigenic work showed reduced ferret serum reactivity to subclade K versus the vaccine reference strains — in some assays large fold‑reductions were seen — signaling antigenic drift ^[2]. Yet the UKHSA test‑negative clinical data indicate that diminished neutralization in the lab has not translated into a collapse of vaccine protection against severe or medically attended outcomes, particularly in children, a pattern also emphasised by Gavi and CDC summaries ^{[2] [1] [3]}.

3. Japan and Canada: circulation documented, VE estimates not yet robust

Both Japan and Canada have reported rapid dominance of subclade K in sequencing panels and early clinical concern, and Canadian teams published warnings that the strain is mismatched relative to the vaccine strain ^{[5] [6]}. However, none of the sources reviewed provide a peer‑reviewed, national test‑negative VE point estimate from Japan or Canada against subclade K equivalent to the UKHSA analysis; Canadian reporting has called for enhanced surveillance rather than offering a stable VE number ^{[5] [7]}.

4. United States: early surveillance, no definitive test‑negative VE against K yet

U.S. authorities and reporting have noted the spread of K into North America and that many U.S. H3N2 sequences include K, but available U.S. material cites the UKHSA estimates and offers modelling and caution rather than an independent, fully analysed test‑negative VE estimate for the U.S. population to date ^{[3] [8]}. Media summaries and CDC commentary have reiterated the England findings as provisional reassurance while warning that U.S. VE may change as the season progresses and as vaccine‑to‑infection interval lengthens ^{[3] [5]}.

5. Caveats, uncertainties, and alternative interpretations

These early VE figures come from test‑negative designs and preprints or surveillance reports early in the season; UKHSA’s work remains preliminary and partly based on periods when subclade K dominated sampling, and experts caution that VE can change as seasons mature, immunity wanes, and geographic patterns vary ^{[2] [5]}. Laboratory antigenic assays show reduced recognition of K by ferret antisera — a mechanistic reason to expect lower VE against infection — yet clinical endpoints (hospital attendance) track protection better for severe outcomes, creating a split between neutralization data and real‑world clinical protection ^{[2] [9]}. Canadian researchers and some commentators urge heightened surveillance because early UK estimates may not generalise or hold later in the season ^{[5] [7]}.

6. Bottom line: what the test‑negative studies say right now

Test‑negative surveillance to date gives a consistent picture from England that current vaccines still substantially reduce the risk of medically attended H3N2/subclade K illness in children (around 70–75%) and provide modest protection in adults (roughly 30–40%); analogous, directly measured test‑negative VE estimates for Japan, Canada and the United States are either not yet published or remain preliminary and reliant on extrapolation from UK data and sequence surveillance ^{[1] [2] [5] [3]}.