IMMUNE RESPONSES TO MRNA AND INACTIVATED COVID-19 VACCINES AMONG HEALTHCARE WORKERS IN HONG KONG

INTRO: COVID-19 vaccines against the earlier strains of SARS-CoV-2 are now available. However, breakthrough infections can still occur due to waning antibodies and immune escape by new variants. We assessed humoral immune responses to the mRNA (BNT162b2) and inactivated (CoronaVac) vaccines in our healthcare worker cohort (HCW). METHODS: We recruited HCWs from public and private healthcare institutions across Hong Kong and collected blood samples at enrolment and every 6 months from June 2020 to June 2022. A subset of volunteers provided blood samples between 10 – 42 days after each dose of vaccine. Immune responses to vaccination were measured as SARS-CoV-2 binding antibodies detected by an enzyme-linked immunosorbent assay (ELISA) and SARS-CoV-2 neutralising antibodies by surrogate virus neutralization test (sVNT) and plaque reduction neutralization test (PRNT). FINDINGS: Among the 1,736 HCWs enrolled in our cohort, 252 HCWs provided pre- and post-vaccination blood samples after each dose of either vaccine. Two doses of BNT162b2 generated levels of neutralizing antibodies (sVNT inhibition = 96.8%, range = 42.8%, 98.2%) comparable to those generated by natural infections in the first wave (sVNT inhibition = 84.0%, range = 32.9%, 93.8%). Similar levels were achieved with three doses of CoronaVac (sVNT inhibition = 95.3%, range = 64.7%, 98.3%) and heterologous vaccination with two doses of CoronaVac followed by a booster dose of BNT162b2 vaccine (sVNT inhibition = 97.0%, range = 85.8%, 97.7%). These antibody levels waned faster after second doses and slower after third doses for both vaccines. CONCLUSION: The BNT162b2 vaccine and CoronaVac vaccines can generate robust antibody responses comparable to natural infections. Three doses of the CoronaVac vaccine, or a heterologous boost with the BNT162b2 vaccine following two doses of the CoronaVac vaccine are required to achieve similar levels of neutralising antibodies in vaccinees who received two doses of the BNT162b2 vaccine.