In Korea, vaccination programs with ChAdOx1 and BNT162b2 were initiated for healthcare workers (HCWs) with priority in March 2021. The anti-S and neutralizing antibody titers of HCWs vaccinated with primary series were significantly higher in the BNT162b2-vaccinated group than the ChAdOx1-vaccinated group.3 Choi SH and his colleagues4 recently published the article about immune responses and the occurrence of omicron breakthrough infection after booster vaccination of BNT162b2. They compared the humoral and cellular immune responses and the occurrence of omicron breakthough infection between booster vaccination of BNT162b2 after primary series of BNT162b2 (BBB group) and ChAdOx1 (CCB group). They demonstrated that omicron breakthrough infections occurred faster in the CCB group than in the BBB group because of lower humoral and cellular immune responses in the CCB group. Although the results of this study need to be carefully interpreted because of the small number of participants with incomplete test results, it is rare to investigate both humoral and cellular immune responses and the subsequent omicron breakthrough infections after booster COVID-19 vaccination.

There is a growing concern about the emergence of subvariants of the omicron variant and further immune escape. They may have the potential to evade the immunity generated by vaccines and previous infections. They have a high number of mutations, particularly in the spike protein, which is the target of most COVID-19 vaccines. The use of sequence-adapted vaccines may provide protection against them. Pfizer/BioNTech and Moderna have developed a bivalent mRNA vaccine capable of generating a protective immune response against the S protein of wild-type SARS-CoV-2 and the S protein of the omicron variant. As of February 2023, a bivalent vaccine against BA.1 or BA.4/5, a subvariant of the omicron variant, is available in Korea.1 Compared to the monovalent booster vaccination, the bivalent booster vaccination showed a superior immunogenicity against omicron subvariants and was more effective in preventing hospitalization or death.

COVID-19 vaccines have been shown to stimulate a robust T cell response in addition to the production of neutralizing antibodies. The T cell response to the vaccine appears to be directed against multiple viral proteins, including the spike protein and other viral proteins, which may help provide broader protection against different strains of the virus. T cell immunity generated by COVID-19 vaccines may play an important role in preventing severe disease and reducing the risk of hospitalization and death, even in individuals who have breakthrough infections.5 It is important to note that the level and duration of T cell immunity generated by the vaccine may vary between individuals and may be influenced by factors such as age, underlying health conditions, and prior exposure to the virus. Ongoing studies are continuing to investigate the long-term durability of T cell immunity generated by COVID-19 vaccines. In the present study, due to the small number of participants and incomplete test results, it was insufficient to interpret the roles of T-cell immunity generated by COVID-19 vaccines.4

The Korea Disease Control and Prevention Agency (KCDA) announced the basic direction for COVID-19 vaccination on March 22, 2023. The KCDA actively recommends vaccination once (twice for immunocompromised host) between October and November, 2023, especially for high-risk groups. It was also stated that the vaccination plan may change depending on the situation of the COVID-19 epidemic and the emergence of variants. There is still a lot of uncertainty surrounding COVID-19 vaccination, and there is a need for various studies and the development of effective vaccines in Korea.