CU Medicine has conducted what is, to date, the largest sample (about 100 million subjects), worldwide study of 24 combinations of COVID-19 vaccines covering seven vaccine types. It shows that, whilst three doses of an mRNA vaccine greatly reduce COVID-19 infection risks in immunocompromised and elderly patients compared to only two doses of vaccines of any kind, an mRNA booster to two doses of any other vaccine protects them against non-severe COVID-19 infections. Very importantly, say the findings, a third dose of mRNA vaccine is necessary to protect against Omicron infection.