Investigating changes in mental health services utilisation in England and their impact on health outcomes and wellbeing during the COVID-19 pandemic: Protocol for a health data-linkage study.

This study comprises a deprived population-based cohort of people having referred or self-referred to NHS-funded secondary mental health services or Improving Access to Psychological Therapies (IAPT) services over an extended period of the lockdown in England (2019-2022).This study will utilise a new longitudinal data resource that will link together detailed data from a cohort of individual participants and retrospective administrative data relating to the use of primary, secondary, and community care services.The study period covers pre-lockdown, different lockdown and post-lockdown, and out of lockdown periods up to March 2022.Routinely collected administrative data contain limited contextual information and represent an underestimate of total health outcomes for these individuals.Routinely collected datasets can often been incomplete or contain missing data, which can make it difficult to accurately analyse the data and draw meaningful conclusions.Intervention and treatment for mental health conditions are not wholly captured across these data sources and may impact health outcomes.