Protection from severe disease and symptomatic infection induced by index virus-based vaccines and BA.1- or BA.4/5-containing bivalent mRNA vaccines declines over time.
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然而，对重症的防护比对有症状感染的防护持续时间更长。
However, protection from severe disease is maintained longer than protection from symptomatic infection.
As compared to index virus-based vaccines, booster doses of BA.1- or BA.4/5-containing bivalent mRNA vaccines may modestly increase vaccine effectiveness against symptomatic disease, while the small number of studies assessing severe outcomes show similar estimates of vaccine effectiveness.
Both BA.1- and BA.4/5-containing bivalent mRNA vaccines enhance the magnitude and elicit greater breadth of cross-reactiveimmune responses to SARS-CoV-2 variants when used as a booster dose, as compared to the index virus-based vaccines.
BA.4/5-containing bivalent mRNA vaccines induced higher neutralizing antibody titres against recent descendent lineages of Omicron (BQ.1, XBB.1) as compared to BA.1-containing bivalent mRNA vaccines, when used as a booster dose.
There is in vitro evidence to show that immune imprinting, also known as original antigenic sin – a phenomenon in which immune memory recall biases the immune response towards previously encountered antigen – occurs with repeated exposure to the same antigen.
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然而，由于数据有限和可能存在偏倚，迄今为止免疫印记在观察性流行病学研究中的临床影响尚不清楚。
However, the clinical impact of immune imprinting in observational epidemiological studies to date is unclear, due to limited data and the possibility of bias.
As previously recommended by the TAG-CO-VAC in its statement published in June 2022 , achieving broader cross-reactive vaccine-induced immune responses remains prudent in the context of continued SARS-CoV-2 evolution.
In upcoming meetings of the TAG-CO-VAC, vaccine antigen composition will be considered, including an assessment as to whether the inclusion of the index virus is warranted in future vaccine formulations.