BUSINESSES IN ENGLAND had been eagerly anticipating June 21st—the “freedom day” when all coronavirus restrictions were to be lifted. But on June 14th Boris Johnson dashed their hopes. It was “time to ease off the accelerator” said Boris Johnson, Britain’s prime minister. Cases, hospitalisations and admissions to intensive care were rising. Ireland, France and Germany are among countries that have tightened restrictions on travellers from Britain. The cause for concern is the delta variant of SARS-CoV-2 (the virus which causes covid-19).

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Previously known as B.1.617, the variant was first discovered in India in February. It contributed to the country’s horrific wave of infections which started in the spring: The Economist estimates that there have been more than 1m covid deaths in India this year. The delta variant has spread rapidly across borders, too. According to GISAID, a data-sharing initiative for corona- and influenza virus sequences, the mutation has been identified in at least 70 countries. It seems to be the most prevalent SARS-CoV-2 strain in places including Canada, Indonesia, Pakistan, Portugal and Russia. Britain was slow to close its borders to travellers from India after the new variant was detected there; as a consequence the variant has spread more quickly than it might otherwise have done.

Like the alpha variant before it (first discovered in Britain), the main reason people worry about the mutation is transmissibility. In Britain the delta variant already represents more than 90% of new cases. A study published this month by Public Health England (PHE), a government agency, suggests that it could spread nearly 2.5 times as quickly as the original SARS-CoV-2 virus did (though the estimate has a wide confidence interval). That is a big step up from the alpha variant, which was about 1.5 times as transmissible as the original strain. In fact, delta is probably more contagious than any other variant, including gamma (P.1), first discovered and still spreading in Brazil; beta (B.1.351), which is prevalent in South Africa; and epsilon (B.1.427-9), first discovered in America.

As well as being more transmissible, the delta variant seems to lead to more severe disease. On the basis of lab experiments, America’s Centres for Disease Control and Prevention suspects that both problems could be linked to the delta variant’s ability to circumvent antibody defences, regardless of whether antibodies were induced by past infection or vaccination. PHE reported that the risk of hospitalisation from the delta variant increases by between 32% and 289% compared with alpha. That may make it more deadly, too.

The vaccine findings were less alarming. Compared with the alpha mutation, delta lowered protection from symptomatic infection from 50% to 33% after one dose of Pfizer-BioNTech or AstraZeneca, but only from 88% to 80% after the second, with vaccine protection expected to increase further over time. That inoculation still works so well highlights the urgency of administering jabs, particularly in the poor world. Lower protection afforded by past infections—be it from SARS-CoV-2 or other coronaviruses—is terrible news for countries such as Afghanistan, where about a third of the population was infected by August last year, according to antibody surveys. Hospitals in Afghanistan are currently overrun with covid-19 patients, with delta as the suspected cause. That makes it essential to jab more people more quickly. Without widespread vaccination, it could only be a matter of time before a mutation that is more resistant to vaccination develops.