epidemiology

Nine-year-old Danielle had Covid in January and, ever since, she’d been experiencing bouts of sharp chest pains and a racing heart. Her dad sounded sensible and pragmatic. They hadn’t been unduly concerned because the episodes were infrequent and short-lived; mostly she was her usual, active self. “But they’ve been coming on lots more over the past ten days. She’s getting them three or four times a day now.” I confirmed his assumption that these pains likely represented Long Covid: ongoing symptoms that can persist for months (and, in some cases, years). The recent exacerbation appeared to be linked to a new viral illness – runny nose, sore throat, slight cough – that Danielle had come down with shortly before the Easter holidays. I said the change was probably due to the strain of fighting off the new infection. “And it could be Covid again, of course,” I added. He was surprised; he hadn’t thought of that – she was much less unwell than in January. But one of the hallmarks of the current Omicron wave – and in particular the prolonged uptick caused by the BA.2 variant from which we are only just beginning to emerge – is the number of reinfections. During the week Danielle became unwell the second time, the ONS estimated that one in 13 people had Covid. Neither previous infection nor vaccination prevents someone reacquiring the virus, though both protect against hospitalisation and death. [See also: Devi Sridhar on the next pandemic and why she abandoned “zero Covid”] As it affects a susceptible population, the evolution of a virus proceeds in two directions. First is transmissibility. A variant that can spread more effectively will gain the upper hand, displacing previous versions to become the dominant strain. This is what we saw with Alpha, Delta and Omicron. But as upper limits of transmissibility are approached, and particularly as population immunity – whether through infection or vaccination – grows, immune evasion becomes more strongly selected for. Variants that can side-step existing protection have the advantage. Reinfection becomes common. This is the equilibrium humankind has reached with the handful of other coronaviruses we live alongside, which cause about a third of common colds. We repeatedly fight them off, but, as viral evolution progresses and immunity wanes, we become susceptible again. And maybe this is where we’ll end up in several years’ time, with Covid as just another seasonal nuisance. But we aren’t there yet. Omicron can still prove serious and even fatal; broadly speaking, though, it is less virulent than previous variants, preferentially infecting the upper airways rather than the lungs. But while there is a link between diminished disease severity and heightened transmissibility – viruses that cause milder symptoms leave hosts fit enough to spread them more widely – there is nothing that trammels viral evolution down more benign routes. Mutations that confer immune evasion and coincidentally increase virulence would make Covid more lethal again. The UK government has chosen to allow Covid to run free because it is content that the metrics of ICU occupancy and mortality have been greatly blunted compared with earlier waves. One problem with this approach is being felt by those who have developed Long Covid – 1.7 million people in the UK, according to the latest ONS estimate, many with far more debilitating symptoms than Danielle. In sectors such as health and social care, high prevalence is causing untold disruption to service delivery. And if a subsequent wave (and there will be repeated waves) is caused by a variant with enhanced virulence, it may well have bolted before we realise there’s a stable door that needed shutting. These are the risks we’re running in order to return to “normality”. Booster campaigns along with surveillance for new variants and monitoring their virulence are the mitigations it’s hoped will keep us out of trouble. Whether this is the right call, we shall simply have to wait and see. [See also: What is monkeypox and how worried should we be?]

epidemiology

Covid might seem less lethal, but the pandemic is far from over

Kate Pickett: If governments had followed our science, we’d have fewer deaths