To the Editor—Healthcare workers (HCWs) in Hong Kong are among the priority groups to receive coronavirus disease 2019 (COVID-19) vaccination. We recruited HCWs who enrolled for COVID-19 vaccination from 22 February to 30 April 2021 for serial measurement of their anti-spike immunoglobulin M (IgM)/immunoglobulin G (IgG)/total antibody and surrogate neutralising antibody using Abbott SARS-CoV-2 IgM/IgG II Quant assay; Roche Elecsys® Anti-SARS-CoV-2 S, and GenScript cPass SARS-CoV-2 Surrogate Virus Neutralization Test Kit. The key exclusion criteria were history of polymerase chain reaction–confirmed COVID-19 or positive test for severe acute respiratory syndrome coronavirus 2–specific IgG or IgM in the serum. The clinical trial protocol was approved by the Research Ethics Committee of Hong Kong Sanatorium and Hospital Medical Group (Ref: RC-2021-07).

Of the 457 HCWs recruited, 220 (48.1%) selected an inactivated vaccine (CoronaVac; Sinovac Life Sciences, Beijing, China) and 237 (51.9%) selected an mRNA vaccine (BNT162b2/Comirnaty; Fosun-BioNTech Pharma), based on their personal preference. The CoronaVac arm was older (mean age=49.11 vs 44.06 years; P<0.0001) and had a higher prevalence of having at least one medical co-morbidity (31.6% vs 22.22%; P=0.0318) [Table 1].

At the time of writing, 210 participants have received two doses of CoronaVac and 92 have received two doses of BNT162b2. After dose 1, more BNT162b2 recipients had positive anti-spike IgG than did CoronaVac recipients (99.1% vs 64.7%; P<0.0001). However, the majority developed anti-spike IgG after dose 2 with no significant difference between the two arms (100% vs 99%; P=1) [Table 2]. Of 289 samples taken after receiving dose 2, only two were negative for anti-spike IgG. These two non-responders were both immunocompromised, one with psoriatic arthritis receiving methotrexate treatment, and the other with chronic lymphocytic leukaemia. The IgG and total antibody levels induced by BNT162b2 were higher than those induced by CoronaVac after dose 1 (P<0.0001) and after dose 2 (P<0.0001) [Fig]. After dose 2, more BNT162b2 recipients had positive surrogate neutralising antibody (100% vs 94%; P<0.0194).

Both CoronaVac and BNT162b2 are immunogenic in these HCWs. Our findings underscore the importance of maintaining social distancing and other infection control measures until 4 weeks after completing the two-dose regimen. Although most vaccine recipients developed antibodies after the second dose, the level of antibody or neutralising activity required to confer protection against future infection is currently not well defined. More research is needed for a better understanding of serology after vaccination. Data collection is ongoing and new findings will be published when available.

JST Zee drafted the letter. All authors contributed to the concept or design of the study, acquisition of the data, analysis or interpretation of the data, and critical revision of the letter for important intellectual content. All authors had full access to the data, contributed to the study, approved the final version for publication, and take responsibility for its accuracy and integrity.

All authors have disclosed no conflicts of interest.