Homologous or Heterologous COVID-19 Boosters Regimens Significantly Impact Sero-neutralization of SARS-CoV-2 Virus and Variants

We determined the levels of neutralizing antibodies against the SARS-CoV-2 ancestral strain, Delta and Omicron variants of concern (VOCs) in 125 healthcare workers, who received Coro-naVac as their primary vaccination and later received either a single ChAdOx1 or a combination of two consecutive boosters using either two ChAdOx1 doses or a ChAdOx1 or BNT162b2 as the primary and second boosters, respectively, or 2 doses of BNT162b2. The titers 12 weeks after primary vaccination were inadequate to neutralize the all strains. After a single ChAdOx1 boost-er, the levels of neutralization at Day 30 varied significantly, only a small proportion of partici-pants developing neutralizing titers against Omicron at Day 7 and 30. The two doses of ChA-dOx1 as the booster induced lowest activity. A combination ChAdOx1 and BNT162b2 induced greater neutralization than by two doses of ChAdOx1. Two doses of BNT162b2 as the booster had the maximal activity against Omicron VOC.