According to

https://www.dictionary.com/browse/secondary-disease

a secondary disease is ""A disease that follows and results from an earlier disease, injury, or event".

https://medical-dictionary.thefreedictionary.com/ has

"a morbid condition subsequent to or a consequence of another disease".

The point seems to be reporting a continued sick leave which is longer than anticipated. In that case, "reporting a follow-up or prolonged sick leave" may be more straightforward than a wording including "sickness" ("continued" may actually be a useful option as well.)

in line with @Anne Schulz:

-> Thank you, Andres, "medical leave extension" seems to be worth an answer on its own.

medical leave extension / extension of sick leave

sources:

Could Denying A Request for an Extension of Medical Leave Violate ...
https://www.cwm-law.com/.../denying-request-extension-medica...
27 sep. 2017 - You have an employee who has been out on medical leave because of a serious health condition. But before his scheduled ...

How Many Extensions of a Medical Leave Must An Employer Grant ...
https://www.hopkinscarley.com/.../how-many-extensions-of-a-m...
Managing employees on medical leave of absence is one of the most vexing human resource problems a company must manage.

Multiple Requests for Extension of Medical Leave – Three Strikes and ...
https://www.laboremploymentperspectives.com/.../multiple-req...
24 nov. 2014 - A recent Americans with Disabilities Act case is a reminder the employers must proceed carefully before denying repeated requests for ...

ARTICLE 12 EXTENSION OF SICK LEAVE Section 1. Five (5) working ...
https://www.umass.edu/.../ESL Article 12 web.pdf
EXTENSION OF SICK LEAVE. Section 1. Five (5) working days after a unit member has exhausted all of his/her sick leave, vacation leave, and personal leave, ...