Leiden University Scholarly Publications

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Dijk, W.J. van (2026)

Pandemic visits a doctor

Doctoral Thesis
The COVID-19 pandemic revealed global vulnerability to infectious diseases and highlighted the need for pandemic preparedness built on three pillars: slowing transmission, optimizing healthcare capacity, and ensuring safe, effective vaccination. This thesis addresses these pillars in two parts. Part 1 focuses on early-pandemic surveillance using population-derived data. The COVID RADAR app enabled large-scale symptom and behavior reporting, correlating strongly with national case trends and predicting primary care demand when diagnostic resources were limited. App data also showed that 5–10% of community cases experienced post-acute COVID symptoms 100 days after infection, with distinct symptom clusters suggesting multiple underlying mechanisms.

Part 2 examines coagulation and thrombotic events related to SARS-CoV-2 infection and vaccination. Pre-existing coagulation potential did not increase infection risk but may worsen outcomes in older adults with comorbidities...Show more The COVID-19 pandemic revealed global vulnerability to infectious diseases and highlighted the need for pandemic preparedness built on three pillars: slowing transmission, optimizing healthcare capacity, and ensuring safe, effective vaccination. This thesis addresses these pillars in two parts. Part 1 focuses on early-pandemic surveillance using population-derived data. The COVID RADAR app enabled large-scale symptom and behavior reporting, correlating strongly with national case trends and predicting primary care demand when diagnostic resources were limited. App data also showed that 5–10% of community cases experienced post-acute COVID symptoms 100 days after infection, with distinct symptom clusters suggesting multiple underlying mechanisms.

Part 2 examines coagulation and thrombotic events related to SARS-CoV-2 infection and vaccination. Pre-existing coagulation potential did not increase infection risk but may worsen outcomes in older adults with comorbidities. Both intradermal and intramuscular vaccination temporarily increased inflammatory and coagulation markers, more strongly after intramuscular dosing. Vaccines were associated with venous thromboembolism (VTE), though overall vaccination reduced VTE events by lowering COVID-19 incidence. The thesis highlights strengths and limitations of syndromic surveillance and vaccine-safety methods, recommending case-control designs during pandemics. Finally, it stresses the need for improved public communication to strengthen vaccine acceptance and promote collective responsibility.
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COVID
Vaccination
Citizen science
App
E-health
Post-covid
Venous thrombosis
All authors
Dijk, W.J. van
Supervisor
Rosendaal, F.R.; Kiefte - de Jong, J.C.
Co-supervisor
Hylckama Vlieg, A. van
Committee
Groenwold, R.H.H.; Bonten, M.J.M; Hof, S. van den; Kruip, M.J.H.A.
Qualification
Doctor (dr.)
Awarding Institution
Faculty of Medicine, Leiden University Medical Center (LUMC), Leiden Univesity
Date
2026-01-07
DOI
doi:10.60602/1887-4286964