The emergency was triggered just hours before US Vice-President Kamala Harris arrived in Vietnam on Tuesday. Very little is known about the Havana Syndrome and symptoms can include dizziness, nausea, migraines and memory lapses.
Josh Lederman from NBC News tweeted: “At least 2 US personnel in Hanoi, Vietnam will be medevacked out of country after Havana Syndrome incidents over the weekend.
“Hours before VP Harris arrived in Vietnam, US staff informed the incidents involved strange sounds, senior US officials tell me & @mitchellreports.”
Vice-President Harris arrived in Vietnam on Tuesday after a three-hour delay in Singapore.
The US Embassy in Vietnam blamed “a recent possible anomalous health incident” for the late departure.
The term is often used by the US government to describe Havana Syndrome.
The US Embassy said a decision was made to proceed with the foreign visit despite the health concerns.
It said in a statement: “The Vice-President’s office was made aware of a report of a recent possible anomalous health incident in Hanoi, Vietnam.
“After careful assessment, the decision was made to continue with the Vice-President’s trip.”
Havana Syndrome was first was reported in 2016 by US officials based in the US embassy in Cuba.
Around 100 CIA officers and family members are among some 200 US officials who have been struck by the destabilising condition since 2016, CIA Director William Burns said.
It is believed the illness is derived as the direct result of targeted sonic or microwave attacks.
READ MORE: Havana syndrome MAPPED: Bizarre illness with Russian links causing havoc for US diplomats
In December 2020, a report by the National Academies of Sciences, Engineering, and Medicine said the most likely origin of the illness was “directed, pulsed radiofrequency energy”.
Ms Harris’ seven-day trip to Asia comes amid growing tensions between the US and China.
Speaking in Singapore, the US Vice-President condemned Beijing’s growing influence and control over the disputed South China Sea.
She said: “We know that Beijing continues to coerce, to intimidate and to make claims to the vast majority of the South China Sea.
“These unlawful claims have been rejected by the 2016 arbitral tribunal decision, and Beijing’s actions continue to undermine the rules-based order and threaten the sovereignty of nations.”
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In response, China’s foreign ministry spokesman, Wang Wenbin, said the “order” that the US wanted was one in which it could “wilfully slander, oppress, coerce and bully other countries and not have to pay any price”.
China has established military outposts on artificial islands in the waters, which are crossed by vital shipping lanes and also contain gas fields and rich fishing grounds.
The US Navy regularly conducts “freedom of navigation” operations through the disputed waters, which China objects to.
On the eve of the US Vice-President’s trip to Hanoi, the Vietnam government insisted Prime Minister Pham Minh Chinh would take a neutral stance on the strained relations.
In a statement, the Vietnamese government said: “The Prime Minister affirmed that Vietnam adheres to an independent, self-reliant, multilateral, and diverse foreign policy and is a responsible member of the international community.
“Vietnam does not align itself with one country against another.”