What makes delta so dangerous?

IIn May 2020, Australian biologist Edward Holmes dared to speculate. Holmes is an expert in the evolution of viruses, and in a lecture on SARS-CoV-2, he outlined his ideas about how the virus might change in the future: Little would change in how contagious the virus is, Holmes suspects, but the risk should decrease over time. In the long run, due to the increased immune protection of the population, the pathogen can change in such a way that it can better escape from the human immune system and become recurrent winter viruses, like other seasonal coronaviruses. In fact, SARS-CoV-2 has already succeeded in at least partially bypassing immune protection for some people. Otherwise, he was completely wrong, says Edward Holmes a year later. Because the virus has become more dangerous – and much more contagious.

Delta, the variant of the virus that is currently spreading rapidly around the world and is already responsible for three-quarters of all infections in Germany, is not the same virus that emerged in the Chinese city of Wuhan at the end of 2019. This pathogen initially had a reproduction number of 2 to 3, which means: an infected person infects an average of 2 to 3 others. But then it infected more and more people, and the virus spread through the population, changed, and at some point – most likely in China – a variant appeared, marked by a small mutation called D614G, which spread better. The number of copies rose slightly, and D614G spread around the world – the basis for nearly all SARS-CoV-2 variants that people currently infect. Then in October, a new variant of the virus was observed in southern England: B.1.1.7 or Alpha, as it is now called in the new designation of the World Health Organization. The virus appeared to be about fifty percent more contagious than the type that had been prevalent until then. Meanwhile, delta, a forty to sixty percent more infectious variant of alpha that may cause a severe course in infected persons, is spreading.