TAG-CO-VAC published an interim report that the vaccines based on the index virus is equally effective for later variants including Omicron. 

However, considering the antigens distance and uncertainity of viral evolution, the WHO body has emphasised on updating the vaccine antigen composition by including Omicron, as the most antigenically distinct SARS-CoV-2 variant thus far, for administration as a booster dose. 

This has lead to the development of bivalent mRNA vaccine including the index as well as the Ba.1 - or BA.4/5. 

The latest meeting of the group concluded:-  Vaccines containing index virus continue to provide high level of protection against severe disease and death from all descendents of COVID-19. 

However, the protection from severe disease is maintained longer as compared to protection from symptomatic infection. 

Bivalent vaccine show modestly increased effectiveness against symptomatic disease while the severe outcome shows similar effectiveness.