COVID-19自2019年年末起造成全世界的大流行。自從疫苗開始大量施打後，才使得疫情逐漸得以控制。SARS-CoV-2在整個Covid-19疫情期間不斷演變和突變，至今為止已出現五種高關注變異株，而每一種皆為全世界帶來新的一波疫情。Omicron變異株自2021年11月被發現後，迅速成為主要流行的變異株。Omicron變異株相較於以往的變異株具有更多的突變，因此帶有更強的免疫逃脫特性。以野生株為藍本所製作的疫苗，在面對Omicron變異株時效果有所折扣。為了改善疫苗的保護力，施打追加劑的疫苗策略在各國間被廣泛推動。除了增加疫苗的接種次數外，疫苗製造商亦推出了雙價疫苗。其成分除了原有的野生株外，額外新增Omicron變異株的BA.1亞型或BA.4/BA.5亞型，以提升疫苗對於Omicron變異株的效果。

The COVID-19 pandemic began since December 2019. Due to widespread vaccination, new recorded COVID19 cases had declined. However, SARS-CoV-2 has mutated over time, and there had been five SARS-CoV-2 variants of concern (VOC), which caused waves of the pandemic. The Omicron variant emerged in late 2021, and became the dominant strain in the world rapidly. The Omicron variant had more mutations than previous SARSCoV-2 variants, making it become a more immune escape variant. Vaccines designed based on wild-type viral strains were less effective against the Omicron variant. Therefore, booster doses were administered to increase vaccine effectiveness in most countries. Besides the strategy of repeating booster doses against Omicron variant, vaccine companies also developed bivalent mRNA vaccine, including wild type/Omicron BA.1 and wild type/Omicron BA.4/5 vaccine.