The head of the WHO Europe was Friday pessimistic about vaccines' ability to put an end to the Covid pandemic, as new variants dash hopes of reaching herd immunity.Faced with the possibility that the virus may be around for many years, health officials must now "anticipate how to gradually adapt our vaccination strategy," in particular on the question of additional doses, Hans Kluge told reporters.In May, the WHO director had said "the pandemic will be over once we reach 70 percent minimum coverage in vaccination."Asked by AFP if that figure was still a target or whether more people would need to be vaccinated, Kluge acknowledged that the situation had changed due to new, more transmissible variants, such as Delta."I think it brings us to the point that the aim of a vaccination is first and foremost to prevent more serious disease, and that's mortality," he said."If we consider that Covid will continue to mutate and remain with us, the way influenza is, then we should anticipate how to gradually adapt our vaccination strategy to endemic transmission and gather really precious knowledge about the impact of additional jabs," he added.Epidemiologists now suggest that it is unrealistic that herd immunity can be reached solely with the use of vaccines, though they remain crucial to contain the pandemic.High vaccination rates are also necessary to "to unload the pressure from healthcare systems" that desperately need to treat other diseases pushed to the backburner by Covid, Kluge said.The Delta variant is considered to be 60 percent more transmissible than the previous dominant variant Alpha, and twice as contagious as the original virus.The more contagious the virus, the higher the bar for reaching herd immunity, which is when enough people are immune that the virus stops circulating. That can be obtained either by vaccination or natural infection.