The original strain of SARS-CoV-2 has an R0 of 2·5, while the delta variant (B.1.617.2) has an R0 of just under 7. Martin Hibberd, professor of emerging infectious diseases at London School of Hygiene & Tropical Medicine (London, UK), reckons omicron's R0 could be as high as 10. In the UK, cases of omicron are doubling every 2–3 days, which puts it on track to supplant delta as the dominant variant in the country by mid-December. It also complicates control efforts. "Contact tracing works well if you have about a week between one infection and the next", explained Hibberd, "But it is almost impossible to make it work if you only have 2 or 3 days between infections.