Serological response to mRNA and inactivated COVID-19 vaccine in healthcare workers in Hong Kong: decline in antibodies 12 weeks after two doses

Serological response to mRNA and inactivated COVID-19 vaccine in healthcare workers in Hong Kong: decline in antibodies 12 weeks after two doses To the Editor-We previously reported serological findings of 302 healthcare workers (HCWs) who completed two doses of mRNA (BNT162b2/ Comirnaty; Fosun-BioNTech Pharma) and inactivated COVID-19 vaccine (CoronaVac; Sinovac Life Sciences, Beijing, China). 1 Both vaccines were found to be immunogenic in the majority of HCWs.The BNT162b2 resulted in a 11-fold higher level of anti-spike IgG (Abbott SARS-CoV-2 IgG II Quant assay, mean=11572.6 AU/mL vs 1005.2AU/mL; P<0.001) and a higher surrogate neutralising antibody (sNAb) [GenScript cPass SARS-CoV-2 Surrogate Virus Neutralization Test Kit] positive rate (100% vs 94.4%; P<0.001).We report week 12 serological data of our cohort.Among 197 CoronaVac and 100 BNT162b2 recipients, baseline characteristics of the two Hong Kong Med J 2021;27:380-3 https://doi.org/10.12809/hkmj219744vaccine arms were comparable except sex (60.9% and 38% female in CoronaVac and BNT162b2, respectively) [online supplementary Table 1].There was no difference in anti-spike immunoglobulin G (IgG) positive rate at week 12 (98.5% in CoronaVac vs 99% in BNT162b2; P=1) [Table 1].Waning of IgG level was observed in both vaccine arms with a larger magnitude of decline in BNT162b2 (-72% vs -64.6%;P<0.001).Despite the more pronounced decline, the median anti-spike IgG of BNT162b2 remained 11-fold higher than that of CoronaVac at week 12 (2840.25AU/mL vs 253.60 AU/mL; P<0.001).Decline in sNAb was also observed in both arms but the magnitude was significantly smaller in BNT162b2 (-28.3% in CoronaVac vs -2.3% in BNT162b2; P<0.001).Using the manufacturer's positive cut-off at 30% signal inhibition or above,