An attenuated vaccine is composed of a live pathogen that has had its virulence reduced. The virulence of a pathogen can be decreased by serial passaging, where susceptible cells are infected repeatedly. By growing the pathogen outside its optimal growth temperature, the speed at which the pathogen will replicate in a warm human is diminished. Crispr approaches could also be used to remove virulent genes, if these are well understood.
Killed pathogen vaccines use the pathogen that causes the disease, inactivating them so they cannot replicate. Inactivation could be through heat or chemical processes. These produce a less strong immune response than attenuated vaccines, however they can give good and long-lasting immunity. Occasionally, the inactivation process has gone wrong. A notable example was an oral polio vaccine, where patients were administered the live poliovirus. Testing is now in place to ensure the pathogen cannot replicate.
There are many different types of subunit vaccine, using part of the pathogen to obtain an immune response. Due to the absence of the entire pathogen, these vaccines can be administered to individuals with malfunctioning immune systems. Less strong immune responses can be obtained with these vaccines, and an adjuvant (a compound improving an immune response) may be required to obtain long lasting immunity. Booster doses are also common with subunit vaccines. mRNA vaccines, such as those used for SARS-CoV-2, involve intramuscular injection of mRNA coding for the spike protein on the shell of the virus. The spike protein is then synthesised in situ, allowing an immune response to form. Conjugate vaccines are also used, such as for Haemophilus influenzae type B. Part of the pathogen is taken, in this case the polysaccharide capsule, and is linked to another component that elicits a strong immune response. Other forms of conjugate vaccine include DNA, recombinant protein, and toxoid.