Since December 2021, the SARS-CoV-2 B.1.1.529 (Omicron) variant started spreading in Hong
Kong, leading to an unprecedented 5th wave. It is known that the Omicron variant may
partially evade immunity from past vaccination and infection although both appear to protect
from severe disease and death. As Hong Kong emerges from the 5th wave, it will be critically
important for public health policy to assess the proportion of the population with evidence
of natural infection and those with evidence of detectable neutralizing antibody to the
Omicron variant, either though natural infection or vaccination. Our project aims to assess
these parameters.

We recruited healthy blood donors by convenience sampling at the five largest blood donation
centres (Mongkok, Causeway, Kwun Tong, Tsuen Wan and Shatin) of the Hong Kong Red Cross Blood
Transfusion Service (HKRCBTS). Blood donors were matched by the HKRCBTS and the Hong Kong
Department of Health with official vaccination records via unique Blood Transfusion Service
donor identification numbers. The records were then anonymised and provided to the study
team. Blood donors were also provided with the option of self-reporting their vaccination and
COVID-19 infection history.

Since we need to distinguish past infection from vaccine induced immunity, we used
serological test strategies that would differentiate natural infection from vaccine immunity.
We have previously shown that IgG antibody to the Nucleocapsid protein of SARS-CoV-2 (whole
protein and the C-terminal domain) provides evidence of past infection and is not elicited by
BNT162b2 vaccine but may be elicited by CoronaVac vaccine (Wu et al. 2021, Mok et al. 2021) .
We have also shown IgG antibody to ORF8 is elicited by past infection and not by either
CoronaVac or BNT162b2 (Hachim et al. 2020). We used these assays to assess evidence of
natural infection in blood donor samples.

Through the above procedures, we obtained detailed serial cross-sectional (weekly between
mid-April and the end of July, followed by monthly until the end of 2022) estimates of the
age-specific distribution of SARS-CoV-2 Omicron variant-specific antibody positivity levels.
We derived age-specific infection attack rates and age-specific population immunity levels
and tracked the magnitude of individual-level and population-level waning immunity over time.