‘Subunit’ vaccines involve using pieces of the virus. This fragment could be a bacterial toxin or even a laboratory-made part of a pathogen. They are sometimes given in combination with an immune-boosting agent called an ‘adjuvant’ to trigger a stronger immune response. These include the hepatitis B vaccine along with the Human Papilloma Virus vaccine, that is now being offered to adolescent girls to prevent cervical cancer. American developer Novavax has a COVID-19 candidate in Phases One-Two of clinical trials.