Omicron BA.1, BA.2 and COVID-19 Booster Vaccination

TO THE EDITOR—In reply to the correspondence from Tjan et al [1] regarding the heterologous booster in healthy adults previously immunized with 2 doses of CoronaVac [2], the additional knowledge on immunogenicity of the booster (third dose) with BNT162b2 in BNT162b2primed individuals against the BA.2 omicron variant will help promote the vaccine uptake and coverage in themidst of a surge in BA.2 omicron worldwide. As of February 2022, the omicron variant was further classified into 4 main sublineages, BA.1, BA.1.1, BA.2, and BA.3. Moreover, an epidemiological surveillance on coronavirus disease 2019 (COVID-19) showed that BA.2 sublineages have become the dominant variant globally [3]. In Thailand, the BA.2 sublineages have been detected since the end of January 2022. The proportion of BA.2 sublineages rapidly increased and accounted for more than 90% of positive cases reported in March 2022 [Puenpa J et al. unpublished]. Potent serum neutralizing antibody (nAbs) against BA.1 was observed after the heterologous booster in individuals previously immunized with 2 doses of CoronaVac [2]. Considering the waning immunity after primary series vaccination and the high transmissibility and potential immune escape of BA.2 sublineage, we further determined the immunogenicity of the mRNA-1273 booster against BA.1 and BA.2 in AZD1222-primed individuals using the 50% focus reduction neutralization test (FRNT50) as previously described [2].