Omicron (B.1.1.529) variant has spread worldwide rapidly with largely evading immunity. The effectiveness of antibodies produced by prior infections with other SARS-COV-2 lineages or by vaccination against Omicron raises concerns. Natural infection induces strong protection against reinfection with the Alpha (B.1.1.7), Beta (B.1.351), and Delta (B.1.617.2) variants. When Omicron variant first emerged in South Africa, it is estimated that hazard ratio for reinfection versus prior infection was 2.39, much higher than 0.75 (wave2) and 0.71 (wave3) in south Africa. Based on several studies from UK and Qatar, there is a unified conclusion that Omicron induced a higher risk of reinfection compared with Delta and other previous variants. Protection against reinfection with Omicron range from 19% to 56% in different studies. Vaccine induced immunity usually peaks two weeks after second dose. After that, a substantial waning of VE is widely observed. For Delta and previous variants, VE against infection six months after second dose ranged from 53.8% to 77%. Estimates of VE against Omicron are gained from several countries, including England, Denmark, South Africa ,Canada and Israel. Genarally a substantial decrease of VE was observed as time flow, even negative VE was observed in Canada and England.