Health Minister Jane Hutt has said there are no grounds for suspecting any fault with the BCG vaccine which left 13 pupils at an Aberystwyth school needing hospital treatment.
The pupils were all taken to Bronglais Hospital
One pupil Dominic Hamer, 13, from Ysgol Gyfun Penweddig, suffered a severe allergic reaction to the vaccination.
After a night in intensive care, the teenager was moved to a general ward at Bronglais Hospital on Wednesday.
Two other pupils, who were also treated overnight, were also discharged.
Another 10 students were also treated in hospital after suffering "adverse effects" to the injections.
Nearly 120 pupils at the comprehensive school were being injected with the BCG vaccine, which combats tuberculosis.
Ms Hutt told assembly members at Cardiff Bay: "The National Public Health Service is investigating the circumstances and the initial indications are that there are no grounds for suspecting that there is any fault with this batch of the vaccine."
The minister added that the vaccine batch had been used extensively without any reaction of the kind experienced at the Aberystwyth school.
She said that the Medicines and Healthcare Regulatory Agency's initial assessment also indicated that, "this is an isolated incident and there's no reason to suspect there's a problem with the vaccine."
Ms Hutt was responding to a question on the tabled by
Nick Bourne, Welsh Conservative leader in the assembly.
"There are serious concerns if this batch is still being used elsewhere in Wales or in the UK," he said.
In reply, Ms Hutt said: "The vaccine has been used extensively, as you know, without any reaction of this kind...there is no reason to suspect there is a problem
with the vaccination."
Ceredigion NHS Trust is also investigating and the batch of vaccine has been withdrawn.
Ysgol Penweddig's head teacher, Arwel George, said a letter had been sent to parents explaining what had happened.
"It was upsetting for us all and especially the children who knew the children who were affected," he said.
"They were worried too and feared they would have to go to hospital.
"We sent a note to all parents on Tuesday night explaining the situation."
"The vaccination is normally routine and we've never suffered anything like this before."
Dominic was said to have had an anaphylactic shock, which is defined as a very sudden serious physical reaction caused by an allergy.
The trust has said it is working closely with the school to ensure that pupils and parents receive any information or support they require.
Another of the pupils who had a reaction to the vaccination was the daughter of Plaid Cymru MP Simon Thomas.
He said the mater needed to be examined "as a matter of urgency".