Contribution of low population immunity to the severe Omicron BA.2 outbreak in Hong Kong

Abstract

Monitoring population protective immunity against SARS-CoV-2 variants is critical for risk assessment. In this serosurveillance study, older adults show much lower seropositive rates of neutralizing antibody (NAb) against ancestral virus than the younger population. The increase in NAb seopositive rate generally follows the population vaccination uptake rate, but older adults have a much lower NAb seropositive rate than vaccination uptake rate. For all age groups, the seropositive rates of NAb against Omicron variant are much lower than those against the ancestral virus. During the fifth wave of COVID-19 in Hong Kong which is dominated by Omicron sublineage BA.2, the case-fatality rate is exceptionally high in the ≥80 year-old age group (9.2%). Our study suggests that the severe BA.2 outbreak in Hong Kong can be attributed by the lack of protective immunity in the population, especially among the vulnerable older adults, and highlights the importance of continual surveillance of protective immunity against emerging variants of SARS-CoV-2.

Vaccine effectiveness of one, two, and three doses of BNT162b2 and CoronaVac against COVID-19 in Hong Kong: a population-based observational study

“…Although the precise relationship between immune response and clinical outcome is uncertain, the Hong Kong population had little pre-existing naturally or vaccine-derived humoral immunity to the omicron sublineage BA.2 before the beginning of the fifth wave. 21 Previous SARS-COV-2 infection has been shown to reduce fatality due to delta or omicron by approximately half (hazard ratio 0•47 [95% CI 0•32-0•68]) in vaccinated individuals and approximately five times (0•18 [0•06-0•57]) in unvaccinated individuals. 22 Therefore, the high death rates observed in Hong Kong might be at least partly attributed to the older population remaining largely unvaccinated and infection-naive, combined with healthsystem congestion.…”

Emergence of SARS-CoV-2 Omicron lineages BA.4 and BA.5 in South Africa

“…Despite being associated with a modest prolongation of the fourth wave, the displacement of BA.1 by BA.2 in South Africa was not associated with a substantial resurgence in cases, hospital admissions or deaths. This pattern was not consistent worldwide, however, and, in some countries, BA.2 was responsible for a greater share of cases, hospitalizations and deaths during the Omicron wave [2][3][4] .We recently identified two new Omicron lineages that have been designated BA.4 and BA.5 by the Pango Network and pango-designation version 1.3, a system of naming and classifying SARS-CoV-2 lineages (Fig. 1a) 5,6 .…”

Population-based sero-epidemiological estimates of real-world vaccine effectiveness against Omicron infection in an infection-naive population, Hong Kong, January to July 2022

“…Furthermore, most VE estimates hitherto have estimated protection against symptomatic disease, hospitalization or death but not against all infections including asymptomatic infections. Our estimates of VE against Omicron infection are robust against the abovementioned limitations because Hong Kong had negligible infection-derived immunity against any SARS-CoV-2 prior to January 2022 and the infection history of our subjects were individually inferred based on their serological measurements (irrespective of history of symptoms, case confirmation or contact) [12].…”