All subunit vaccines are made using living organisms, such as bacteria and yeast, which require substrates on which to grow them, and strict hygiene to avoid contamination with other organisms. This makes them more expensive to produce than chemically-synthesised vaccines, such as RNA vaccines. The precise manufacturing method depends on the type of subunit vaccine being produced. Protein subunit vaccines, such as the recombinant hepatitis B vaccine, are made by inserting the genetic code for the antigen into yeast cells, which are relatively easy to grow and capable of synthesising large amounts of protein. The yeast is grown in large fermentation tanks, and then split open, allowing the antigen to be harvested. This purified protein is then added to other vaccine components, such as preservatives to keep it stable, and adjuvants to boost the immune response – in this case alum. For polysaccharide or conjugate vaccines, the polysaccharide is produced by growing bacteria in industrial bioreactors, before splitting them open and harvesting the polysaccharide from their cell walls. In the case of conjugate vaccines, the protein that the polysaccharide is attached to must also be prepared by growing a different type of bacteria in separate bioreactors. Once its proteins are harvested, they are chemically attached to the polysaccharide, and then the remaining vaccine components added.