The COVID-19 pandemic posed significant challenges to healthcare systems globally, including Tanzania, disrupting essential health services, including child basic immunisation programmes. Using an interrupted time series (ITS) analysis, we analysed how the pandemic influenced child fundamental immunisation trends across three key tears: 2020 (partial lockdown), 2021 (introduction of COVID-19 vaccination programmes), and 2022 (post-pandemic recovery phase). The results reveal a slight decline in immunisation coverage in 2020, with no significant impacts when this year is used as an intervention point. However, a sharp decline in 2021 indicates a more substantial disruption in immunisation services, coinciding with the implementation of COVID-19 vaccination campaigns. When 2021 is considered the intervention year, the study finds significant immediate and prolonged impacts, suggesting that the combination of pandemic fatigue, the reallocation of health resources, and public fear may have exacerbated the decline in immunisation rates. By 2022, immunisation coverage showed signs of recovery. These findings highlight the complex and evolving nature of the pandemic’s impact on child immunisation services. This study highlights the need for targeted interventions to restore and maintain essential health services, particularly in global health emergencies. Recommendations include improving healthcare resource allocation during crises, strengthening public health messaging, and expanding community outreach to mitigate long-term service disruptions.