What is 'Havana syndrome', due to which Kamala Harris' trip to Vietnam was delayed?

US Vice-President Kamala Harris delayed her trip to Vietnam due to a health incident related to Havana syndrome. It was first reported in Havana in 2016, with US diplomats hearing piercing sounds and experiencing odd physical sensation, nausea and headache. National Academies of Sciences, Engineering, and Medicine proposed it was being caused by directed, pulsed radio frequency energy device.