Speaking at First Minister’s Questions on Thursday, Mr Sarwar claimed the deaths of two more children were now linked to infections at Glasgow’s Queen Elizabeth University Hospital (QEUH).
And the Scottish Labour leader again called on the Scottish Government to take control of the hospital and fire the heads of NHS Greater Glasgow and Clyde (NHSGGC).
He told Nicola Sturgeon he had been contacted by the doctor of a child treated at QEUH who had died after contracting the common fungus, aspergillus, which can be dangerous if it infects those with a weaker immune system.
Mr Sarwar also said a second clinician had told him that a second child had died of a waterborne infection, akin to that which is believed to have killed ten-year-old patient Millie Main in 2017, in the past two months.
The ages of the two children are not known.
Mr Slorance, who was head of the Scottish Government’s response and communication unit, went into hospital to be treated for cancer in October last year and died in December. The 49-year-old had been fighting a rare and incurable cancer – mantle cell lymphoma – for the previous five years.
During his time in hospital, Mr Slorance tested positive for Covid-19 and another life-threatening infection, both of which his widow, Louise Slorance, believes he contracted while at QEUH.
Ms Slorance only discovered the fact her husband had been infected with the fungus when she requested a copy of his medical records after his death.
Mr Sarwar said: “Since raising this in the chamber last week, I've been contacted by a senior clinician at the hospital who has revealed there was another case of Aspergillus in a child cancer patient around the same time and in the same ward as Andrew. That child tragically died.
"When a hospital reports a serious infection like Aspergillus, a red report should be filed and the health secretary informed. Did this happen, were you aware of this death and what action was taken?”
Ms Sturgeon said she did not have details of the cases raised by Mr Sarwar, but said she would look into the case “as a matter of urgency”.
She said she had written to Mrs Slorance on Thursday to tell her what action had been taken, including an independent external review of her husband’s case notes. The health secretary has asked Health Improvement Scotland to carry out a wider review into infections at the QUEH.
Mr Sarwar said: “Nicola Sturgeon's holding answers are no longer good enough. She has been in charge of this scandal from start to finish.
"This is gross negligence. The health board has failed. The Scottish Government oversight board has failed. And the First Minister continues to fail.
"She must sack the leadership of the health board – today. Sack the oversight board – today. And use emergency powers to take control of this hospital. Otherwise more families will have to pay the price for Nicola Sturgeon failing to do the right thing."
Ms Sturgeon said there would be a “proper and full investigation” into whether there were relationships between infections and insisted that NHS Greater Glasgow and Clyde was in “what is often referred to as special measures”.
She said: “Sacking the health board does not change overnight the practice in a hospital. When concerns are raised about the cause of someone's death, then that has to be properly investigated so that the action that is then taken as a result of that is the right action.
"And it is not right to say that no action has been taken over four years.
“Anas Sarwar says to me ‘use your emergency powers to take control of the hospital’. NHS Greater Glasgow and Clyde is at the highest level of escalation and will remain there and while all of these issues are investigating and action is taken.”
Ms Sturgeon added: "They are serious matters all of us should take seriously, but we do not do justice to the families concerned if we simply call for action that is not based on proper investigation, proper scrutiny and proper consideration and that's the duty of government and that is the duty we will continue to take seriously.”
However, Scottish Labour MSP Jackie Baillie stepped in to correct Ms Sturgeon as a point of order.
She said: “The First Minister said that Greater Glasgow and Clyde was at the highest level of escalation. That is simply incorrect and I am genuinely surprised that she got it wrong, given that she is the former health secretary.
"Greater Glasgow and Clyde is at stage four of the escalation framework. The highest level is stage five, and involves the Cabinet secretary for health using ministerial powers of intervention under the National Health Service Act of 1978.
"The last time this was used was in 2018 to remove the chief executive of NHS Tayside. So the First Minister is wrong. Will she correct the record? And will she now act before families are devastated by the loss of loved ones?”
Following FMQs, Mr Sarwar published comments made to him by senior clinicians at QEUH.
One said: “I think there are serious questions for the health board and the Scottish Government. Are they doing enough to keep people safe?
"There was another case of aspergillus around the same time as Andrew Slorance and in the same ward. A child cancer patient died after contracting the infection in November 2020.
"It begs the question – if there was a case as far back as the 4th of November, what did the health board do to investigate it? Did they look for an environmental source and could future infections have been prevented?"
A second doctor said: “Despite the reassurances from the health board and the Scottish Government, there continue to be cases of infection linked to water and the environment including Stenotrophomonas. There is a cultural of denial and the absence of proper investigations into these cases.
“The result is inaction with potentially fatal consequences."
Asked if Ms Sturgeon would continue to have confidence in the health board if it emerged it had been keeping details from ministers, a spokesman for the First Minister later said: "Anas Sarwar has asserted things or claimed things.
"Let's see where we get to in terms of investigating exactly the factual basis of what he has outlined."