Can you catch the coronavirus a second time? That remains unclear. A key question is whether antibodies produced by the body following an infection with the coronavirus provide some level of immunity, and if so, for how long. But we do have some clues. “We know from ‘normal’ coronavirus studies done in the past you can infect people after about a year following an initial infection,” said Dr Ben Killingley, consultant in acute medicine and infectious diseases at University College London hospital. Dr Joshua Schiffer, an expert in infectious diseases at Fred Hutchinson Cancer Research Center in the US, added that any signs of reinfection would require detailed scrutiny. “I have yet to see a definitive case of reinfection reported in the scientific literature [to date]. To truly prove reinfection, and discriminate from prolonged viral shedding related to the first infection, would require sequencing of both the first and second viruses and demonstration that the two viruses are genetically different,” he said, adding it will also be important to look at symptoms and how long reinfection lasts. “It is also important to recognise that documentation of one or a handful of reinfections does not prove that this is a common, epidemiologically important event.” What happened in South Korea, where patients tested positive after having recovered from Covid-19? This initially caused concerns, as experts feared the results might suggest patients had been reinfected. However, the World Health Organization has since said the results were actually false positives, a result of the test picking up particles of the virus within dead lung cells – but this is not active virus. That’s because the PCR (or “have-you-got-it”) test is based on detecting genetic material from the virus – on its own it does not reveal whether that virus is active, and infectious, or not. Why do some people have long-lasting symptoms? Some people with Covid-19 experience complications following an initial infection. “In some persons they begin to feel well again and signs and symptoms including fever decrease, but some then go on to develop respiratory distress and must be provided oxygen in hospital,” said David Heymann, professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine. “It appears to be a delayed immune response that is more serious in some persons and that reacts to remaining virus in various organs.” Even some people who have had a mild experience of Covid-19 have reported experiencing symptoms for several weeks or even months, although experts say only time and testing will tell what the long-term implications are and how common this is. Killingley said it was unclear what was behind such ongoing symptoms, but there are a range of possibilities – including inflammation caused by the virus – although the mechanisms and risks are, as yet, unclear. Immunology professor Daniel Altmann, of Imperial College London, said such cases are important to study. “[We are] at the beginning of describing what may be a complex picture of chronic disease that may ensue from the initial infection – coming and going in relapsing waves, sometimes almost like a kind of chronic fatigue syndrome,” he said. “[These cases] may reflect examples of virus not fully cleared, or alternatively, some kind of damaging post-hoc disturbance to immune or inflammatory function,” he said. Schiffer agreed such cases are cause for concern. “In the case of Sars CoV-2, my colleagues and I are particularly worried about the possibility of prolonged periods of lung inflammation and/or scarring,” he said. Could the coronavirus lie dormant in the body, and then reactivate? Experts say it is unlikely. “I know some have discussed ‘reactivation’ of virus, but this seems unhelpful and unsupported to me,” said Altmann. “It’s a term that’s borrowed from other viruses, especially the herpes virus family, that can hide in the body in a latent state to reactivate years later. [There is] no evidence of that at all for coronaviruses.” How long are people with Covid-19 infectious for? A recent study of 60 coronavirus patients in China showed 10 patients tested positive for Covid-19 after discharge from hospital. However, once again, experts say this could be down to the test picking up genetic material from inactive virus, with other studies, including work in South Korea, suggesting those who test positive with Covid-19 after recovering from the disease are not infectious. “It’s not uncommon to find virus in the nose or throat for up to four weeks after initial infection, but tests to establish whether this is live infectious virus – as opposed to just genetic material detection – are not normally positive for much longer than a week,” said Killingley, adding: “I haven’t come across relapse cases whereby the initial infection acquired weeks ago reignites itself into an infectious case.” Schiffer had doubts: “In my view, the presence of Sars CoV-2 RNA many weeks after initial infection may represent ongoing viral replication in cells, which also implies the persistence of small amounts of “active” virus,” he said. “However, quantities are tiny and may represent the dying embers of infection in the body.” Altmann said that, asymptomatic cases aside, infectiousness correlates strongly with acute symptoms, which typically last for up to 12 days, with a tailout to 28 days. “My sense is that in the people who are feeling unwell long after this, it may not be virus, but some kind of immune or inflammatory perturbation,” he said.