COVID-19 is an acute respiratory infection caused by a new strain of Coronavirus (SARS-COV2).
From 15 March 2023, the WHO variant tracking system will consider the classification of Omicron sublineages independently as variants under monitoring (VUMs), VOIs, or VOCs.
With these changes factored in, Alpha, Beta, Gamma, Delta as well as the Omicron parent lineage (B.1.1.529) are considered previously circulating VOCs. WHO has now classified XBB.1.5 as a VOI.
You can re-infect COVID-19 with different variants or even old variants because your antibody amount decreases over time. Despite being lighter than Delta, Omicron spreads 5 times faster than Delta and has the highest risk of re-infection.