The US Centers for Disease Control and Prevention (CDC) is currently examining five cases of hospitalization among individuals who recently received the chikungunya vaccine, which helps prevent diseases linked to the chikungunya virus.

In a notice released Tuesday on its vaccine information page for healthcare professionals, the CDC highlighted that all affected individuals were aged 65 and older, hospitalized due to cardiac or neurological issues following vaccination with IXCHIQ, the only chikungunya vaccine approved in the United States.

CNN has initiated contact with the CDC for further comments.

The chikungunya virus is primarily transmitted to humans through infected mosquito bites. The vaccine is not widely administered to adults and is generally recommended for certain travelers at higher risk of exposure to this mosquito-borne illness.

According to the CDC’s online announcement, “This matter will be addressed at an upcoming meeting of the Advisory Committee on Immunization Practices (ACIP). Healthcare providers should assess the advantages and disadvantages of vaccination for travelers based on their age, destination, trip length, and planned activities.”

Topics surrounding the chikungunya vaccine and monitoring for adverse events were initially on the agenda for an ACIP meeting set for Wednesday, but that meeting is now postponed.

Dr. William Schaffner, a specialist in infectious diseases at Vanderbilt University Medical Center and an ACIP liaison, described the CDC’s ongoing investigation as a prime example of the agency’s effective vaccine monitoring system in action, indicating that it is successfully identifying rare occurrences that may warrant further inquiry.

He commented, “The CDC and local healthcare providers currently attending to these patients are taking appropriate steps. This is precisely how the system is designed to function.”

Dr. Schaffner added that it is crucial to ascertain whether these incidents indicate a causal link or are merely coincidental. “We should not reach any conclusions at this time; that’s precisely why this investigation is underway,” he stated.

The IXCHIQ vaccine, administered as a single intramuscular shot, contains a live, attenuated form of the chikungunya virus. Developed by the biotech firm Valneva, it was the first vaccine approved by the US FDA to prevent chikungunya-related illnesses, receiving approval in November 2023 for individuals aged 18 and older at an elevated risk of exposure.

Dr. Schaffner noted, “It’s recommended in a limited capacity for travelers heading to areas with ongoing outbreaks of chikungunya virus, as well as to those visiting regions where outbreaks have occurred in the last five years, and for laboratory personnel working with the virus.”

CNN has also reached out to Valneva for a statement. The prescribing information for the IXCHIQ vaccine indicates that “Vaccination may lead to significant or extended chikungunya-like adverse reactions.”

The chikungunya virus exists in tropical and subtropical climates. The CDC states that there have been no locally transmitted infections reported in the U.S. since 2019. However, outbreaks and cases have been documented in over 100 countries across the Americas, Africa, Asia, Europe, and the islands of the Indian and Pacific Oceans.

Symptoms of chikungunya infections may include fever, joint pain, headaches, muscle discomfort, swelling in joints, or rashes, according to the CDC. The best preventive measure is to avoid mosquito bites. Currently, there is no specific medication to treat chikungunya; patients may find relief through rest, hydration, and over-the-counter pain relievers.

In its Tuesday notice, the CDC reported that common side effects observed in more than 10% of vaccinated individuals during clinical trials included tenderness, headaches, fatigue, muscle pain, joint discomfort, fever, and nausea. In certain cases, “severe or prolonged chikungunya-like adverse reactions” were noted.

Dr. Dan Barouch, the director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center and not part of the clinical studies, stated in a Wednesday email that chikungunya-like side effects included “fever, joint discomfort, headaches, rashes, along with serious cardiac and neurological conditions in two cases.”

The CDC encourages healthcare providers to “report all adverse events that may be linked to vaccinations to the CDC/FDA Vaccine Adverse Events Reporting System (VAERS)” either by filing an online report or utilizing a PDF form.

Dr. Barouch noted, “The robust vaccine safety monitoring system frequently detects exceptionally rare adverse effects of vaccines after they have been authorized.”

When the vaccine was authorized in 2023, the FDA indicated that its safety had been assessed through two clinical trials in North America, involving approximately 3,500 adults where one study included about 1,000 subjects who received a placebo. Severe chikungunya-like side effects were recorded in 1.6% of IXCHIQ recipients, with no occurrences in the placebo group, as per the FDA.

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