Zhao Lu'na, a migrant worker in Beijing and mother to a 6-year-old son, admits that optional immunization is too expensive. She and her husband make only a few thousand yuan per month, but optional vaccines can cost from about 100 ($16.20) to nearly 1,000 yuan.

"I believe vaccination protects the child, but we cannot afford optional vaccines," Zhao says.

In her neighborhood, where there are mainly migrant residents, most children do not get the extra inoculations because people think they cost too much and they are not a necessity, Zhao adds.

Even when money is not a problem, many parents choose not to get optional vaccination because they are fearful of health risks and undesirable results.

"I have no choice with the mandatory vaccinations because it is government regulations. But I will not let my child get any optional inoculation," says Bian Jianling, a Beijinger with a 6-year-old daughter. "No one can be 100 percent sure that the vaccines are completely safe."

In the eyes of Bian and those who question the safety and necessity of optional vaccinations, their concerns are not groundless. There have been occasions when they have been alarmed by news headlines about children who developed severe side effects after getting vaccinated, with some paralyzed or even killed.

In 2010, bad vaccine scandals occurred in Shanxi, Guangdong and Jiangsu provinces, generating widespread panic among parents.

According to doctors, however, people should thoroughly consider their options before they decline all optional vaccination.

"Like any other medication, there are possible risks and side effects associated with vaccination," says Zhang Wenli, a senior pediatrician with No 3 Hospital of Peking University.

"The most common side effects like fever or rashes are mild, and go away within days. Vaccines can prevent serious or deadly diseases."

Zhang quotes an outbreak of polio in Xinjiang Uygur autonomous region in 2011 which killed one person and paralyzed another 17 patients, including eight children. The infection had been brought in from abroad.

"If more than 85 percent of Xinjiang residents had been vaccinated against polio, that tragedy could have been avoided," Zhang Wenli says.

While it can be tragic for the rare individual who develops a severe reaction to a vaccine, generally the chance of that occurring is less than one in a million, Zhang adds.

"It is understandable that people are unwilling to put their children at risk, or they think it is unnecessary to spend money on optional vaccination, but nothing is better than efficient prevention."

Zhang Wenli suggests parents should carefully weigh the possibility of the disease against the risks of vaccination and decide whether to get their children vaccinated, rather than to turn down all optional vaccines.