Recombinant vaccines

A vaccine is a killed or weakened pathogen, or a part of one, that prepares the immune system to fight the real disease. Edward Jenner coined the word 'vaccine' (from Latin vacca, cow) after using cowpox to protect people from smallpox; Louis Pasteur coined 'vaccination' in 1881. Older vaccines carry some risk: a live vaccine might cause infection, or an inactivated toxin might become active again. Recombinant vaccines avoid these risks.

Types of recombinant vaccine:

• Live genetically modified vaccines: genes that make the pathogen harmful are deleted or switched off, so it is weakened but still triggers immunity (a Salmonella vaccine for sheep and poultry; a pseudorabies vaccine for pigs).
• Vector vaccines: a harmless virus carries antigen genes from other pathogens. The vaccinia virus (of the pox family, with about 200 genes) has been given antigen genes from hepatitis B, herpes simplex and influenza, making one polyvalent vaccine against several diseases.
• Subunit vaccines: only a part of the pathogen, usually one protein. The gene for the hepatitis B surface antigen (HBsAg) was placed after a yeast promoter (ADH) in the yeast plasmid pMA56, and expressed in yeast, Saccharomyces cerevisiae. It was licensed in 1987 (Recombivax, Engerix-B).
• DNA vaccines: a plasmid carrying the antigen gene is injected into muscle (with a gene gun) or given as a nasal spray. The body's own cells make the antigen, which triggers both antibody (humoral) and cellular immunity.
• RNA vaccines: mRNA for the antigen, wrapped in tiny lipid nanoparticles, is taken up by immune cells and translated into the antigen. They can be updated quickly for new variants, are made without cells, and cannot join our DNA; but they must be stored frozen.

Edible vaccines: antigen genes are put into food plants such as banana, using Agrobacterium or particle bombardment. Eating the plant stimulates immunity in the gut lining (mucosal immunity). Banana is ideal: it is easily digested, eaten raw (so the antigen is not destroyed by cooking), cheap, and available all year.