What are the different types of vaccines?

Vaccines can be classified by their composition: 1) Live attenuated - weakened pathogens that replicate but don't cause disease (MMR, oral polio). 2) Inactivated/killed - whole pathogens killed by heat or chemicals (flu shot, hepatitis A). 3) Subunit/protein - purified antigens or recombinant proteins (hepatitis B, HPV). 4) Toxoid - inactivated toxins (tetanus, diphtheria). 5) Conjugate - polysaccharide antigens linked to carrier proteins for better immune response (pneumococcal, Hib). 6) mRNA - genetic instructions to produce antigen (COVID-19 Pfizer, Moderna). 7) Viral vector - modified virus delivers antigen genes (COVID-19 AstraZeneca, J&J). Each type has advantages in terms of safety, efficacy, production complexity, and cold chain requirements.