The vaccine is safe. Prior to approval for practical use, the quality and safety of any vaccine is verified through a series of experimental steps, and quality control is carried out on a regular basis, even after it is released to the market. Researchers also use a variety of sources to monitor any reactions or side effects associated with the vaccine. The majority of vaccine reactions are mild or transient, such as pain at the injection site and mild fever. In very rare cases, serious reactions may occur, in which case the case should be investigated immediately.
The risk of vaccination is much lower than that of the disease. For example, the polio virus can cause paralysis, measles can cause encephalitis and blindness, and some injectable infections can be fatal. Although vaccine-related morbidity and mortality are rare, the importance of vaccination is negligible compared to the risk, and many diseases and deaths that cannot be prevented by vaccines continue to plague humanity.