What Vaccine Types Currently Exist and How Each One Works

Live-attenuated vaccines contain a weakened — but still living — form of the pathogen. Because the virus or bacterium can still replicate briefly in the body, the immune response closely mimics a natural infection, typically producing robust and long-lasting immunity, often from just one or two doses.

Examples include the measles-mumps-rubella (MMR) vaccine, the chickenpox vaccine, and the yellow fever vaccine. The trade-off is that because these vaccines contain live organisms, they are generally not recommended for people with severely compromised immune systems or during pregnancy. Learn more about who should avoid live vaccines and why the distinction matters for travel and clinical care.

Inactivated vaccines are made by killing the pathogen entirely — usually with heat or chemicals — so it cannot replicate in the body. The immune system still learns to recognize the pathogen's proteins, building a protective memory, but because there is no live organism present, these vaccines are generally considered safe for immunocompromised individuals.

The flu shot (in its injected form), the inactivated polio vaccine (IPV), and the hepatitis A vaccine are common examples. Inactivated vaccines frequently require booster doses because the immune response they generate can be less robust than that from live vaccines. Adjuvants — immune-stimulating additives — are sometimes included to enhance the response.

Messenger RNA (mRNA) vaccines deliver a short strand of genetic code that instructs your own cells to temporarily produce a specific protein — typically a surface protein of the target pathogen. Your immune system recognizes this foreign protein and mounts a response, building memory without you ever being exposed to the actual pathogen.

The mRNA itself breaks down within days and does not interact with or alter DNA. This platform allows vaccines to be designed and updated rapidly once a pathogen's genetic sequence is known. COVID-19 vaccines developed by Pfizer-BioNTech and Moderna introduced this technology to a wide public audience, though the platform had been studied for years prior to that.

Viral vector vaccines use a modified, harmless virus — often an adenovirus — as a delivery vehicle to carry genetic instructions into your cells. Once inside, those instructions prompt your cells to produce a protein from the target pathogen, triggering an immune response. The vector virus itself is engineered so it cannot cause disease or replicate.

The Johnson & Johnson COVID-19 vaccine and the Oxford-AstraZeneca vaccine are well-known examples of this platform. Viral vector technology has also been used in approved vaccines for Ebola. One practical consideration: if a person has pre-existing immunity to the vector virus, it can sometimes blunt the vaccine's effectiveness — a factor researchers account for in development.

Rather than using whole pathogens or genetic instructions, protein subunit vaccines inject specific, purified pieces of the pathogen — usually surface proteins — that the immune system can learn to recognize. Because only fragments are introduced, there is no risk of infection and these vaccines tend to be very well tolerated.

The hepatitis B vaccine and the shingles vaccine (Shingrix) are examples of protein subunit vaccines. Virus-like particle (VLP) vaccines take a related approach, assembling empty shells that mimic the pathogen's outer structure without containing any genetic material. The HPV vaccines Gardasil and Cervarix use this VLP approach. Both types often require adjuvants to generate a sufficiently strong immune response.