With the Queen approaching her 96th birthday in April, there was always going to be concern about her contracting Covid, but the monarch has tested positive against a radically different backdrop from when the virus arrived in the UK.

In addition to the protection afforded by her vaccinations – and she is understood to have had a booster – she could also be given antiviral drugs approved by UK authorities as recently as December.

While engaging in “light duties”, she will be cared for at Windsor Castle by the medical household – staff who are part of the royal household.

At the forefront of decisions about her care will be the Queen’s physician, Sir Huw Thomas, who is a consultant at St Mary’s hospital in London and professor of gastrointestinal genetics at Imperial College London, as well as the head of the medical household.

He may choose to prescribe one or more antivirals designed to protect the most vulnerable, a decision that potentially might have to be made within days.

Despite the press and public being told that the Queen is suffering from “mild, cold-like symptoms”, experts stress that nearly all severe covid infections can begin with mild symptoms.

Prof Paul Hunter, an infectious disease expert at the University of East Anglia, said that with anyone in their mid-90s, even if triple-vaccinated, there would be concerns that they could gradually deteriorate.

“You would, I think, almost certainly be considering giving antiviral drugs, of which there are a number around at the moment.”

Drugs that could be considered include Ronapreve, which was approved in August last year, and which contains types of proteins called monoclonal antibodies. It has been shown to reduce the risk of hospital admission or death by 70% in those with mild to moderate Covid-19.

Other options are two drugs approved in the UK in the last part of last year: Molnupiravir, which clinical trials suggest reduces the risk of hospital admission or death by 30%; and Pfizer’s Paxlovid, which has been shown to be around 90% effective in preventing the need for hospital admission.

There are three other drugs – Sotovimab, Remdesivir and Dexamethasone – though Hunter said that the first two would need to be given within three days for them to have the best impact.

The Queen has been known for her strong constitution and infrequent ill health and as recently as June 2020 was pictured riding a Fell pony at Windsor while isolating.

On Wednesday, the sovereign, standing holding a walking stick, remarked during an in-person audience: “Well, as you can see, I can not move.”

Buckingham Palace declined to comment, but the Queen was understood to have been feeling slightly stiff, rather than having injured herself or being unwell.

She missed the Remembrance Sunday event at the Cenotaph on 14 November last year due to a sprained back. Before that, she attended a busy reception for business leaders at Windsor Castle on 19 October. But the next day she canceled a trip to Northern Ireland on medical advice and was admitted to hospital for a night to undergo preliminary tests.

She then spent more than three months only carrying out light duties on doctors’ orders. That hospital stay was her first in eight years. In 2013, she was treated at the private clinic for a bout of gastroenteritis, and she also stayed for one night.

Nevertheless, as Age UK has noted, age is not the only characteristic that influences individual risk from Covid, which should be considered alongside other characteristics that influence risk, including sex, ethnicity, and some pre-existing medical conditions.

The charity also pointed out that, during the last few months of last year, older people had remained less likely than other age groups to have coronavirus. While reiterating that there was evidence that the protection afforded by vaccines waned over time, the charity also said that protection against serious illness remained very high for at least five months, and urged the elderly to get a booster.

Buckingham Palace has declined to say when the Queen received a booster, or indeed confirm if she has had one. The monarch herself expressed enthusiasm in public last year for vaccinations as she encouraged people to “think about other people rather than themselves” and get the Covid-19 jab, in a video call with health officials leading the deployment of the vaccine across the UK.

“Once you’ve had the vaccine, you have a feeling of ‘you know, you’re protected, which is I think very important,” she said.