Chikungunya is a mosquito-borne disease caused by the chikungunya virus, endemic in parts of Africa, Asia, and the Americas. It causes a fever roughly a week after a person has been bitten by an infected Aedes mosquito. Other symptoms include headaches, fatigue, nausea, severe muscle pain, and debilitating joint pain. Older people and infants are most at risk. Chikungunya's medical and economic burden is expected to grow as global warming increases the mosquitoes' geographic spread. Currently, there are no approved preventative vaccines or effective antiviral treatments.

The researchers investigated the safety and efficacy of Valneva's vaccine candidate VLA1553. Their study enrolled 4,115 healthy adults across 43 U.S. study sites. One dose of VLA1553 was administered to 3,082 participants via arm injection; 1,033 were given a placebo. All participants were included in the safety analysis, but the immune response was only tested in a subgroup of 362 participants; 266 received the vaccine and 96 the placebo. Participants had their immune responses assessed one week, 28 days, three months, and six months after vaccination. They also recorded adverse events in an electronic diary for 11 days after vaccination.

After a single vaccination, VLA1553 induced an antibody response at a level considered protective against disease among 99% of participants. The immune response did not vary with age. Neutralizing antibody levels declined 28 days after vaccination, but seroprotection persisted in more than 96% of participants after six months.

VLA1553 was generally well tolerated across all age groups; most adverse events were mild or moderate. Among vaccinated participants, the most common adverse events were headaches (experienced in 32%), fatigue (29%), muscle pain (24%), joint pain (18%), and pain at the injection site (13%).

Because the study did not take place in an endemic region, participants' preexisting immunity to the chikungunya virus was unknown, as was the safety of the vaccine in endemic populations. An additional limitation is that the vaccine is made from a weakened version of the live virus, so is likely unsuitable for immune-compromised people and pregnant women. For effective endemic disease control, chikungunya vaccines would need to be administered to children. To determine safety and efficacy in this age group, a study of adolescents in endemic areas of Brazil is currently underway.

"Chikungunya is currently regarded as one of the viruses most likely to spread globally, and studies have shown that climate change is driving the spread of the mosquitoes that carry it into new areas of the world," study author Katrin Dubischar, Valneva's chikungunya vaccine program director, noted in a statement. "Therefore, having an effective vaccine is important for preparedness for future outbreaks."

Disclosure: Most authors, including Katrin Dubischar, are Valneva employees and own Valneva stock and share options.