When Hu Haitao watched his son Hu Bing swallow the small white pills meant to protect him from polio, he never imagined what the end result would be.

14 days after taking the vaccine, the four-and-a-half-month-old was on life support.

"My baby experienced diarrhea and respiratory failure," said Hu. "He was critically ill and had to stay in the ICU."

What followed next is every parent's worst nightmare.

Young Hu slipped into a coma and experienced heart failure.

After three days of treatment, the baby recovered, but his left leg was left paralyzed.

Staff at the hospital in Jinan, in the eastern Chinese province of Shandong said the baby was suffering from polio, but did not give a more specific explanation.

It is a story too close for comfort for mother Ma Yongmin.

Her daughter Zhang Liyang received the polio vaccine in December 2012. Nine days later, the six-year-old was also in hospital.

After taking the vaccine, the youngster complained about a pain in her back. Like most mums, at first Ma thought her daughter just did not want to go to school. But she kept complaining, so her father Zhang Gongxian took her to their local hospital.

"The doctor asked me if she had received any vaccinations, I told him "yes, she had a varicella vaccination," Zhang said. "The doctor told me that the vaccine can possibly lead to illness, but we didn't think it would become so bad."

Little Zhang then lost feeling below the waist.

Desperate for answers, her father took her to a major hospital, also in Jinan, where doctors diagnosed her with myelitis, or inflammation of the bone marrow in her spinal cord.

"She suffers from incontinence and her legs cannot feel any pain or temperature changes," Shi Jiming, the girl's physician said. "From a medical perspective, she is paralyzed from the waist down."

Once a happy six-year-old who loved dancing and singing, Zhang is now confined to her bed. Her days are spent watching cartoons. Sometimes her parents tie her to a makeshift rack so she can experience the sensation of standing.

Young Zhang has been receiving therapy for her condition, but is not expected to walk again.

Ma believes the varicella vaccination given to her daughter is the cause of her paralysis.

A vaccination works by administrating an antigenic material to stimulate an individual's immune system. The immune system then develops immunity to the pathogen being targeted. The benefits of vaccination have been widely accepted globally. Although safe for the vast majority of the population, those with an impaired immune system may suffer from complications.

Dr Wang Yu, an expert with the Chinese Center for Disease Control and Prevention, explains the magnitude of these adverse effects.

"If a person's immune system doesn't work properly, it is unable to stimulate antibodies," Wang said. "Therefore, rather than being destroyed in the body, the pathogens attack the nervous system and actually cause the disease that they are supposed to prevent."

Eleven types of free vaccines are currently on offer to children born in China. Experts say the greater the number of vaccines, the greater the risk of complications.

Wang says multi-function vaccines are currently being used in developed countries, helping to lower the risks posed by multiple vaccinations. But these new technologies are not being used in China yet.

He also warns some vaccines are being produced in China using outdated methods, some dating back 50 years.

According to Wang, unwanted reactions to vaccinations can vary. Some recipients experience sudden swelling of the organs, which can be cured with timely treatment. Some however develop Epilepsy. Others suffer irreversible damage to their bodies, an effect referred to as vaccine sequelae.

Zhang's father had never heard of vaccine sequelae until his daughter became ill.

"I had never considered such terrible consequences," he said. "The doctor only told me how good the vaccine would be, but never said anything about possible side effects."

Wang Yu said vaccine sequelae can theoretically be prevented through preliminary testing to test a person's risk of developing an adverse reaction. However, these tests are not technically feasible. It is a question that baffles the whole world.

According to the CCDCP, around 500 million vaccinations were given in China last year. Adverse reactions accounted for roughly one in every one million vaccinations, affecting about 500 people.

Dr Wang warns it can take several days to several months for a patient to present symptoms of an adverse reaction, making it even more difficult to identify sequelae.

It was only after discussing similarities between his son's illness and those of a friend's child that Hu stumbled upon the truth.

After his son's vaccination in 2005 and the subsequent results, he carried out exhaustive research on adverse effects. Hu decided to have his child tested to see if his illness was caused by the shot.

14 months later he received the result. He cried for two hours.

"I have very mixed feelings," Hu said. "On one hand, I was happy to get the test results. On the other hand, they mean that my son will be permanently disabled."

Zhang is still working to get the test results for his daughter. He has already spent more than 100,000 yuan, or 16,338 US dollars, on medical treatment. But debt is not the family's biggest concern.

"Compared to the debt, we are more worried about our child's future," Zhang said. "What can she do if her condition doesn't improve?"

The Zhang family's days are now filled with doctor appointments and multiple therapy treatments in their never-ending battle to make her walk again.

Their mornings start early with Zhang taking his daughter to physical therapy. They then travel to another clinic for acupuncture, before another round of physical therapy in the afternoon.

Despite the magnitude of their sad stories, the Hu and Zhang families are not alone.

Other parents are seeking both help and compensation from the government, particularly those whose children who are still in the early stages of their illnesses.

The Chinese government provides means-tested compensation to sequelae victims, the maximum compensation ranges from 100,000 to 300,000 yuan based on local annual income. However, many families say the compensation is not enough.

Hu's family has spent more than 200,000 yuan treating their son so far. He is not sure how much they will have to spend in the future.

"My son will undergo his second operation soon," Hu said. "The doctor told us he is expected to require 15 operations from now until he turns 40. We can't imagine how much money we will need."