Coronavirus safety posters are displayed in the window of the Sondheim Theatre on June 14, 2021 in London, England. Prime Minister Boris Johnson has confirmed a four-week delay to the final easing of coronavirus restrictions following concern over the virus’s Delta variant and rising infection rates.Rob Pinney | Getty ImagesThe highly contagious delta variant is the fastest and fittest coronavirus strain yet, and it will “pick off” the most vulnerable people, especially in places with low Covid-19 vaccination rates, World Health Organization officials warned Monday.Delta, first identified in India, has the potential “to be more lethal because it’s more efficient in the way it transmits between humans and it will eventually find those vulnerable individuals who will become severely ill have to be hospitalized and potentially die,” Dr. Mike Ryan, executive director of the WHO’s emergencies program, said during a news conference.Ryan said world leaders and public health officials can help defend the most vulnerable through the donation and distribution of Covid vaccines.”We can protect those vulnerable people, those frontline workers,” Ryan said, “and the fact that we haven’t, as Director-General (Tedros Adhanom Ghebreyesus) has said, again and again, is a catastrophic moral failure at a global level.”The WHO said Friday that delta is becoming the dominant variant of the disease worldwide.The agency declared delta a “variant of concern” last month. A variant can be labeled as “of concern” if it has been shown to be more contagious, more deadly or more resistant to current vaccines and treatments, according to the health organization.Delta is now replacing alpha, the highly contagious variant that swept across Europe and later the U.S. earlier this year, Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia, said in a recent interview.Studies suggest it is around 60% more transmissible than alpha, which was more contagious than the original strain that emerged from Wuhan, China, in late 2019.Delta has now spread to 92 countries, Maria Van Kerkhove, the WHO’s technical lead for Covid, said Monday. It now makes up at least 10% of all new cases in the United States, according to the Centers for Disease Control and Prevention, and is on its way to becoming the dominant variant in the nation.The United Kingdom recently saw delta become the dominant variant there, surpassing its native alpha variant, which was first detected in the country last fall. The delta variant now makes up more than 60% of new cases in the U.K.WHO officials have said there were reports that the delta variant also causes more severe symptoms, but that more research is needed to confirm those conclusions. Still, there are signs that the delta strain could provoke different symptoms than other variants.”This particular delta variant is faster, it is fitter, it will pick off the more vulnerable more efficiently than previous variants, and therefore if there are people left without vaccination, they remain even a further risk,” Ryan said Monday.