After her death in May 2023 her family and friends set up a campaign to raise awareness of heart problems among young people and to provide screening so others do not suffer her fate. "I will do whatever I can to prevent what has happened to my family happening to others," said Nicholls, who also has two sons aged 23 and 21.
One in every 250 people is thought to have a genetic heart condition, affecting a total of 260,000 in the UK and 32 million worldwide.
The former Bolton Wanderers footballer Fabrice Muamba narrowly survived a cardiac arrest at the age of 23 during an FA cup match in 2012, caused by hypertrophic cardiomyopathy which gave him a thickened heart wall. The England cricketer James Taylor was forced to retire in 2016 at 26 after he was found to have ACM.
More often these conditions -- which can be exacerbated by exercise -- go undiscovered until death. Every week in the UK 12 people under 35 die from an undiagnosed heart condition and up to 80 per cent of them had no prior symptoms.
Clarissa's Campaign has raised more than £100,000 to offer heart screening to Cambridge students and wants electrocardiogram (ECG) scans to be offered to all young people taking part in organised sport.
The awareness raised by the tragedy, however, has already made a difference. Sixteen months after Clarissa's death, Cai La Trobe-Roberts, 22, an economics student at Cambridge and star striker for the university football team, started feeling unwell.
"I was in Cambridge before term started for pre-season training and I had a bit of pain in the front of the chest and the back of the throat," said Cai, who is from Bangor, north Wales. "Clarissa's story was really in my mind."
Cai had never met Clarissa, but her death had rocked the student population. As well as being a keen long-distance runner, she had represented the university at hockey, and her story was particularly well known among the sports teams.
Cai went to his GP and was sent to A&E with suspected kidney problems. But a kidney function test showed up nothing and a duty doctor told him he was probably just suffering from anxiety.
The young man was insistent he needed further testing. "I said, 'I need you to scan my heart, I need an ECG', and I told them about what happened to Clarissa."
The doctor finally agreed and the scan revealed a serious condition: myocardial and pericardial damage. Cai had small holes in the sac that surrounds the heart.
"I can't say for sure whether it saved my life or not," said Cai. "But it transformed my life, absolutely."
He spent three weeks in hospital undergoing treatment before being discharged with strict orders to do no exercise until fully recovered. "We never found what the cause was but it can be pretty serious," he said, "and if you exercise it can get much worse. If it hadn't been found I would definitely have carried on as normal."
* Father, 31, died of cardiac arrest after heatstroke at Ironman event
Cai, who completed his studies this summer and is now working as a consultant in a financial business in London, has gradually recovered over the last year. He is back to full strength, running, cycling and playing football again.
Last Wednesday students queued at the University of Cambridge sports centre to have their hearts tested, funded by Clarissa's Campaign. "I saw a TikTok video about Clarissa and the offer of screening and thought I'd come along," said Mathieu Arnaud, 19, a history student and rower. Ten electrodes were connected to his chest, arms and legs and the results given to a cardiologist on site, who took the student's medical and family history, giving him the all clear.
Kilian Schulz, 24, studying for a master's in computer science, said he heard about the screening on his running club WhatsApp group. "I have had a low pulse and have been getting dizzy so I thought I'd get it checked out," he said. "I'm glad I came -- the doctor said it is within the normal range so there's nothing to worry about."
Nicholls, who lives in south London with her husband Simon, 51, travelled to Cambridge to help with the screening.
"I've come for all the screening days we've put on," she said. "I like to be here in Cambridge, where Clarissa was, to be with her friends. The tragedy is that she was coming to that point in life where she was blossoming. She would probably have gone into the law -- that's what she was interested in. And that future was just wiped away."
The campaign started funding screening days last year at £7,000 a day, with about 100 students tested a day. The screening is provided by the charity CRY -- Cardiac Risk in the Young -- which subsidises the cost.
So far 424 students have been screened and 42 have been referred for further tests. On average, CRY expects about one heart condition for every 100 young people it screens, and for one in every 300 a life-threatening heart condition is diagnosed.
For many, drugs can help manage their condition. For others, lifestyle changes, such as giving up vigorous sport, will keep them alive. Some may need a defibrillator implanted.
By the end of the year, 1,000 students will have been screened, with further days planned next year.
Nicholls wants to go further. "There's a huge problem in this country. There's no pathway for asymptomatic young people to have an ECG apart from through charities or paying privately, which costs at least £250."
She pointed to Italy, where anyone taking part in organised sport, whether a marathon or children's Sunday football league, has to have an annual heart scan. Sudden cardiac deaths among young people in Italy have dropped 89 per cent as a result.
Nicholls does not think a mandatory scheme is necessary, but said: "Everyone should have the option. Elite athletes get screened here but we need it for amateur sport as well."
She has submitted a proposal to the National Screening Committee for heart tests to be made available via sports clubs.
A Department of Health and Social Care spokesman said: "Our thoughts are with the family of Clarissa Nicholls following her tragic death. The UK National Screening Committee is currently examining the evidence for screening for risks of sudden cardiac death and will open a public consultation on this in due course."