evidence: Types of Vaccine Ingredients Antigens can be considered the active ingredients in vaccines because they are the parts of the vaccine to which an immune response is generated. Most often these components are whole viruses or bacteria, parts of the viruses or bacteria, or products made by bacteria, called toxins. Some vaccines against COVID-19 also use nucleic acids. Learn more about how vaccines are made. [...] Because vaccines are made from viruses and bacteria, some chemicals and cell byproducts used during vaccine production may remain in the final preparation in minute quantities. Some examples include antibiotics, DNA, egg proteins, fetal tissues, formaldehyde, human proteins, and yeast.