Salmonella enterica is one of the most common blood stream infections in many parts of sub-Saharan Africa. An estimated 3.4 million people develop iNTS disease annually and the case fatality rate is as high as 20-25% in areas with limited resources. Around 11-20 million people are diagnosed with typhoid fever every year with 128,000 to 161,000 losing their lives to the disease. For typhoid fever, novel typhoid conjugate vaccines (TCV) are already developed and prequalified through the World Health Organization (WHO) while several vaccines against iNTS disease are currently undergoing clinical testing. Throughout the development process of new vaccines, different surveillance methods need to be deployed to allow an initial accurate assessment of disease burden, detailed knowledge on disease epidemiology, endemic areas for the target disease as well as the highest risk groups for infection. Post-vaccination, the impact of vaccines needs to be determined, which requires a different set of surveillance.

Through this work, I investigate the use of disease surveillance for public health decision-making, using an existing bacterial surveillance infrastructure that was setup in the past 15 years in various African countries. This thesis investigates the epidemiology and transmission of invasive Salmonellosis, particularly iNTS disease, evaluates trends identified through the now over 10 year running surveillance, including potential relationships with other pathogens (i.e., Plasmodium falciparum malaria and COVID-19), and eventually disaggregates the use of surveillance and obtained data for public health decision making to use TCV and upcoming iNTS vaccines in endemic settings.

The research presented here relies mainly on data collected through Severe Typhoid in Africa program conducted from 2016 to 2019, as well as the Typhoid Fever Surveillance in Africa Program (TSAP) from 2011-2014 for long-term trends determination.

I report incidence of iNTS disease estimated from 2016 to 2019 in five sub-Saharan African countries, with the focus on young children considering the potential iNTS vaccine introduction in these settings. iNTS remains one of the major febrile illnesses in sSA, and the biggest burden lies on the young children under the age of two. I further investigated the severity of the iNTS disease and AMR profiles in different countries, and assessed the association between iNTS disease and P. falciparum malaria, one of the known risk factors to iNTS disease. In addition, I used phylogenetic analysis to investigate the different S. Typhi strains circulating in Madagascar for the past ten years, and participated in updating the pan-African S. Typhimurium phylogenetic tree. Findings from these work can have significant potential impact on public health as they show how such surveillance data can help make the informed decision. Lastly, with the Covid-19 pandemic which started during my PhD work, I evaluated how COVID-19 had an impact on the surveillance activities and severity of disease in Madagascar.

Overall, I demonstrated how disease surveillance can make an impact on the public health decision making with an example of the invasive Salmonellosis surveillance in sSA. Data collected from surveillance not only show the burden of the disease and the extend of AMR, but also guide the way for the policy makers to make an informed decision on the public health matters including vaccination. Well-established, sustainable surveillance system can monitor the impact of interventions such as vaccination and detect other new pathogens and outbreak in real-time as it was witnessed in our study sites during the pandemic.