The report published by Public Health Wales also identifies marked declines in hospital care during the coronavirus outbreak among those clinically extremely vulnerable (CEV) to severe Covid symptoms.
The study was based on more than 127,000 people who were advised to shield in Wales, representing approximately four per cent of the population.
The most common underlying conditions among the group were respiratory, followed by immunosuppression and cancer.
Overall levels of healthcare use are usually much higher amongst the CEV population than the general population.
Yet in 2020, compared to 2019, the greatest declines in emergency healthcare use were among the CEV population.
Emergency department attendances decreased by 24 per cent among the CEV population and 20 per cent among the general population.
While emergency hospital admissions decreased by 26 per cent among the CEV population and 13 per cent among the general population.
Covid-19 also contributed to patients potentially delaying planned care to a time when they felt less at risk.
Jiao Song, principal statistician for research and evaluation in Public Health Wales, said: “This is the first quantitative study examining changes in healthcare presentation specifically for mental ill health amongst the CEV population.
"We found that from March to September 2020, one in 50 of the CEV population had a clinical record of depression and/or anxiety, and of them nearly one in five had no previous history of mental ill health.”
The study was led by the Networked Data Lab Wales (NDL Wales), a collaborative programme between Public Health Wales, Digital Health and Care Wales, SAIL Databank and Social Care Wales and funded by the Health Foundation.
Ashley Akbari, senior research manager and data scientist and Swansea University, said, “Working together, sharing and building expertise has been part of our core values and strengths in Wales for a long time, and central to the NDL Wales and the wider NDL collaboration as a whole.
"Understanding patterns of healthcare use and needs amongst those shielding is essential to ensure that any further population interventions of this nature, in the future, learn from the Covid-19 pandemic.”
Alisha Davies, lead for NDL Wales and head of research and development at Public Health Wales, added: “These are important findings to help the healthcare response supporting those who have been shielding over the past year.
"Given such a significant change in hospital care and evidence of the inverse care law, insights from this study and others are needed to provide more in depth understanding of the CEV and general populations’ unmet needs.
"Such information can help inform effective, efficient and equitable delivery of future care."