This guidance is for the person reporting, usually the employer (known as the 'responsible person'). The HSE advise that you should follow the RIDDOR reporting of COVID-19 guidance on what to report before using this detailed guidance.
The HSE says it accepts that these are not easy criteria to apply in the unusual circumstances presented by the Coronavirus (COVID-19) pandemic.
This guidance gives further detail to help determine if you need to make a report and provides some principles to use when assessing the information available and making your judgement. It has been developed to assist responsible persons in determining whether reports need to be made.
It covers the following in terms of legal requirements:
dangerous occurrences
cases of disease, exposure to a biological agent
work-related deaths due to occupational exposure to a biological agent.
The document is available here.
HSE March 2021