Serum neutralization of SARS coronavirus 2 Omicron sublineages BA.1 and BA.2 and cellular immune responses 3 months after booster vaccination

We investigated serum neutralizing activity against BA.1 and BA.2 Omicron sublineages and T cell response before and 3 months after administration of the booster vaccine in healthcare workers (HCWs).HCWs aged 18-65 years who were vaccinated and received booster doses of the BNT162b2 vaccine were included. Anti-SARS coronavirus 2 IgG levels and cellular response (through interferon γ ELISpot assay) were evaluated in all participants, and neutralizing antibodies against Delta, BA.1, and BA.2 were evaluated in participants with at least one follow-up visit 1 or 3 months after the administration of the booster dose.Among 118 HCWs who received the booster dose, 102 and 84 participants attended the 1-month and 3-month visits, respectively. Before the booster vaccine dose, a low serum neutralizing activity against Delta, BA.1, and BA.2 was detectable in only 39/102 (38.2%), 8/102 (7.8%), and 12/102 (11.8%) participants, respectively. At 3 months, neutralizing antibodies against Delta, BA.1, and BA.2 were detected in 84/84 (100%), 79/84 (94%), and 77/84 (92%) participants, respectively. Geometric mean titres of neutralizing antibodies against BA.1 and BA.2 were 2.2-fold and 2.8-fold reduced compared with those for Delta. From 1 to 3 months after the administration of the booster dose, participants with a recent history of SARS coronavirus 2 infection (n = 21/84) had persistent levels of S1 reactive specific T cells and neutralizing antibodies against Delta and BA.2 and 2.2-fold increase in neutralizing antibodies against BA.1 (p 0.014). Conversely, neutralizing antibody titres against Delta (2.5-fold decrease, p 0.0001), BA.1 (1.5-fold, p 0.02), and BA.2 (2-fold, p 0.0001) declined from 1 to 3 months after the administration of the booster dose in individuals without any recent infection.The booster vaccine dose provided significant and similar response against BA.1 and BA.2 Omicron sublineages; however, the immune response declined in the absence of recent infection.