At least two US personnel will be flown out of Hanoi after incidents of Havana syndrome, reports say.
NBC reporter Josh Lederman said on Twitter that the action was being taken after "incidents over the weekend".
The illnesses come amid vice-president Kamala Harris' trip to Vietnam, which is reportedly being used to try and woo allies.
Havana syndrome is so-named because it was first reported by officials based in the US embassy in Cuba in 2016.
It has symptoms such as dizziness, nausea, migraines and memory lapses.
Mr Lederman wrote: "At least 2 US personnel in Hanoi, Vietnam will be medevacked out of country after Havana Syndrome incidents over the weekend.
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"Hours before VP Harris arrived in Vietnam, US staff informed the incidents involved strange sounds, senior US officials [say]."
He added in a follow-up tweet: "This is not the first time Havana syndrome incidents have been reported in Vietnam, sources tell me.
"The previous ones involved sound too. They were investigated but nothing was able to be confirmed."
Harris' trip to Southeast Asia was delayed by the health scare, which officials initially called an "anomalous health incident".
The term is often used by the US government to describe Havana syndrome, reports news agency Reuters.
A spokesperson for Harris declined to comment on the reason for the delay.
A US National Academy of Sciences panel in December found that a plausible theory is that "directed energy" beams cause the syndrome, CIA Director William Burns has said.
The CIA sees a "very strong possibility" that the syndrome is intentionally caused, and that Russia could be responsible, but is withholding definitive conclusions pending further investigation.
Moscow denies involvement.
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