The UK Health Security Agency are to continue its investigation into the incident that occurred at Kindred Nursery in Brampton last week after a number of tests carried out all returned negative results.
Tests were carried out in the nursery on water and air samples after some of the staff and children had developed mild symptoms, including a rash and a cough.
The nursery then closed the next day to conduct a deep clean before reopening on Monday, February 7.
The UKHSA East communications office said: "Along with colleagues in Environmental Health, UKHSA East will continue to try and determine the cause of the symptoms experienced by staff and children."
Thrapston Road was closed, and emergency services were called to the incident as a precaution.
Cambridgeshire Fire and Rescue Service attended to investigate and eliminate potential causes before leaving the scene by 6 pm on Thursday.
A clinician and a matron were also on-site to assess the children so that they could return home to their parents and guardians.
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