Chikungunya: What we know about "serious adverse events" during the vaccination campaign

Health authorities announced this Saturday, April 26 , that they were withdrawing people aged 65 and over from the chikungunya vaccination campaign using the Ixchiq vaccine in Mayotte and Réunion after three "serious adverse events," including one death.

"Vaccination remains open to people aged 18 to 64 with comorbidities," the Ministry of Health stated in a press release, while the chikungunya epidemic has so far caused the deaths of nine people in Reunion Island.

In March, the French High Authority for Health, which was urgently contacted by the government, recommended that priority be given to vaccinating those over 65, adults with comorbidities (high blood pressure, diabetes, cardiovascular disease, etc.), and mosquito control workers on this French island in the Indian Ocean. The government followed these recommendations , starting the campaign on April 7 .

The health authorities' decision comes after they were informed on April 23 of two "serious adverse events," including one death, and then of a third on April 25.

The three "confirmed serious adverse events" occurred in Réunion "following vaccination against chikungunya with Valneva's Ixchiq vaccine, in people over 80 years old with comorbidities," the Ministry of Health stated on its website .

"Two people developed symptoms similar to those of a severe form of chikungunya a few days after vaccination, one of whom died. The third was discharged from hospital," the ministry added.

This disease manifests itself through fever, joint and muscle pain, headaches, and a rash. "In some cases, severe neurological forms can occur, including meningoencephalitis and peripheral nerve damage," which can lead to death, according to the Pasteur Institute .

These adverse events can be explained by the nature of the Ixchiq vaccine , explains Jean-Daniel Lelièvre, head of the clinical immunology and infectious diseases department at Henri Mondor Hospital, to BFMTV.com. This vaccine against chikungunya is a live attenuated vaccine.

These vaccines, like BCG or MMR, are "made up of attenuated infectious agents (viruses, bacteria)" and therefore create "a minimal infection," according to the official vaccination info service website . They "induce immune protection similar to that following a natural infection: rapid and generally long-lasting," it adds. However, since they contain a live infectious agent, they are contraindicated in immunocompromised individuals.

And beyond these people, "there are more adverse events for live attenuated vaccines in seniors in general, as we saw with the yellow fever vaccine, for example," explains Jean-Daniel Lelièvre. Health authorities are therefore seeking, for these people, to strike a balance between the risks posed by the vaccine and the benefits it can bring to these populations vulnerable to diseases like chikungunya.

For the Ixchiq vaccine, "we had a lot of data on younger people, but in those over 65, there was relatively little data, around 400 people. This allows us to eliminate very common adverse effects, but not the rarer ones," adds the immunologist, a member of the HAS technical vaccination committee. This vaccine received European marketing authorization (MA) on June 28, 2024, and began to be administered in Réunion in April.

"And we must not forget that very elderly people often have comorbidities: diabetes, heart failure, etc.," emphasizes Jean-Daniel Lelièvre. This makes them more at risk from chikungunya, but also, in the case of "a live attenuated vaccine, which causes a low-level infection," this "may be enough in people who are already very fragile to decompensate a pre-existing precarious state." All these hypotheses will, however, have to be validated after investigation by the health authorities.

Authorities, however, are encouraging vaccination for those under 65 in Réunion. "If you have kidney failure, heart failure, respiratory failure, diabetes, obesity, or are between 18 and 65 years old, it is recommended that you get vaccinated as I speak," Health Minister Yannick Neuder said on France 3 this Sunday. And in the event of "particularly intense" side effects after vaccination, the ministry recommends contacting a doctor promptly.

Interviewed Saturday on Réunion La 1ère, the head of the ARS stressed that stopping the vaccination campaign (for 18-64 year-olds with comorbidities) was "not envisaged," as the vaccine could "lead to herd immunity." The "serious adverse events" recorded in Réunion concern people over 80, and for those under 65, "we have more data" on this vaccine, asserts Jean-Daniel Lelièvre.

Until now, the recommendations targeted those over 65 "because we didn't have enough doses, so we targeted those most at risk of developing a severe form of chikungunya (hospitalization, decompensation of pre-existing illnesses)...", explains Jean-Daniel Lelièvre.

"This doesn't mean that younger people can't develop severe forms," ​​he warns. Younger people are particularly at risk of experiencing long-term repercussions from this disease, with persistent joint pain. This can occur in 30 to 40% of patients and last for several months or even years in some patients, according to Public Health France .

While he considers it "wise" to have stopped the campaign for those over 65, Jean-Daniel Lelièvre emphasizes that "science is never black or white." "When we recommended this vaccine, we did so because the disease was flaring up in Réunion, there were deaths, several hundred hospitalizations... With the adverse events today, we have a little trouble perceiving the benefit-risk balance, so we are rather in favor of avoiding putting people at risk," explains the member of the HAS technical vaccination commission. This is particularly true in the current context, where the epidemic is "stabilizing," even if it remains at "a high level," according to the latest bulletin from Public Health France.

But "we must be very clear": "this was not a full-scale trial in Réunion," the immunologist assures. The Director General of Health, Dr. Grégory Emery, said on Sunday that the campaign had not been launched too quickly, as "all the stages" of control had been "respected."

But the argument doesn't convince everyone on the island, home to more than 885,000 people. "I feel like we're being used as guinea pigs; this case will further diminish the credibility of health authorities in the eyes of the population," Henri, 69, a retired salesman in Saint-Benoît (eastern Réunion), told AFP.

Jean-Daniel Lelièvre also points out that "it is difficult to prevent chikungunya, transmitted by tiger mosquitoes that bite when you can't hear them." For now, this vaccine is the only one available. Nearly 120,000 people have been infected with the virus since the beginning of the year, according to estimates by the Regional Health Agency (ARS). Nine deaths of people over 70 with comorbidities have been recorded by Public Health France. Nine other deaths, including that of an infant, are currently under investigation.