Naturally, fans reacted with shock and concern, but it seems that Kate will likely be fine given she has also been fully vaccinated, according to HELLO!.
That said, it was absolutely right to take the measure she did to remain quarantined at home to avoid any chance of spreading the illness to others.
Ironically, Kate's event she was supposed to attend on Monday was in celebration of the very people who have been at the forefront of the battle against COVID-19.
The NHS celebrated its 73rd anniversary on Monday, and to mark the day, the Duke and Duchess of Cambridge were set to host key staffers from the Health Service for a cup of tea at Buckingham Palace.
Instead, William did all the hosting alone - and he did it well, might we add.
The Prince wandered about the Tea Party speaking to guest staff ranging from nurses, care workers, counsellors to catering managers and housekeeping coordinators.
He passed on his thanks on behalf of the royals and the UK as he listened to their experiences from the past year and half.
To add, The Queen also shared her gratitude for the NHS by awarding the National Health Service with the George Cross.
"It is with great pleasure, on behalf of a grateful nation, that I award the George Cross to the National Health Services of the United Kingdom," she said in a statement.
"This award recognises all NHS staff, past and present, across all disciplines and all four nations.
"Over more than seven decades, and especially in recent times, you have supported the people of our country with courage, compassion and dedication, demonstrating the highest standards of public service."
She finished: "You have our enduring thanks and heartfelt appreciation."
This story oringally appeared on our sister site, Now To Love.