Scheduled to arrive in Vietnam on Tuesday (Aug 24), the second leg of a trip to South-east Asia after Singapore, United States Vice-President Kamala Harris was delayed by several hours due to investigations in Hanoi related to the mysterious Havana syndrome.
The incident upstaged a bid by US President Joe Biden's top deputy to woo the allies Washington hopes will help it challenge China's assertive foreign policy in the region.
Here is more information about the obscure illness:
What is it
According to the BBC, the syndrome first affected US diplomats in Cuba at the US embassy in Havana in 2016 to 2017, with a US government report by the National Academies of Sciences, Engineering and Medicine stating that the illness was caused by directed microwave radiation.
A separate study conducted in 2019 in the Journal Of The American Medical Association said that the diplomats who suffered the effects of the syndrome showed "brain abnormalities" which could affect the brain's ability to send messages as well as other changes affecting auditory and spatial functions.
Patients suffer clinical symptoms with cases beginning with a sudden onset of a loud noise accompanied by pain in one or both ears or across a broad region of the head.
Further symptoms include a sensation of head pressure or vibration, dizziness, followed in some cases by tinnitus, visual problems, vertigo, and cognitive difficulties.
Nausea, fatigue, insomnia and sluggishness are other symptoms.
The Wall Street Journal has cited a source as saying that the symptoms were preceded by piercing ear pain, high-pitched electronic noise and pressure in the ears.
What may have caused it
The 2020 report by the Academies found a "directed, pulsed radio frequency (RF) energy"- a type of radiation that includes microwaves - the most plausible explanation for the symptoms experienced by diplomats who suffer from Havana syndrome.
While the US government has not outrightly addressed the cause of the syndrome, initial speculations said it could be the result of an attack with a covert sonic or acoustic weapon.
Speaking to The Guardian, Mr James Lin, the leading US authority on the biological impact of microwave energy, said that for a weapon to inflict neurological damage, it need not be large, but would need to be heated a minute amount to cause "a thermoelastic pressure wave" that travels through the brain.
The pressure wave would initially be experienced by the target as sound, while the weapon could be carried around in a van or "in a couple of big suitcases", he said.
However, The Economist has cast doubt on the theory, reporting that sound waves at frequencies outside of the range of human hearing cannot cause concussion-like symptoms.
Sceptics of the microwave weapon theory have also pointed to decades of US efforts to build such a device during the Cold War and since, without any confirmed success.
More than 200 people have reported symptoms of Havana syndrome, reported The Economist, with cases having emerged in Australia, Austria, Colombia, Russia and Uzbekistan.
In August, at least two US officials stationed in Germany sought medical treatment after developing symptoms of Havana syndrome, reported the WSJ.
They were the first cases to be reported in a Nato country that hosts US troops and nuclear weapons.
US diplomats said similar incidents had been registered among American officials stationed in other European nations and intelligence officers or diplomats working on Russia-related issue, but declined to provide any detail.
According to three diplomats, employees of the US State Department have set up an informal self-help group for Havana syndrome patients, as they claim that the US government, while providing care and other support, has failed to recognise their condition nor taken adequate measures to protect government officials posted abroad.