So you’ve recovered from Covid – and hopefully you’ve escaped largely unscathed. Maybe you’ve still got a lingering cough, or persistent fatigue, but the worst is over, and a little winter sun will surely vanquish those niggling maladies.

But before you book a much-needed holiday, be warned: despite having shaken the virus, up to 30 per cent of people still test positive on a PCR test 30 days after their initial infection, and some continue to do so even three months afterwards, experts warn. 

It’s worrying news for travellers, then, that amid the rise of omicron, more countries are now calling for pre-travel PCR tests – including the likes of Switzerland, Dubai and the Maldives. And of course, everybody must now take a Day 2 PCR on arrival in the UK. But if you’ve recently recovered from Covid, the risk of a ‘false positive’ PCR test means your holiday could be over before it has even begun.

“Three weeks after a mild case of Covid this summer, my teenage daughter had completely recovered – and had taken several lateral flow tests which were negative,” says Mark Hodson, editor of travel website 101 Holidays. “She was due to go to Barcelona [which required a PCR test at the time], but when she took a PCR test two days before she was due to fly, it came back positive – so she had to cancel the trip altogether.”

Hodson’s daughter is far from alone; Telegraph Travel has been contacted by several readers whose PCR tests have come back positive weeks after their initial infection. “In September, we travelled to Barbados for a family wedding with our daughter Ellie who had caught Covid a month earlier and recovered,” writes Rob Clark. “Despite having a GP letter confirming her recovery from the virus, she tested positive via PCR [on arrival] and was taken to a quarantine centre, a former prison, the next day.”

So, what causes a ‘false positive’ PCR result? According to epidemiologists, fragments of viral genetic material – not live virus – can still be present in the nose and throat long after the infectious period is over, a common occurrence known as “viral RNA shedding”. Dr Alasdair Scott, clinical director of test provider C19 Testing, explains: “People tend to clear the virus but may still shed the RNA. Even a very small amount of RNA can be detected [by a PCR test] and will give a positive result. But a positive result just means that the person has viral RNA in their system: it doesn’t mean they are infected with the SARS-CoV-2 virus, and it certainly does not mean they have Covid.”

Dr Richard Dawood, medical director at London’s Fleet Street Clinic, agrees: “Traces of viral RNA can be detected after a person ceases to produce live virus and is no longer infective to others. A good analogy might be detection of human DNA at a crime scene, which we know can persist long afterwards.”

This issue particularly affects PCR tests, because they are designed to detect the presence of RNA – so if you can choose which type of test to take, make it an antigen, advises Dr Scott. “PCR works by using probes to detect tiny fragments of RNA that is specific for SARS-CoV-2 – whereas antigen tests detect fragments of the viral protein ‘wall’, usually the nucleocapsid protein. So you can think of antigens as detecting the virus itself, rather than the RNA. So while a PCR test may remain positive, an antigen test [would be] negative.”

The presence – and impact – of RNA could affect almost a third of people post-Covid, triggering a misleading PCR result up to three months after recovery. “Estimates suggest that 10-30 per cent of individuals may test positive up to a month after initial infection,” says Dr Scott, “and a smaller proportion may test positive up to 90 days [afterwards].”

It is a cruel coincidence, then, that many of our favourite winter destinations currently mandate PCR tests before or after arrival – or both – regardless of a traveller’s vaccination status. The likes of the UAE, Thailand and Barbados will turn away, or forcibly quarantine, anybody who fails to present a negative PCR, which is worrying news for those who’ve recently recovered from the virus. 

“Some countries know and understand [the issues with PCR tests], and have translated this into their entry requirements for travellers,” says Dr Dawood; “Some accept proof of a recent positive result and recovery as an alternative to a PCR test. Other countries do not, and there is a risk of being stuck somewhere with a positive result, waiting for it to turn negative, especially if a less sensitive test (such as an antigen or lateral flow test) has been used in the other direction.”

When I tested positive for Covid last month, I received a phone call from NHS Test and Trace instructing me not to take a PCR test within 90 days. I didn’t have the energy to question it then, but the Government does indeed state that “fragments of inactive virus can be persistently detected by PCR in respiratory tract samples following infection, and for some time after a person has completed their isolation period and is no longer infectious.” The Government advises that those who’ve contracted Covid “should be exempt from routine re-testing within 90 days.” But what if an exemption isn’t possible? The likes of the Maldives, Thailand, et al, accept no alternatives to PCR tests – so you must take your chances, or choose another destination entirely.

I had planned to visit Thailand this winter, but am now uncertain whether to risk a PCR – which is the only option for visitors. Yes, I could pay for a private one as an experiment, but if it returned a positive result I would have to isolate yet again, despite not posing a risk to others. It is wiser, perhaps, to eschew a PCR-dependent destination for somewhere that accepts an antigen test or proof of recovery.

According to studies, those who have had a more severe symptomatic infection, longer duration of illness, or immunosuppression may be more likely to return a false-positive PCR – but research is ongoing, and the health experts are quick to point out that each case is unique. “But the longer you leave it since you had Covid, the lower the risk [of a false-positive],” advises Dr Scott, “so if you have an option to leave it longer before taking a test you may want to.”

Even amid the heightened caution around omicron, there are still relatively few barriers against holidays – at least compared with December 2020 – but the risk of a false-positive PCR is all-too real. It feels especially unjust, then, that its sufferers pose no threat to others at all – and that our newly boosted immunity isn’t quite the golden ticket we were promised.