More and more parents from around the country are recounting horrific flu jab experiences amid mounting controversy over adverse reactions to the seasonal flu vaccine.

Australia's chief medical officer, Professor Jim Bishop, has urged doctors not to use the vaccine on children under five, following more than 250 reports of adverse reactions - a figure experts and parents fear is being severely underestimated.

In Queensland, health authorities are also investigating the death of a two-year-old Brisbane girl, which may be linked to the flu vaccine.

As the vaccine saga unfolds, whether or not to vaccinate has also been a hot topic in online parenting forums such as Essential Baby, where hundreds of mothers and fathers have expressed their fears and detailed nightmare stories of their children's reactions.

Perth mother Marrisa Moir told ABC News Online her two-year-old son began gagging and squealing in the bath about four hours after having the flu vaccine on April 9.

She says Rohan, who has Global Developmental Delay with hypotonia - a condition where milestones are not met at expected ages - has regressed since the ordeal.

"He started staring off into space with this weird look on his face then his eyes rolled back and started going bright red and watering," she said.

"I grabbed him out of the bath and then he started shaking uncontrollably. He couldn't stand or hold anything, he was shaking that much.

"He curled his arms over his chest and kept gagging and letting out squeals and I had no idea what the hell was going on."

Ms Moir says she drove "like a mad woman" to get her son to the Princess Margaret Hospital. She said he passed out on the way there.

"The doctor said it was a febrile convulsion and that 30 minutes was pretty long," she said.

"She [the doctor] thought it was related to the flu needle he'd had and also said they have had quite a few cases of the same thing happening."

Ms Moir says since the post-flu vaccine seizure, Rohan has continued to experience mild seizures lasting about 10 seconds.

"We had an EEG [electroencephalogram] today to monitor that," she said.

"He has gone backwards since having the major seizure in regards to his development and his muscles are a bit weaker.

"We are working on physio and play skills again to get him up to speed, but I've been told it could take some time."

'It was horrible'

Another mother, from North Ryde in New South Wales, says her 18-month-old daughter woke in the middle of the night in a puddle of vomit with a soaring temperature and high heart rate.

Lydia says her daughter Sophie had the flu vaccine on the morning of March 30.

"I was assured by the GP that Sophie would probably not have any side effects and, at worst, she would develop a mild temperature," Lydia said.

"At 3.00pm Sophie started shuddering, became very distressed and felt hot. I gave her Panadol and put her in a tepid bath about 20 minutes later as she was so hot.

"At about 3.40pm her temperature reached 40 degrees. She then started vomiting. I telephoned the GP who told me to give her Nurofen and Panadol and to take her to hospital if her temperature did not come down in half an hour.

"Her temperature, thankfully, started to come down ... but at about 8.30pm, Sophie woke up in a huge pile of vomit, was very distressed and had a high temperature. She also had a rash on areas of her arms that looked like hives.

"My husband noticed her heart was racing so we took her to Royal North Shore Hospital [St Leonards, Sydney]."

Lydia said the nurse at the hospital told her she had seen "many" children with high temperatures and vomiting after receiving the flu injection.

"The doctor on duty also informed us that he had seen three other children that day alone with similar symptoms following the injection," she said.

Lydia says she and her husband are concerned about whether the vaccine will continue to affect Sophie in the future.

"My husband and I were very worried. We have never seen her that sick in her whole life. She has never had a 40 degree temperature," Lydia said.

"She also has never thrown up as much as she did that day and night. She was so distressed. It was horrible. I would have never got this done to my perfectly healthy child if I was informed about the possible reactions."

And it seems the adverse reactions are not just occurring in children under the age of five.

A mother from Bardon, in Brisbane's west, says her three children aged four, six and eight all suffered rapidly rising temperatures in excess of 39 degrees Celsius, severe headaches and repeated vomiting.

She says the shots were administered by a GP, who "was very much of the opinion that the reactions were a response to the vaccine".

Reporting channels

The Federal Government says it has a national reporting line set up for individuals and health professionals to report adverse drug and vaccine reactions, which it says has been in place for decades.

Queensland and Western Australia's health departments both say the onus is on doctors to record adverse reactions to vaccines.

But some parents have told ABC News Online that health authorities dismissed their concerns that their child's reaction may be linked to the flu vaccine.

Jim Hocking from Armadale, near Perth, says both his children had to be hospitalised within days of receiving the flu vaccine in June last year.

"We've ended up with two really sick kids in the space of a week. We haven't taken them to any playgroups, they haven't been around any kids that have been notably sick and now they've absolutely crashed in a day or two," he said.

Mr Hocking said he and his wife told doctors at the Armadale hospital that their children had just had the flu vaccine, but he says there has never been any follow-up and he has no idea if their cases were ever reported.

"You're at a public hospital, you're being dealt with as an emergency case. The only avenue we've been able to raise it with is with our local GP," he said.

"At the end of the day we haven't had any follow-up from anybody."

Mr Hocking says he is worried that health authorities may view cases in isolation if there are no clear channels of reporting reactions.

"It is worrying ... if they don't know that there's a problem. I guess with any injection they give it would be an advantage to have someone to call to say there's been a reaction. It would make sense to certainly have an easier way to report your concerns," he said.

Earlier this week, Kirsten Maloney from the ACT complained that her daughter's reaction to the flu vaccine had so far not been recorded by ACT officials.

The ACT's chief medical officer, Dr Charles Guest, has said there are no reports of serious reactions to the vaccine in the ACT.

But Ms Maloney says her daughter was admitted to hospital after she received the injection.

"There was an adverse drug reaction report filled out by one of the pharmacists at Canberra Hospital, and all the doctors that we talked to and the paediatricians that we spoke to acknowledged that it was most likely the reaction to the flu shot," she said.

"I don't understand how [Dr Guest] doesn't know about that."

Brisbane mother Jodi Hahn said her son reacted 12 hours after having the swine flu vaccine, but authorities told her he probably had croup.

Five weeks after her son was rushed to hospital with febrile convulsions, vomiting and fever, Queensland Health has finally agreed to officially record his case as a reaction to the vaccine.

Peter Collignon, a professor of infectious diseases from the Australian National University, says the number of recorded cases of adverse reactions is probably underestimated, given authorities have no system in place to monitor people's reactions.

He says an effective surveillance system should monitor thousands of people for one or two weeks after vaccination before rolling out the vaccine to the entire population.

"We need a better system than voluntary notification to the TGA [Therapeutic Goods Administration] that there's a problem," he said.

"Because whenever you do that you really underestimate how much of a problem there is."

Health authorities are investigating the recent spike in reports of reactions.