The Queen reached the milestone of 70 years on the throne on February 6, the anniversary of the 1952 death of her father, King George VI. She will turn 96 on April 21.
The Queen's eldest son, Prince Charles, 73, and her 74-year-old daughter-in-law, Camilla, Duchess of Cornwall, contracted Covid-19 this month. Charles has since returned to work. There are also thought to have been several recent virus cases among the staff at Windsor Castle, where the Queen is.
Elizabeth spent a night in a London hospital for unspecified tests in October and until recently had been under doctors' orders to rest and only undertake light duties.
She cancelled various major engagements late last year but returned this month to public duties, and has held audiences virtually and in person with diplomats, politicians and senior military officers.