HANOI (AP) — U.S. officials are continuing to investigate two possible cases of so-called Havana Syndrome health incidents that delayed Vice President Kamala Harris' trip from Singapore to Vietnam.

The investigation was in its early stages and officials deemed it safe for Harris to make her scheduled stop in Vietnam, after initially hitting pause for a few hours on Tuesday. Havana Syndrome is the name for a rash of mysterious health incidents first reported by American diplomats and other government employees in the Cuban capital beginning in 2016. Harris on her trip is reassuring Asian allies after the tumultuous evacuation of U.S. forces from Afghanistan.

U.S. officials had not yet confirmed the latest reported Havana Syndrome case, and it did not involve anyone traveling with Harris, White House Press Secretary Jen Psaki said Tuesday. In light of the reports, “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.

There have been two separate cases of unexplained health incidents reported by U.S. personnel in Vietnam within the past week, U.S. officials said. It was not immediately clear who was impacted by the syndrome, though officials said it was not someone who worked for the vice president or the White House, according to the officials, who were not authorized to speak publicly about an ongoing investigation.

On Wednesday, Harris appeared before U.S. diplomatic staff in Hanoi to sign a lease to a new embassy there. She didn't weigh in directly on the Havana Syndrome situation but expressed gratitude to those working for the U.S. across the globe.

“Here’s my message to embassy staff: thank you. The people who work in our embassies around the world are extraordinary public servants who represent the best of what the United States believes itself to be and aspires to be, which is a good neighbor for our partners and our allies around the globe," she said.

On Wednesday Harris was highlighting the announcement that the U.S. will send 1 million additional doses of the Pfizer vaccine to Vietnam, bringing the total U.S. vaccine donation to that country to 6 million doses.

The U.S. will also provide $23 million to help Vietnam expand distribution and access to vaccines, combat the pandemic and prepare for future disease threats. The Defense Department is also delivering 77 freezers to store vaccines throughout the country.

Some of those impacted by Havana Syndrome report hearing a loud piercing sound and feeling intense pressure in the face. Pain, nausea, and dizziness sometimes follow.

Similar, unexplained health ailments have since been reported by Americans serving in other countries, including Germany, Austria, Russia and China. A variety of theories have been floated to explain the incidents, including targeted microwaves or sonic attack, perhaps as part of an espionage or hacking effort.

Particularly alarming are revelations of at least two possible incidents in the Washington area, including one case near the White House in November in which an official reported dizziness. Administration officials have speculated that Russia may be involved, a suggestion Moscow has denied.

Congress has raised alarms over such incidents, finding rare bipartisan support in the House and Senate for continued government-wide investigation into the syndrome, response as well as support for American personnel receiving medical monitoring and treatment.

The Biden administration is facing new pressure to resolve the mystery as the number of reported cases of possible attack has sharply grown. But scientists and government officials aren’t yet certain about who might have been behind any attacks, if the symptoms could have been caused inadvertently by surveillance equipment — or if the incidents were actually attacks.

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Lemire reported from Lowell, Mass. Additional reporting contributed by Associated Press writers Matthew Lee and Lisa Mascaro in Washington