new coronavirus variant that was first sequenced in India has been doubling about every eight days in England since early April. The Delta variant is now dominant in England, and according to a recent risk assessment from Public Health England (PHE), it has a substantially increased rate of transmissibility compared with the previously dominant variant Alpha, which was first sequenced in the UK. Having just a single dose of vaccination is less effective against Delta, and it may lead to a surge in hospital cases. We always knew variants were a risk to England’s roadmap out of lockdown. In January, the science advisory group Sage warned that England’s red-list border policy of “geographically targeted travel bans” was unlikely to keep new variants out, not least because we didn’t know where the next one would come from. England spent the winter trying to keep out the Beta variant, first sequenced in South Africa, and the Gamma variant, first detected in Japan in travellers from Brazil. The effort mostly succeeded, but the government’s red-list policy entirely missed the rise of a dangerous new variant in India. India, a country with close travel links to the UK, first reported a concerning new variant on 24 March. Cases had more than doubled in the country over the preceding two weeks. On 9 April, the UK put Pakistan and Bangladesh on its travel red list, but despite mounting cases and a concerning new variant, India wasn’t included. It finally added India to the red list on 23 April. Imported cases dropped soon afterwards, but the damage was already done. The new Delta variant has mutations that are associated with vaccine resistance – so why didn’t PHE immediately escalate Delta to a variant of concern in March? Part of the reason was a lack of firm evidence showing the new variant was more transmissible, more severe or more vaccine resistant. Yet, as the scientific adage puts it, “the absence of evidence is not evidence of absence”. Establishing the evidence takes weeks of careful science. If a variant is spreading rapidly, the answer isn’t to wait, but to act decisively and quickly. In a document prepared for Sage on 11 May, researchers highlighted the risk that incontrovertible evidence showing Delta was more transmissible “may come too late”. “In the face of uncertain evidence,” they warned, “the risk of overreacting seems small compared to the potential benefit of delaying a third wave until more people are vaccinated.” Their concerns were justified. In early April, the Delta variant accounted for 0.2% of sequenced cases in England. Two weeks later, this had grown to more than 3%. By 15 May, a mere seven weeks after the variant was first identified in England, it accounted for 38% of sequenced Covid-19 cases. Many explanations for its spread were offered: it was mainly among travellers; there was something unique about the communities where it was spreading; we couldn’t infer anything from rapid spread in India because the country’s sequencing was poor or its situation did not translate to ours. Moreover the continued decline in the previously dominant Alpha variant masked the rise of Delta: people could point to overall case numbers as proof that there was nothing to be concerned about. In fact, we were living through two epidemics – one declining and one rising. This was obvious to anyone watching the relentless exponential growth of Delta during April. By the middle of May, after PHE eventually escalated Delta to a variant of concern, members of Sage were becoming extremely worried. Modelling from a Sage subgroup showed that a variant that was 40% more transmissible than Alpha could cause a huge surge in hospitalisations – even if vaccines were still effective against it. Delta may be 50% more transmissible than Alpha, according to Sage minutes from 13 May. The scientific group warned that progressing to step 3 of the roadmap on 17 May could result in a January-level surge, and that the principles of “earlier, harder, broader” responses were key. Sage could hardly have been clearer about its serious concerns, but the government went ahead with plans to loosen restrictions. Almost all indoor spaces were opened up on 17 May, indoor mixing of households was allowed and mask mandates dropped in secondary schools. Meanwhile the Delta variant continued its remorseless rise. By the week of 22 May, it comprised nearly 60% of sequenced cases in England, with new hotspots emerging in Bolton, Blackburn and Bedford. Firm evidence of the dangers posed by the Delta variant has now been established, but not in time for us to prevent its dominance. Meanwhile new research from the Francis Crick Institute shows that protection from two doses of the Pfizer vaccine may wane significantly against the Delta variant after a few months and in older people. Our most vulnerable populations are just now reaching the three-month post-second dose milestone. Through its poorly applied border policy, England managed to keep out the Beta variant but let in an equally resistant, more transmissible and more severe distant cousin. The new dominance of Delta is already making itself felt. Cases in England soared in the week prior to 4 June. There have been many headlines warning of the start of a third wave – but in truth, the third wave started eight weeks ago, when Delta began its march to dominance. The lessons are clear. We have a leaky and inefficient border policy. As so many times before, the government did not act on early signs of exponential growth. Every week I’ve become more alarmed by the growth of the Delta variant. Three questions remain: given the evidence we finally now have, will the government act to curb spread? Will we finally adopt a border policy that protects our vaccination programme? And, if a new, worse variant emerges here, will we be able to stop it in time? Christina Pagel is director of UCL’s Clinical Operational Research Unit