The cause of Havana syndrome is not known clearly but it may be caused by  directed microwave radiation. It first affected people at the US and Canadian embassies in Havana in 2016 and 2017, and, hence, the name.

On Tuesday, Harris was in Singapore when the incident occurred in Hanoi.

"Earlier this evening, the Vice President's traveling delegation was delayed from departing Singapore because 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," said Rachael Chen, spokeswoman for the US embassy in Hanoi, in a statement, according to a CNN report.

Harris eventually took off from Singapore's Paya Lebar Air Base at 7:32 p.m. local time, after a more than three-hour delay.

"Well, all is fine and looking forward to meetings in Hanoi tomorrow," Harris' chief spokeswoman Symone Sanders told traveling reporters aboard Air Force 2.

 "This has nothing to do with the vice president's health," according to a pool report, it added.

During the White House press briefing Tuesday afternoon, White House press secretary Jen Psaki told reporters that  Harris "wouldn't travel further to a country if there wasn't confidence in her security on the ground."

But she did not reveal the security measures to keep Harris safe during her travel and added that no additional assessments had been made when asked if Harris or her staff were the target of a possible attack, reported CNN.

"This is not a confirmed case at this point in time. We take any reported incident, which was recent and was reported publicly, I will note, quite seriously. As a result, there was an assessment done of the safety of the vice president, and there was a decision made that she could continue travel along with her staff," Psaki said.

Psaki confirmed that those affected were not traveling with Harris but declined to say how many individuals fell ill, according to the CNN report.

Harris was not medically evaluated as she was not present at the time the incident was reported.

Victims of Havana syndrome have reported a different set of symptoms and physical sensations, including sudden vertigo, nausea, headaches and head pressure, sometimes accompanied by a "piercing directional noise."

Some have been diagnosed with traumatic brain injuries and continue to suffer from debilitating headaches and other health issues years later.

Since the syndrome was first reported in Cuba in 2016, cases of the condition have been reported elsewhere in the world, including China and, last month, Austria.

Hundreds of US diplomats, spies and other personnel have reportedly fallen ill with symptoms including ear ringing, nausea, and severe headaches, according to a BBC report.

A 2019 US academic study found "brain abnormalities" in the diplomats who had fallen ill in Cuba, it said.

A report last year from the US National Academies of Science found the mystery illness was most likely to be caused by directed microwave radiation, it added.

In June, US Secretary of State Antony Blinken announced a review into the causes of the illness.