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The Centers for Disease Control and Prevention (CDC) is currently looking into five cases of hospitalization involving individuals who recently received the chikungunya vaccine.

In an update shared on Tuesday through a vaccine information webpage directed at healthcare professionals, the CDC revealed that all affected individuals, aged 65 and over, were hospitalized due to cardiac or neurologic complications after being vaccinated with IXCHIQ, the sole chikungunya vaccine currently approved in the United States.

CNN has contacted the CDC for further insights.

Chikungunya is primarily transmitted through the bites of infected mosquitoes. While the vaccine is not commonly given to most adults, it is recommended for specific travelers who may be more susceptible to this mosquito-borne virus.

“This topic will be addressed in a forthcoming meeting of the Advisory Committee on Immunization Practices (ACIP). Healthcare providers should evaluate the risks and benefits of vaccination for travelers based on various factors including their age, destination, trip duration, and planned activities,” the online notice from the CDC stated.

Discussion regarding the chikungunya vaccine and monitoring for adverse effects was initially scheduled for an ACIP meeting this Wednesday, according to preliminary agendas, but it has since been postponed.

Dr. William Schaffner, an infectious disease expert from Vanderbilt University Medical Center and ACIP liaison, highlighted that the CDC’s investigation exemplifies the agency’s vaccine safety monitoring system efficiently identifying rare adverse events that could warrant additional scrutiny.

“The CDC, along with local health authorities caring for these patients, are actively addressing the situation, which is entirely appropriate. This is how the system is designed to function,” Schaffner remarked.

“We need to determine if there is a direct causal link or if the events are coincidental,” he added regarding the investigation, in which he is not directly participating. “It’s premature to reach any conclusions at this stage. That’s why the investigation is ongoing.”

The IXCHIQ vaccine, which involves a single-dose intramuscular injection, features a weakened live version of the chikungunya virus. Developed by the biotechnology firm Valneva, it stands as the first vaccine authorized by the FDA to combat chikungunya virus diseases. This approval, obtained in November 2023, applies to individuals aged 18 and older who face increased exposure risks to the virus.

“Vaccination is advised in a limited manner for travelers heading to regions currently experiencing chikungunya outbreaks, or to areas where outbreaks have occurred within the past five years,” Schaffner explained. “It is also recommended for laboratory personnel working with the chikungunya virus.”

According to a Valneva spokesperson, the company acknowledges the CDC’s ongoing review of post-marketing safety data as part of its standard vaccine monitoring protocols.

“We learned from the CDC that four out of the five affected individuals had also received additional vaccinations, and most had significant preexisting health conditions. Valneva has not pinpointed any safety concerns in its post-marketing evaluations, including periodic safety assessments, and IXCHIQ’s safety profile remains consistent with data from clinical trials,” the statement clarified. “No deviations have been noted in the established safety profile from clinical trials, where participants have been monitored for over three years following vaccination. Valneva continues to work with health authorities to uphold the highest vaccine safety standards.”

Prescribing information for the IXCHIQ vaccine indicates that “vaccination may result in severe or extended chikungunya-like adverse reactions.”

The chikungunya virus is prevalent in tropical and subtropical regions. In the United States, no locally transmitted cases have been documented since 2019, according to the CDC. However, cases and outbreaks have been reported in over 100 countries across the Americas, Africa, Asia, Europe, and the Indian and Pacific Oceans.

Symptoms of chikungunya infection can include fever, joint discomfort, headaches, muscle pain, swollen joints, and rash, as per the CDC. To prevent chikungunya, avoiding mosquito bites is crucial. Currently, there are no specific treatments for the disease, but rest, hydration, and over-the-counter medications for pain and fever may help alleviate symptoms.

According to the CDC’s notice from Tuesday, common side effects reported by over 10% of vaccinated individuals in clinical trials included tenderness at the injection site, headaches, fatigue, muscle aches, joint pain, fever, and nausea. Some individuals experienced “severe or prolonged chikungunya-like adverse reactions.”

“Chikungunya-like reactions encompassed fever, joint pain, headaches, rashes, and in two instances, serious cardiac and neurologic conditions,” stated Dr. Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center, who did not partake in the clinical trials, in an email on Wednesday.

The CDC encouraged healthcare providers to report any adverse reactions potentially linked to the vaccine through the CDC/FDA Vaccine Adverse Events Reporting System (VAERS), either by submitting a report online or by using a PDF form.

“The comprehensive vaccine safety surveillance system frequently detects very rare side effects post-approval,” remarked Barouch, an infectious disease expert not involved in the CDC investigation.

Upon the vaccine’s approval in 2023, the FDA stated that its safety was assessed through two clinical trials conducted in North America involving roughly 3,500 adults receiving the vaccine, including about 1,000 participants who received a placebo. Severe chikungunya-like adverse reactions were documented in 1.6% of IXCHIQ recipients, with 0% in the placebo group, according to the FDA.