Britain had 73 chikungunya cases last summer. Not one came from a British mosquito.

Between January and June 2025, the UK Health Security Agency recorded 73 cases of chikungunya in England. In the same six months of 2024 it recorded 27. The annual figure has gone from 45 in 2023 to 112 in 2024 to a trajectory that, on the H1 evidence, was set to exceed 140 in 2025. Most of the patients had been to Sri Lanka, India or Mauritius. Most lived in London. Every single one of the 73 had been bitten on holiday.

That last sentence is the only one that matters.

Chikungunya is an arbovirus. It does not spread person-to-person. It needs a mosquito — specifically Aedes aegypti or Aedes albopictus, the latter known to British naturalists as the Asian tiger mosquito — to take blood from an infected human, incubate the virus for a week or so, and then bite somebody else. Without the vector in the right place at the right time, the chain breaks. Every imported case is a terminal node on the transmission graph. The patient suffers, recovers, and the virus dies with the antibodies.

Britain does not yet have Aedes albopictus in established populations. The Channel is doing the work the National Health Service has not yet had to do.

This will not last. The tiger mosquito has spent the last quarter-century colonising Europe at a rate that varies by year but never reverses. It is now resident in France, Italy, Spain, Germany, Austria, the Netherlands, Belgium, Switzerland and the Czech Republic, and was confirmed in coastal Brittany in early 2026 by the Observatoire de l'environnement en Bretagne — a region long considered too cool and too damp. The European Centre for Disease Prevention and Control's most recent chikungunya worldwide monthly update, published 30 March 2026 with data to 28 February, notes that all 2026 chikungunya clusters in France and Italy are now closed. Closed, but not impossible. The 2025 French season produced 805 locally transmitted cases — an order of magnitude above the cumulative European total of the previous decade. The 2026 European clusters were caught early. The vector is in place; only the introduction of a viraemic traveller is required.

England's 73 cases in the first half of 2025 are the introduction pool. Wales, Scotland and Northern Ireland reported few or none. London was the centre, predictably: the city with the highest density of returning travellers from active outbreak regions is the city with the highest exposure. Dr Philip Veal, Consultant in Public Health at UKHSA, put it on 14 August 2025 with the institutional understatement of his trade. "Chikungunya can be a nasty disease and we're seeing a worrying increase in cases among travellers returning to the UK."

It is worth being clear about what "a nasty disease" means in this context. Chikungunya does not kill many people. The case-fatality rate is well under one percent. What it does is hurt, for a long time. The name comes from a Makonde verb meaning "to become contorted" — a reference to the joint pain that characterises the acute phase. About a third of patients develop persistent polyarthralgia that can last for months or, in some patients, years. The Conversation's coverage of the 2025 data was written by Paul Hunter, Professor of Medicine at the University of East Anglia, who described it in plain terms: "All of these infections were associated with travel to regions which have had ongoing outbreaks of the virus, including Sri Lanka, India and Mauritius."

In July 2025 the Joint Committee on Vaccination and Immunisation issued formal advice on chikungunya vaccines for UK travellers. Two are licensed in Britain. Valneva's IXCHIQ is approved for adults aged 18 to 59; Bavarian Nordic's Vimkunya is approved from age 12. Both are single-dose; both are available privately through travel clinics, following a clinician's risk assessment. JCVI did not recommend either for the NHS schedule. The committee's reasoning was straightforward: the risk to the average traveller is low; the risk to specific travellers (older, going for extended stays, going to active outbreaks) is real and quantifiable; and a clinician-led private route gets the vaccine to the people who actually need it without diluting NHS capacity.

This is the unglamorous, defensible British answer. The Lancet Countdown on Health and Climate Change Europe 2026 puts the European dengue suitability indicator at +297% over the 1981–2010 baseline, with chikungunya, Zika and West Nile virus on similar curves. The biological fact of climate change has now run far ahead of British political attention to it. But the British public health architecture still works: a national surveillance system that publishes its numbers; a vaccination advisory committee that issues clear, evidence-based guidance with a year's warning; a travel-health website that names the destinations of risk; and a vector ecology that — by accident of geography and a few decades of grace — has not yet caught up with the rest of western Europe.

What an ordinary returning traveller can do about chikungunya in 2026 is not mysterious. Avoid bites in the destination country: long sleeves, long trousers, EPA-registered repellents (DEET, picaridin, IR3535, oil of lemon eucalyptus), permethrin-treated clothing, and a treated bed net under which to sleep. Look at the destination on the NaTHNaC TravelHealthPro site before booking. If you are over 65, immunocompromised, pregnant, or planning extended travel to an active outbreak country, raise it with a travel clinic — and consider the vaccine. The most important domestic action, for those returning, is to keep the next bite from happening: even though Aedes albopictus is not yet established in the UK, the principle of source reduction — emptying water from saucers under flowerpots, plant pots, gutters, paddling pools, disused buckets and birdbaths each week through the warm months — is what stops the mosquito from establishing in the first place. The geography is not a defence forever. It is a deadline.

What to watch in the next twelve months is therefore not the case count alone. It is whether ECDC's monthly chikungunya update logs any 2026 cluster that fails to close cleanly; whether the Observatoire de l'environnement en Bretagne or the Centre for Ecology and Hydrology confirms Aedes albopictus establishment north of the Channel; whether UKHSA's 2026 travel-health figures, due in late summer, run above the 2025 trajectory; and whether the JCVI revisits its advice once another year of European outbreak data is in. The first sustained autochthonous chikungunya cluster on British soil will not be a surprise to anyone reading the surveillance bulletins. It will be a surprise only to people who weren't.

For the moment, though, the British score for 2025 is the one Dr Veal recorded in August. Seventy-three patients, all of them home from somewhere warm, none of them the source of a second case. The vector absence — the simple fact that the right mosquito is not yet in the right back garden — did the work. It bought the public health system a year. It is the kind of buying-time-with-geography fortune that British policymakers are well advised to spend now, while it lasts.

What we know

• 73 chikungunya cases reported in England between January and June 2025, all travel-imported, predominantly from Sri Lanka, India and Mauritius. [UKHSA, 14 August 2025]
• 27 cases in the same six-month window of 2024; 112 cases full-year 2024; 45 cases full-year 2023. [UKHSA via GOV.UK]
• No autochthonous (locally-transmitted) chikungunya cases recorded in the UK in 2025 or to date in 2026; vector Aedes albopictus not yet established in established UK populations. [The Conversation / Hunter; ECDC monthly]
• The Joint Committee on Vaccination and Immunisation issued advice on chikungunya vaccines for UK travellers on 16 July 2025, naming IXCHIQ (Valneva, age 18–59) and Vimkunya (Bavarian Nordic, age 12+). Both available privately via travel clinics, neither on the NHS schedule. [JCVI / GOV.UK]
• All 2026 chikungunya clusters reported in mainland France and Italy are now closed, per the ECDC chikungunya worldwide monthly update of 30 March 2026 (data to 28 February 2026). [ECDC]

Sources cited

1. UK Health Security Agency — Rise in chikungunya cases in UK travellers returning from abroad. GOV.UK news release, 14 August 2025. https://www.gov.uk/government/news/rise-in-chikungunya-cases-in-uk-travellers-returning-from-abroad
2. UK Health Security Agency — What are the symptoms of the chikungunya virus and is there a vaccine? UKHSA Blog, 7 August 2025. https://ukhsa.blog.gov.uk/2025/08/07/what-are-the-symptoms-of-the-chikungunya-virus-and-is-there-a-vaccine/
3. Paul Hunter — Chikungunya: what UK travellers should know about this mosquito-borne virus. The Conversation, 19 August 2025. https://theconversation.com/chikungunya-what-uk-travellers-should-know-about-this-mosquito-borne-virus-263296
4. European Centre for Disease Prevention and Control — Chikungunya Worldwide Monthly Update. Published 30 March 2026; data to 28 February 2026. https://www.ecdc.europa.eu/en/chikungunya-monthly
5. Joint Committee on Vaccination and Immunisation — Chikungunya vaccine in UK travellers: JCVI advice. GOV.UK publication, 16 July 2025. https://www.gov.uk/government/publications/chikungunya-vaccine-for-uk-travellers-jcvi-advice-16-july-2025/chikungunya-vaccine-in-uk-travellers-jcvi-advice
6. Lancet Countdown on Health and Climate Change Europe — 2026 Report. Published 22 April 2026 in The Lancet Public Health. https://www.thelancet.com/journals/lanpub/article/PIIS2468-2667(26)00025-3/fulltext