Near-immediate
People inside buildings or otherwise shielded will be indirectly killed by the blast and heat effects as buildings collapse and all inflammable materials burst into flames. The immediate death rate will be over 90%. Various individual fires will combine to produce a fire storm as all the oxygen is consumed. As the heat rises, air is drawn in from the periphery at or near ground level. This results in lethal, hurricane force winds as well as perpetuating the fire as the fresh oxygen is burnt. Such fire storms have also been produced by intense, large scale conventional bombing in cities such as Hamburg and Tokyo.
People in underground shelters who survive the initial heat flash will die as all the oxygen is sucked out of the atmosphere.
Outside the area of total destruction there will be a gradually increasing percentage of immediate survivors. However most of these will suffer from fatal burns, will be blinded, bleeding from glass splinters and will have suffered massive internal injuries. Many will be trapped in collapsed and burning buildings. The death rate will be higher than in a normal disaster since most emergency services will be incapable of responding due to their equipment being destroyed and staff killed. The sheer scale of the casualties would overwhelm any country's medical resources. The International Red Cross has concluded that the use of a single nuclear weapon in or near a populated area is likely to result in a humanitarian disaster that will be "difficult to address". There is currently no international plan in place to deliver humanitarian assistance to survivors in the case of a nuclear attack. Most casualties would receive at best minimal, palliative treatment. The best they could hope for would be to die in as little pain as possible.