Active vaccine safety surveillance: Experience from a prospective cohort event monitoring study of COVID-19 vaccines in Kenya

Although active vaccine safety surveillance (VSS) can complement passive VSS while overcoming the inherent limitations of spontaneous safety monitoring, it remains rare in sub-Saharan Africa. We conducted post-authorization active VSS of COVID-19 vaccines in Kilifi, Kenya using a cohort event monitoring study design. Participants were followed weekly over 13 weeks for adverse events. A subset was followed daily for one week for solicited systemic reactogenicity events (chills, fatigue, fever, headache, joint pain, malaise, muscle aches, nausea). The daily prevalence of reactogenicity events was compared to the 3-day pre-vaccine average using McNemar’s test. The association of baseline characteristics with reactogenicity events was assessed using logistic regression. Between 28^th September 2022 and 30^th June 2023, 2,440 participants were enrolled into the cohort; 1,000 systematically sampled participants were included in the reactogenicity sub-study. Most were aged 17–39 years (1683; 69.0%) and were female (1895; 77.7%); 535 (28.2%) female participants were pregnant. The three most frequently reported reactogenicity events were fatigue (422; 44.1%), headache (370; 38.7%), and malaise (346; 36.2%); the proportion of severe events ranged from 2.3% (22; nausea) to 5.0% (48; malaise). Except for headache, the prevalence of systemic reactogenicity events was significantly higher in the first two days post-vaccination than pre-vaccination (p-values <0.05). The odds of reactogenicity events were higher among non-pregnant women (adjusted odds ratio [aOR] 1.81; 95% CI 1.28-2.55) and pregnant women (aOR 1.69; 1.03-2.78) than among men, and higher among Johnson & Johnson (aOR 2.05; 1.40-3.00) and Moderna (aOR 4.19; 2.34-7.51) vaccine recipients than among Pfizer vaccine recipients. The prevalence of pregnancy complications was 2.6% (95% CI 1.4-3.5%) against a background prevalence of 3–49%. Reactogenicity events following COVID-19 vaccination were generally non-severe and transient. There was no elevated risk of pregnancy-related complications. Addressing operational barriers is essential for enhancing the utility and feasibility of future active VSS.