Despite this, Kate has been busy in the last week attending events like Wimbledon on Friday, alongside Wimbledon chief executive Sally Bolton and Tim Henman, a former British tennis player.
The Duchess also went to Wembley Stadium last Tuesday to watch a soccer game between England and Germany.
She attended the game with her husband Prince William and son Prince George.
Kate is known to undergo consistent lateral flow tests before any royal engagement, as part of the royal household testing regime.
When Kate was alerted of the close contact, she started isolation at home immediately.
As for William, he is expected to undertake socially-distanced engagements throughout Monday (local time), and as of right now is not expected to isolate.
Both William and Kate were expected to lead celebrations for the 73rd anniversary of the NHS, which included a thanksgiving service at St Paul's Cathedral.
Following that, they were due to host an afternoon tea at Buckingham Palace.