Thus, past contamination with BA.2 in unvaccinated people may not be anticipated to provide much protection from reinfection with BA.5. vaccinated with CoronaVac experienced 8.0, 7.0, 11.8, 12.0 and 12.0 fold GMT reductions and those with two doses of CoronaVac boosted by BNT162b2 experienced 6.1, 6.7, 6,3, 13.0 and VTP-27999 HCl 21.2 fold GMT reductions to these viruses, respectively. Vaccinated individuals with BA.2 breakthrough infections had higher GMT antibody levels vs. BA.4 (36.9) and BA.5 (36.9) than unvaccinated individuals with BA.2 infections (BA.4 GMT 8.2; BA.5 VTP-27999 HCl GMT 11.0). Conclusions BA.4 and BA.5 subvariants were less susceptible to BNT162b2 or CoronaVac vaccine elicited antibody neutralization than subvariants BA.1, BA.2 and BA.2.12.1. Nevertheless, three doses BNT162b2 or booster of BNT162b2 following two doses of CoronaVac elicited detectable BA.4 and BA.5 neutralizing antibody responses while those vaccinated with three doses of CoronaVac largely fail to do so. BA.2 infections in vaccinated individuals led to higher levels of BA.4 or BA.5 neutralizing antibody compared to those who were vaccine-naive. Keywords: COVID-19, SARS-CoV-2, Omicron, Neutralization, Vaccine Abbreviations: SARS-CoV-2, SARS coronavirus 2 1.?Introduction The new variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) within the Pango lineage B.1.1.529 lineage was first recognised in countries in Southern Africa as a newly emerged variant and it was designated as a variant of concern (VOC) Omicron by the WHO on 26th November 2021 [22]. Multiple sub-variants of Omicron were quickly recognised, but BA.1 subvariant, followed by BA.2, were the ones that spread globally, initially [20]. Compared to the wild-type computer virus or other previously circulating VOCs, Omicron variants have over 35 amino acid changes in the spike protein in addition to other amino acid substitutions elsewhere in the computer virus genome [20]. Some of these amino acid substitutions in the spike protein resulted in these viruses evading neutralizing immunity elicited by prior infections or vaccination [5], [6], [7], contributing to greater transmissibility and F2rl1 competitive VTP-27999 HCl advantage to displace previously circulating computer virus variants [5]. BA.2.12.1 is a variant of BA.2 first recognised in the USA, now also seen elsewhere, and contains identical VTP-27999 HCl spike receptor binding domain name (RBD) sequences to BA.2 but with the addition of amino acid substitutions L452Q and S704L which may have capacity for immune evasion [16]. Early in 2022, two other sub-variants of Omicron, BA.4 and BA.5 were recognised [18]. Both BA.4 and BA.5 had spike proteins much like BA.2 with the exception of using a 69C70del (present in the Alpha variant and the BA.1 lineage), L452R (present in the Delta variant), F486V and the reversion to the wild-type amino acid at Q493 [18]. BA.4 and BA.5 have spike proteins identical to each other differ from each other elsewhere in the genome. VTP-27999 HCl The amino acid substitutions in spike protein of Omicron variants relative to the ancestral WT computer virus are summarised in the Supplementary table. BA.5 is now rapidly increasing in prevalence in many parts of the world. It is important to assess the impact of these variants on further immune evasion from immunity elicited by prior contamination or vaccination. In individuals vaccinated with RNA vaccines, studies using pseudovirus neutralization transporting the spike proteins of these respective viruses showed that BA.4/BA.5, and to lesser extent BA.2.12.1 neutralizing antibody titers, were even lower than BA.1 and BA.2 neutralizing antibody titers, suggesting that this SARS-CoV-2 BA.4/5 has continued to evolve with increasing neutralization escape [9,21]. However, pseudotype neutralization assays are known to give varying conclusions, depending upon the computer virus vector used. Pseudotype neutralization also ignores impact of mutations outside of the spike protein which may have impact on viral replication competence, immune innate evasion and neutralizability. Plaque reduction neutralization assays are a gold-standard method to define computer virus neutralization titres and we used this approach to address the relative WT and Omicron subvariant neutralization titres cohorts who were vaccinated and those convalescent from contamination. 2.?Materials and methods 2.1. Clinical specimens Subsets of sera from those previously collected during 11 Aug 2020 to 29 Dec 2021 were randomly selected for any previous study comparing neutralizing antibody to WT, BA.1 and BA.2 subvariants in vaccinated and non-vaccinated individuals [7] and these sera are used in the present study. These cohorts include individuals who experienced no previous SARS-CoV-2 contamination and received 3 doses BNT162b2, 3 doses CoronaVac or 2 doses CoronaVac boosted by a dose of BNT162b2, with blood collected 3C5 weeks after the last vaccine dose. They also included unvaccinated individuals who were convalescent from WT SARS-CoV-2 (143C196 days post contamination). In addition, paired acute and convalescent sera were collected from patients with presumed BA.2 subvariant infection (n?=?24).