A hospital has asked all patients to stay away until further notice, except for women expecting to give birth, due to problems with its oxygen equipment.
Watford General Hospital has declared a critical incident and earlier urged any emergency patients to go to other hospitals with A&E units.
The hospital said the decision was taken as a "result of a technical issue with our hospital's oxygen equipment".
It added that the problem did not "pose any risk to our patients".
"A small number of patients are being transferred to other hospitals in the area, with each patient being fully assessed in line with existing safety guidelines before they are moved," a West Hertfordshire Hospitals NHS Trust spokesman said.
As of 17:00 BST on Thursday, 29 people had died at the NHS trust's hospitals after being diagnosed with Covid-19.
The trust is responsible for Watford General as well as Hemel Hempstead and St Albans City hospitals.
Current NHS advice tells people with coronavirus or suspected symptoms to avoid hospitals and other medical settings like pharmacies.