His infections coincided with the periods when the Alpha, then Delta and Omicron strains became dominant around the world.
"After the second infection, I thought, what are the odds? When I got it the third time, I wondered, what's going on?" says Mr Tan, who was fully vaccinated by February last year and received a booster shot in January this year.
"The silver lining in all of this is that it did get better each time. We have already moved in the direction of endemic Covid-19. I think people still need to take precautions, but it's probably a matter of time before one gets it."
He is glad, at least, that he did not spread the virus to his mother and sister at home, though both caught Covid-19 about two months ago and have since recovered.
Milder but more frequent?
Indeed, Covid-19 reinfections tend to be milder, says Professor Tambyah, a senior consultant at National University Hospital's (NUH) division of infectious diseases and Professor of Medicine at NUS Yong Loo Lin School of Medicine. He adds that this probably extends to infections from future Covid-19 variants.
"All the data suggests that Covid-19 reinfections are milder than the first episode. This is true of most viruses, with the notable exception of dengue," he says. "All known human viruses mutate to become more transmissible and less virulent. As such, the new variants are more likely to be intrinsically milder in the infections that they cause."
Other infectious diseases specialists note, however, that the risk of reinfection is elevated today, which in turn poses more health risks, given the high transmissibility of Omicron, the current dominant strain, and its subvariants.
Dr Ruklanthi de Alwis, deputy director of the Centre for Outbreak Preparedness at Duke-NUS Medical School, notes that immunity levels are waning in highly vaccinated Singapore, with protection by antibodies declining from both vaccination and from previous Covid-19 infection.
In other countries, she says, "we're seeing that the reinfection period can get shorter".
Earlier this month, the Australian state of New South Wales revised the Covid-19 reinfection period from 12 to four weeks. The health authorities cited the surge of the Omicron BA.4 and BA.5 subvariants in the local community in their decision.
Dr Kurup agrees: "If the previous Covid-19 infection was a long time ago, there may be a great difference between the strains and people can have symptoms again."
He says the ongoing development of new vaccines - such as a pan-sarbecovirus vaccine, targeted at different coronaviruses; or mucosal vaccines, which could trigger protective immune responses at predominant sites of infection like the nose - could offer additional protection against Covid-19.
Comedienne Sharul Channa, 35, has had Covid-19 twice, maybe thrice.
In a show with fellow entertainer Kumar, The Sharul and Kumar Show 2, this month, she joked about having enough post-infection immunity to donate antibodies.
"I've had Covid three times. In case anyone hasn't got their booster, you can suck my blood," she quipped.
She says she has not had a chance to get a booster shot yet, given her repeated infections over the past eight months.
In November last year and February this year, she experienced symptoms such as a sore throat, cough, runny nose, tiredness and fever, and tested positive both times on PCR tests at a general practitioner's clinic.
Earlier this month, before her show at the KC Arts Centre, she developed similar symptoms, but tested negative using an ART. Too tired for further testing, she chose to self-isolate.
Days after she emerged from isolation, she checked herself into a hospital when she had sharp pains in her lower abdomen. She attributed it to too much sneezing and coughing. Her doctor diagnosed it as "a viral infection", which was likely her third brush with Covid-19.
Dr de Alwis says it is not uncommon to test negative for Covid-19 using an ART, while testing positive with a PCR test at the same time.
The PCR test, which detects the nucleic acid component of the virus, is more sensitive than the ART, which detects proteins produced by actively replicating virus. However, while PCR swabs can pick up low, inactive amounts of the virus, ARTs are a better indicator of infectiousness, says Dr de Alwis.
Dr Kurup adds that maintaining healthy habits like regular exercise and a nutritious diet, as well as keeping chronic diseases like diabetes under control, can reduce the risk of infection and reinfection.
In the meantime, Channa is focusing on building up her immunity with vitamin C supplements and probiotics from her doctor. She recently resumed her 10km walks and gym workouts, which are less intensive than before.
"I can't afford to get Covid-19 again. I need to be careful," she says.
'The fear of getting Covid-19 is more crippling than Covid-19'
When she caught Covid-19 on two occasions, Mrs Winnie Tomlinson, 41, was more concerned about her family's quarantine arrangements than her own health.
Last September, the Singaporean church worker returned from a trip to the United States with her husband, an American director at an international business school here, and their two sons, Noah, four, and David, five.
She tested positive during their quarantine at a hotel. Minutes before her Covid-19 test result came through, she had taken a pregnancy test as the couple had been trying for a third child.
She recalls: "I found out that I was pregnant practically at the same time I tested positive for Covid-19. It was bittersweet, but we were determined not to let Covid-19 cloud the joy of pregnancy.
"For us, the uncertainty surrounding how long we would be in quarantine, as well as the stress of having PCR tests for our young children, were a lot more stress-inducing than Covid-19 itself."
In the end, she was transferred to another hotel and later, a hospital. There, the doctor instructed her to recover at home instead, together with the rest of her family. Noah tested positive for Covid-19 while in home quarantine, though he was asymptomatic.
In June this year, three weeks after her daughter Maria was born, Mrs Tomlinson's husband Austin came down with Covid-19, which spread to the rest of the family, as well as to their domestic helper.
Mrs Tomlinson and Noah have thus caught Covid-19 twice, and she suspects her three-month-old baby, who had a high fever at one point, may also have been infected. She has not had her booster shot since falling pregnant with Covid-19, while her husband got his a month before he was infected.
Both Mrs Tomlinson's bouts of Covid-19, where she had symptoms like chills, fever and a cold, were mild.
She says: "I've come to realise that the fear of getting Covid-19 is more crippling than Covid-19 itself. The stress is a lot less now, but it's not mentally healthy as a society.
"We've done our part to be socially responsible. Let's move on."
Originally posted on Straits Times (Premium): 'I caught Covid-19 three times': S'poreans reinfected with virus see milder symptoms