Q.Explain, with the example of the Hepatitis B vaccine, how recombinant DNA technology is used to produce a genetically engineered vaccine. Why are such vaccines considered safer than vaccines made from a weakened whole pathogen?

[!TLDR] The Hep-B vaccine gene for one viral surface protein (HBsAg) is expressed in yeast, producing a pure subunit-vaccine antigen with zero infection risk, since the actual virus is never grown or used. …

A vaccine works by training the immune system to recognise and respond to a specific protein, or antigen, found on the surface of a disease-causing pathogen, so that if the actual pathogen is later encountered, the immune system can respond rapidly and effectively. Traditional vaccines achieve this exposure either by using a killed (inactivated) whole pathogen, or by using a live but deliberately weakened (attenuated) strain of the pathogen.

The recombinant Hepatitis B vaccine takes a different route. Rather than growing the Hepatitis B virus itself at any point, the gene coding for a single protein found on the surface of the virus -- the Hepatitis B surface antigen, HBsAg, the specific protein the immune system needs to learn to recognise -- is identified, cut out, and inserted into a suitable host cell, most commonly a yeast cell, using standard recombinant DNA techniques. The engineered yeast cells then express and produce large quantities of this one viral surface protein, which is purified away from all yeast cellular material and formulated into the finished vaccine. …

Contrast the recombinant subunit-vaccine approach directly against traditional killed/attenuated vacc …

Do not say the Hepatitis B vaccine contains 'a weakened form of the virus' -- it contains only one isolated, non-infectious surface protein (HBsAg) …