There was one stand-out response to the video that emerged from the recent anti-vaccination protests in Trafalgar Square in London, in which Kate Shemirani, a former nurse in her fifties, incited the crowd to send in the names of doctors and nurses who supported the vaccine rollout. It came from her son, Sebastian.

The 22-year-old gave a heartbreaking interview to the BBC in which he talked about his gradual alienation from his mother, who is now so mired in conspiracy theories that he has given up all hope of getting her back again.

Kate Shemirani addresses fellow wing nuts at the Unite for Freedom demonstration in Trafalgar Square.

“My mum is definitely beyond help,” he told BBC Radio 4’s Today program, as he talked about her “God complex” that had made it impossible to communicate with her. The relationship is now so strained that he hears from her only a couple of times a year, when she sends him a propaganda video.

For most of us, Shemirani’s antics are worrying; for Sebastian, her ramblings are a personal tragedy playing out on the public stage. While his mother’s profile makes his experience unusual, the basic facts of it are not. Sebastian is one of a growing cohort of young people grappling with something that would have been unthinkable a few years ago: what to do when your parents are radicalized by online conspiracy theories?

“My mum is definitely beyond help.”

The most shocking example of this came not from Trafalgar Square but from the United States, where a pupil at Marjory Stoneman Douglas High School in Parkland, Florida, posted his story on the social media platform Reddit.

The student, whose story was recently authenticated by journalists and is being called Bill, survived a 2018 shooting at the school in which 17 people were killed — nine of them his classmates.

For most of the past three years, his parents have been supportive, but at the start of lockdown his father, in particular, began to change. It started a couple months into the pandemic with the anti-lockdown protests.

“His feelings were so strong it turned into facts for him,” Bill wrote. “So if he didn’t like having to wear masks, it wouldn’t matter what doctors or scientists said. Anything that contradicted his feelings was wrong. So he turned to the Internet to find like-minded people, which led him to QAnon.”

Sebastian Shemirani says he has given up all hope of getting his mother back.

The Q phenomenon began on American message boards where an anonymous poster began to set out a bizarre and convoluted conspiracy casting Donald Trump as the hero in a battle against the “deep state”, which was working with Hillary Clinton to operate a global child-abuse ring involving the world’s elites.

For years it was a niche obsession for a small group of US-based conspiracy theorists; today it has morphed and gone mainstream, escaping onto Facebook and YouTube. The conspiracies tend to join up with antivax material: many believe Covid-19 is a hoax.

The conspiracies have so convinced Bill’s father that he now believes the shooting his son survived was a hoax, and that the shooter, Bill and all his classmates were paid pawns in a grand conspiracy orchestrated by some shadowy force.

It sounds far-fetched, but Bill is not alone. He shared his story in a group called QAnon Casualties, which has 176,000 members. It has become a virtual support group for people whose family members have been sucked into the world of online conspiracies, and almost invariably the problem is Mom or Dad.

Bill’s father now believes the shooting his son survived was a hoax.

“My parents used to be reasonable people,” one post says. “I love them and know they love me, but I can’t maintain a relationship with them when they believe this nonsense.” A doctor posts a draft of a letter he and his sister, a nurse, are planning to send to their mother, who is refusing to be vaccinated. Another says he is at his wits’ end because his mother is emailing his 25-year-old brother warning him the vaccine will kill him.

The pleas for help are heartfelt. They also show that the growing number of people in this position have no idea where else to turn.

Coronavirus deniers and other conspiracy-theorist groups protest outside of Westminster in July.

In the UK, Marianna Spring, 25, the BBC’s disinformation correspondent, who has been covering stories like this for the past year, has become something of an agony aunt. She is contacted all the time by people in a similarly desperate state and has noticed a trend. “From my experience, the number of people who get in touch with me says that this is a very big problem, and those who are talking to me about families breaking down are predominantly young people,” Spring says.

Conspiracy theories are hard to study because they are quick to evolve and hard to pin down. You may be able to determine how many views a viral video has had, but how do you quantify the psychological impact on the person watching it?

Research carried out by King’s College London and Ipsos Mori last year suggested it was the young who were most exposed to antivax propaganda online. But that might not be giving us the whole picture. Spring has found older age groups are far more likely to engage with the movement at a grassroots level, rather than just scrolling past.

Partly, she says, this is because the over-fifties tend to be less social-media literate than younger people, who have grown up online and are less likely to take the information they see online at face value. The pandemic has also made people more isolated and therefore more vulnerable to persuasion in online communities.

Marianna Spring, the BBC’s disinformation correspondent, says it is primarily young people who talk to her about families breaking down.

Spring says that many of those she has spoken to have said that their parents have become increasingly solitary over the past year: “They would sit at home on their computer all day throughout the pandemic. Then they started to attend rallies and protests and find friendship through it.”

From her reporting, she has found that, while effort is being made to crack down on the social media platforms that host the conspiracies, those whose relatives are affected want more help dealing with this. “The thing to remember is that they are more likely to trust their own kids or family members than they are institutions or reputable news sources,” Spring says. “So in many ways that person is the beacon of hope.”

Dr Jovan Byford of the Open University has been talking to conspiracy theorists for 20 years. Conspiracy theories are not about logic but about emotion, he says: “They are stories about good and evil, as much as about what is true.” The key, he says, is to stay calm and be prepared to “de-escalate” the situation when tempers are fraying and it is more important to keep dialogue going than to expect a breakthrough immediately.

The most common advice on the Reddit forum is patience, and calmly challenging the beliefs one by one. And there are glimmers of hope amid the frustration. In one recent post entitled “Mom got vaccinated”, the poster writes: “It’s been a long battle, almost five years now of QAnon hell. It took lots of discussion, lots of researching her points of disinformation and showing her how unreliable these sources were … But once I was able to properly communicate to her that she was actively denying science and medicine and let her know I was doing all of this because I cared about her health, she went out and got the vaccine. Keep your hope alive, they can be saved.”

Rosie Kinchen is an editor for The Times of London