AE Reports Prompt FDA, CDC to Hit Pause on Ixchiq Vaccine

Originally published by our sister publication Infectious Disease Special Edition

By Fran Kritz

This story was updated on May 15 at 4:45 p.m. The CDC director accepted the ACIP recommendations.

The FDA and CDC recently issued a joint advisory statement recommending a pause in the use of the chikungunya vaccine, live (Ixchiq, Valneva SE), in people 60 years of age and older, while they investigate reports of serious adverse events (AEs). 

Ixchiq, approved by the FDA in November 2023, is the first of two vaccines recently approved by the FDA to prevent chikungunya. In February, the FDA approved the chikungunya vaccine, recombinant (Vimkunya, Bavarian Nordic), which was recommended by the CDC Advisory Committee on Immunization Practices (ACIP) in April. Vimkunya has not been paused by either agency.

Chikungunya is a virus that spreads to humans through bites primarily from infected Aedes aegypti and A. albopictus mosquitoes. There is no approved medication to treat the virus.
According to the WHO, chikungunya is not usually fatal and resolves on its own within a few weeks. Symptoms can include fever and severe joint pain—which can last for months to years—joint swelling, muscle pain, headache, nausea, fatigue and rash.

The U.S. pause on the vaccine follows a CDC alert in March 2025 that the agency was investigating five hospitalizations in older adults who received the vaccine and an announcement on May 7 by the European Medicines Agency that it had started a review of the vaccine and halted its use in people 65 and older.

According to the CDA/FDA advisory, as of May 7, 2025, 17 serious AEs, including two that resulted in death, have been reported globally in people 62 through 89 years of age who received Ixchiq.

Six of the reports were from the United States, and most of the serious AEs reported to the Vaccine Adverse Event Reporting System, which is comanaged by the FDA and CDC, have been in people with underlying chronic medical conditions, often on multiple medications. About 80,000 doses of Ixchiq have been distributed globally, according to the FDA.

The 17 post-marketing reports on Ixchiq’s severe AEs “are consistent with severe complications of chikungunya disease, which suggests that some of those vaccinated with the live vaccine had vaccine virus replication in the central nervous system,” said David O. Freedman, MD, a professor emeritus of infectious diseases at the University of Alabama at Birmingham.

At least three post-marketing studies are ongoing, although one is in pregnant women, according to ACIP.

The six U.S. cases of severe AEs were presented during an April 2025 ACIP meeting. The committee found that association of the vaccine with severe AEs is “plausible but causal association for each event was not determined.”

The six cases were all in men, ranging in ages from 67 to 86, and all had underlying health conditions:

1. an 83-year-old with coronary artery disease (CAD), congestive heart failure (CHF) and hypertension who was discharged with a diagnosis of encephalopathy and generalized weakness;
2. a 77-year-old with CHF and hypertension who was discharged with a diagnosis of acute metabolic encephalopathy; 
3. an 86-year-old with diabetes, CHF and hypertension with a diagnosis of toxic metabolic encephalopathy at discharge;
4. a 68-year-old with prostate cancer and hypertension who was diagnosed with meningismus/aseptic meningitis;
5. a 67-year-old with hyperlipidemia who was diagnosed with atrial flutter with rapid ventricular response at discharge and suspected small non-ST segment elevation myocardial infarction; and
6. a 74-year-old with ischemic cardiomyopathy, hypotension, CAD, chronic leukopenia and chronic thrombocytopenia who was diagnosed with an episode of worsened and prolonged hypotension at discharge on the background of preexisting cardiomyopathy and hypotension.
   

The FDA plans to conduct an updated benefit–risk assessment of Ixchiq in 60 individuals and continue the evaluation of post-marketing safety reports.
Choices for Those at Risk

Providers who want to vaccinate older adults against chikungunya retain an option with Vimkunya, Dr. Freedman noted. “The current pause only applies to Ixchiq,” Dr. Freedman said. Vimkunya is not a live vaccine; it’s a virus-like particle (VLP) vaccine, which uses VLPs designed to mimic the chikungunya virus without the ability to infect cells or replicate/cause disease, he explained.

Last month, ACIP recommended the vaccine for people 12 and older traveling to a country or territory with a chikungunya outbreak, who plan an extended stay where an outbreak could occur and for lab workers at risk for exposure. The CDC director adopted the recommendation on May 15, 2025, and they will be published in an upcoming Morbidity and Mortality Weekly Report.

The nominated CDC director has not yet adopted this recommendation. Regardless of the CDC recommendation or FDA indication, most health insurers do not pay for travel vaccines, even if they have a full CDC nod. Still, Vimkunya, which is FDA approved for any provider to give, remains an option that people, including older adults, can pay for out of pocket.

Dr. Freedman reported no relevant financial disclosures.