Two chikungunya vaccines are licensed, but a safety signal splits regulators

For the first time, there are licensed vaccines against chikungunya — two of them — and their trial data are genuinely strong: in the pivotal study of the live-attenuated vaccine, a single shot produced protective neutralising-antibody levels in 98.9% of recipients within a month [s1]. The open question is not whether the vaccines work but whether one of them is safe enough for the group that needs protection most: older adults, in whom post-marketing reports of serious adverse events have pushed regulators to different conclusions [s3].

That tension — high efficacy against a disease that is rarely fatal but often disabling, set against a safety signal concentrated in the elderly — is what makes chikungunya vaccination a harder call than the trial numbers alone suggest.

Why a vaccine now

Chikungunya is spreading into places that had little or no recent experience of it. Between 1 January and 30 September 2025, WHO recorded 445,271 suspected and confirmed cases and 155 deaths across 40 countries, including both locally acquired and travel-imported infections, in what it described as a resurgence [s2]. The virus is carried by Aedes mosquitoes, and its potential range is far larger than its current one: before 2025, autochthonous transmission had been reported from 119 countries and territories, and a further 27 countries have established Aedes aegypti populations but have not yet reported local transmission [s2].

Chikungunya rarely kills, but it is not mild. The acute illness is fever and severe joint pain, and a substantial minority of patients develop arthralgia that persists for months. Until recently, prevention rested entirely on vector control and bite avoidance [s3]. That context — an expanding, debilitating, hard-to-control arbovirus — is our October 2025 read of the global chikungunya situation, and it is what the vaccines are meant to change.

What the trials showed

The live-attenuated vaccine (marketed as Ixchiq) was tested in a double-blind, randomised, placebo-controlled phase 3 trial across 43 US sites [s1]. Of 4,128 participants randomised 3:1, 3,093 received the vaccine and 1,035 received placebo [s1]. Among the immunogenicity subset, a single dose induced seroprotective neutralising-antibody levels — a µPRNT50 titre of at least 150 — in 263 of 266 recipients, or 98.9% (95% CI 96.7-99.8), 28 days after vaccination, with safety and antibody data followed to day 180 [s1].

That is a surrogate endpoint, not a demonstration that vaccinated people avoid disease during an outbreak — the trial measured antibodies, because running a placebo-controlled efficacy trial against an unpredictable arbovirus is impractical. Regulators accepted the antibody threshold as a correlate of protection, which is how both products reached the market [s3]. The second vaccine, a virus-like-particle product (Vimkunya), was licensed on similar immunogenicity grounds, with high neutralising-antibody responses reported in its trials [s3].

The safety divergence

The complication is specific to the live-attenuated vaccine. Because it contains a weakened but replicating virus, it can in principle cause a chikungunya-like illness — and post-marketing safety signals, particularly in older adults, have emerged since licensure [s3]. Those reports have led to divergent regulatory decisions between authorities, with some restricting use of the live vaccine while the virus-like-particle vaccine, which contains no live virus, is not subject to the same concern [s3].

Europe's medicines regulator is among those that narrowed the live vaccine's use; the specifics of that decision are in our coverage of the June 2026 CHMP opinion. The awkwardness is clinical, not just bureaucratic: severe chikungunya and long-lasting joint disease fall disproportionately on older adults and people with underlying conditions, so a safety signal that lands hardest in the elderly collides directly with the population a vaccination programme would most want to reach.

What this means for a reader

This is not medical advice, and the risk-benefit calculation is genuinely individual — it depends on age, health, and how much chikungunya exposure a person actually faces [s3]. The honest summary is that the vaccines are a real advance with an unfinished evidence base: immunogenicity is established, but long-term protection, the durability of antibody responses, and safety in specific populations remain incompletely characterised, and some other candidates in development have been discontinued [s3].

The framing to resist is the one the arrival of any new vaccine invites — that a licensed product settles the question. Here it reopens one. The tool exists; matching it to the people who benefit without exposing the wrong ones to harm is the work that licensure did not finish.

What to watch

Whether real-world effectiveness data — not just antibody titres — accumulate from the countries now in outbreak, and whether the older-adult safety signal is quantified precisely enough to define who should get the live vaccine, who should get the virus-like-particle one, and who should get neither.

Sources

• Safety and immunogenicity of a single-shot live-attenuated chikungunya vaccine: a double-blind, multicentre, randomised, placebo-controlled, phase 3 trial — The Lancet, 2023-06-12
• Chikungunya virus disease - Global situation — World Health Organization, 2025-10-03
• Chikungunya Vaccines in Travel Medicine: From Regulatory Approval to Real-World Challenges — Tropical Medicine and Infectious Disease, 2026-08-07