How easy are they to manufacture?

Different viruses require slightly different production processes, meaning separate equipment and facilities are needed for each one. For instance, the influenza virus is grown inside fertilised chicken eggs-which must themselves be sourced from specialised sterile laying facilities. Polio virus is grown in dishes of cells, which require different handling, while bacteria-based vaccines are grown in vast bioreactors. Growing live pathogens also means stringent precautions must be taken to avoid the virus escaping and making vaccine plant workers sick.

Once large amounts of virus or bacteria have been grown, they must then be isolated, purified and attenuated or inactivated, depending on the vaccine. Each of these steps requires specific equipment, reagents, and stringent procedures to avoid, and check for, contamination, which can further increase costs.