2-dose vaccines are mostly inactivated vaccines, also called whole virus vaccines; 3-dose vaccines are usually genetically engineered recombinant protein vaccines made through genetic engineering methods. Both can prevent the occurrence of infectious diseases, but there are certain differences in production technology and vaccination time: I. Differences in production technology: 1. 2-dose vaccines: Inactivated vaccines are made by culturing and proliferating viruses in cells, and then by inactivation and purification. The composition and structure of such vaccines are similar to natural viruses, which can retain the immunogenicity of viruses and produce better immune responses with fewer side effects; 2. 3-dose vaccines: recombinant protein vaccines, on the other hand, are made by recombinant viral structural proteins through genetic engineering technology. The production of recombinant vaccines does not require intact viruses and is safer for humans. Because recombinant protein vaccines contain fewer components of the virus, 3 doses are usually required. 2. Difference in vaccination time: 1. 2-dose vaccine: the vaccination time between 2 doses should be ≥ 21 days, and the second dose should be completed within 8 weeks after the first vaccination; 2. 3-dose vaccine: each person should receive 1 dose of this product in 0, 1 and 2 months respectively, and the whole process should be 3 doses, and the 3 doses should be completed within 6 months. In addition, it is recommended that patients choose to receive vaccines of the same production technology from the same company to avoid mixing vaccines and producing adverse reactions. In case of special circumstances, vaccines from different companies with the same production technology can be chosen instead.