Red Door has been asked to raise awareness of chronic viral hepatitis B and C through a global programme of events and activities.
For the past three years, the WHA has worked with Fleishman-Hillard on raising awareness of the disease and World Hepatitis Day.
Red Door MD Catherine Oliver said: 'The task to co-ordinate a global day involving 193 World Health Organization member countries is a challenge the team will relish both here in London and around the world, with our partners in The Health Collective Network, who will provide local support in many of the countries taking part.'
This year, World Hepatitis Day became the fourth disease-specific health day to be recognised by the WHO, alongside Aids, TB and malaria.
The campaign will launch in early 2012 and will focus on working with patient organisations and advocacy groups, as well as targeting national governments, health policymakers and the public.
As part of the brief, agencies were asked to 'bear in mind' that the 2012 World Hepatitis Day on 28 July coincides with the first day of the London Olympics.
The agency review by the WHA, exclusively revealed by PRWeek (7 October), triggered a flurry of activity as 15 healthcare agencies fought for the business, followed by a final three-way pitch process.
The decision to repitch is understood to be related to an increased remit for the WHA, which is expanding to cover all 193 WHO members.