The GRB comes suddenly and from a direction almost perfectly north of the planet. It is a one-time event but the exposure is sufficient to expose the vast majority of the population to a lethal dose. Only those who by chance happen to be sufficiently shielded at the time or deep enough underground at the time of the event avoid exposure. As a benchmark a typical individual in their home or apartment or vehicle experiences between 6-10 Grays) of exposure in a matter of moments. There is no lingering radiation afterwards to cause further damage and death is not immediate, however it is inevitable for most. Because of the angle at which the GRB approached Earth there is a rapid tapering off of radiation exposure from approximately the equatorial line with the countries and people that lay below this line suffering no ill-effects or exposure to the radiation of the GRB at all thanks to the bulk of the planet effectively shielding them from it.