Concerns have previously been raised over a rare risk of blood clots in people who take the Oxford/AstraZeneca vaccine.
The insider told the publication: “The princess has been unwell and has sought medical attention.
“It has been a worrying time for those around her.
“It has been difficult for those close to her to see her suffering. She’s really been through it.”
Last year, Princess Michael of Kent was diagnosed with Covid-19 and experienced “extreme fatigue and fevers”.
Following the diagnosis, she spent her self-isolation period at Kensington Palace.
Last November, royal biographer Simon Astaire confirmed she tested positive for the pathogen after her housekeeper “fell ill three weeks ago”.