The U.S. government advised American travelers age 60 and older not get a chikungunya vaccine as it investigates possible side effects.

The Centers for Disease Control and Prevention and the Food and Drug Administration posted notices late last week on the vaccine, Valneva’s Ixchiq.

Chikungunya, spread by the bites of infected mosquitoes, is a debilitating tropical illness marked by fever and joint pain. About 100 to 200 cases are reported annually among U.S. travelers.

Last year, the government began recommending the vaccine, which is made with weakened chikungunya virus, to U.S. adults who travel to countries where chikungunya is a problem.

But last month, a panel of vaccine experts who advise the CDC heard about an investigation into six people 65 and older — most of them with other medical problems — who became ill with heart or brain symptoms less than a week after vaccination. More than 10 other similar cases have been reported in people from other countries.

European regulators are also looking into the issue.

The U.S. advisers voted to issue a precaution for people 65 and older about getting the vaccine.

They also recommended that a second chikungunya vaccine — Bavarian Nordic's Vimkunya — be made available for people age 12 and older who are traveling to countries where outbreaks of the mosquito-borne illness are occurring. CDC officials have not yet announced whether they will accept those recommendations.