Charles, 71, the monarch’s eldest son and heir, tested positive for COVID-19 last month but recovered after a week in isolation at his home in Scotland.
“I have to admit, at first I was quite concerned, he fits the profile of somebody, at the age he is at, which is fairly risky,” his oldest son Prince William told the BBC.
“But my father has had many chest infections, colds and things like that over the years and so I thought to myself if anybody is going to be able to beat this, it is going to be him.”
In the end Charles had only mild symptoms, the 37-year-old said.
The queen, 93, and her husband Prince Philip, 98, have been staying at Windsor Castle near London since the outbreak began.
“Obviously I think very carefully about my grandparents,” William added.
“We are doing everything we can to make sure they are isolated away and protected from this but it does worry me.”
William and his wife Kate, the duke and duchess of Cambridge, spoke to the BBC as part of their work supporting mental health charities.
They urged people to look after their mental as well as their physical well-being during the lockdown.
“There is going to be an ever-increasing need for people to look after their mental health and take it seriously,” William said.
The couple also spoke about their own life at their home in Norfolk, eastern England — including the “challenging” task of homeschooling small children.
Prince George is six, Princess Charlotte four and Prince Louis will be two next week.
The duchess said they had faced “ups and downs” like many families — and also admitted that getting her children to cooperate on video calls with relatives could be tricky.
“It’s quite hectic for them all to say the right thing at the right time without pressing the wrong buttons,” she said.
“But it’s great and it’s nice to keep in touch with everybody.”