Country experiences of responding to the challenges of COVID-19 in 2020 highlighted how critical it is to have strong, in-country health security capacity. The UK government has invested in health security capacity development through various projects and agencies, including the UK Department of Health and Social Care, whose Global Health Security Programme provides funding to Public Health England (PHE) to implement health security support. This article describes the results and conclusions of the midterm evaluation, undertaken by Itad, of one of Public Health England's global health projects: International Health Regulations Strengthening, which operates across six countries and works with the Africa Centres for Disease Control. It also highlights some of the key lessons learned for the benefit of other agencies moving into supporting national health security efforts. The Itad team found strong evidence that the IHR Project is well aligned with, and responding to, partners' capability strengthening needs and that the three workstreams - systems coordination, workforce development and technical systems strengthening are implementing relevant and appropriate action to support national priorities. The IHR Project is also aligned with and complementary to other relevant UK development assistance although the Project could strengthen the strategic collaboration with WHO, US CDC and other UK government projects in countries. The Itad team also found that the IHR Project could be more effective if the technical assistance activities were accompanied by relevant materials and equipment while maintaining its supportive role. There was evidence of where technical assistance in the form of training and follow-up mentoring had led to improvements in practice and in IHR compliance, but these were not being systematically captured by the Project's routine reporting. There was good evidence that the project was doing the right things and aligning its work in the right way, with more limited evidence at the time of the midterm evaluation that it was making progress towards achieving the right results.