Defense Secretary Lloyd Austin said Sunday that he had tested positive for Covid.

In a statement, he said was fully vaccinated, with a booster shot having been administered in October, and was feeling only mild systems as a probable result. Positivity for a fully vaccinated person is characterized as a breakthrough case.

Austin said he will try to attend all necessary meetings virtually for at least the next five days, during which he will isolate himself according to new guidelines from the Centers for Disease Control and Prevention.

"I will retain all authorities," he said. "Deputy Secretary [Kathleen] Hicks will represent me as appropriate in other matters."

Austin suggested that he was positive at a time that would not have exposed President Joe Biden to the coronavirus.

He said his last meeting with the president was on Dec. 21, more than a week before his symptoms prompted him to request a test while he was on holiday leave at home. In addition, Austin said, he wore a mask and kept proper distance during the meeting with Biden.

Austin used the occasion to double down on the armed forces' vaccination requirement. He said his inoculation served him well in confronting a potentially deadly disease.

"The vaccines work and will remain a military medical requirement for our workforce," he said. "I continue to encourage everyone eligible for a booster shot to get one. This remains a readiness issue."

Austin made the announcement the same day Biden spoke to Ukrainian President Volodymyr Zelenskyy, vowing to stand by the country as it fears a possible invasion by Russia.

Russian forces have been expanding along the Ukraine border and now number about 100,000