A serological survey of more than 5,000 blood donors and other volunteers found that that three doses of the Comirnaty or CoronaVac vaccines were, respectively, 48% or 30% effective in preventing Omicron infection shortly after vaccination. Protection improved shortly after a fourth dose. However, it waned significantly over time.
The study appeared this week in Nature Medicine. It was co-led by Professor Joseph Wu Tsz-kei and Malik Peiris, both of the School of Public Health at the University of Hong Kong (HKUMed).
There is extensive data on how COVID-19 vaccines protect against severe disease, hospitalization, and death. However, few studies have investigated the vaccines’ effectiveness in preventing both asymptomatic and symptomatic disease caused by Omicron infection.
From January through July 2022, Hong Kong experienced an unprecedented fifth wave of COVID-19 infections driven predominantly by the Omicron BA.2 variant (B.1.1.529.2). There were 1,341,363 reported cases (18.4% of the total population) and 9,290 deaths (0.7%).
This team used data from that wave in what they describe as “one of the first real-world estimates of vaccine effectiveness against Omicron infections, irrespective of symptoms or severity.”
Combining results from a community-wide serological survey of 5,310 blood donors and volunteers with SARS-CoV-2 viral load data from city-wide wastewater surveillance, they first estimated vaccine effectiveness against all Omicron BA.2 infections conferred by two, three and four homologous doses of the Comirnaty or CoronaVac vaccines for 100 days after each dose.
The researchers estimated three and four doses of Comirnaty were 48% and 69% effective in preventing Omicron infection, respectively, seven days after vaccination, waning to 26% and 35% by 100 days after vaccination. Three and four doses of CoronaVac were 30% and 56% effective after seven days respectively, declining to 6% and 11% by 100 days.
Meanwhile, they also determined that more than 45% of the local population in Hong Kong was infected by SARS-CoV-2 between January 1 and July 31 2022 (i.e., an infection attack rate of 45%). Official case counts, incorporating both Reverse Transcriptase-PCR (RT-PCR) testing and Rapid Antigen Testing (RAT) results, had identified less than 41% of all infections. These researchers developed two in-house ELISA assays detecting IgG antibodies to the nucleocapsid (N) or Open Reading Frame 8 (ORF8) protein of SARS-CoV-2, with the latter assay developed specifically to detect past infection in CoronaVac vaccines.
They argue that booster vaccination, using either the mRNA or inactivated vaccine platforms, is effective in preventing SARS-CoV-2 Omicron BA.2 infection in the short-term, which adds to previous studies demonstrating vaccine effectiveness in preventing severe disease and death.
But they say that surge booster campaigns, particularly with updated bivalent mRNA vaccines, could be strategically used to rapidly boost population immunity when there is risk of future waves of infections arising from a concerning novel virus variant.
The comparatively lower infection attack rate in Hong Kong by July 2022, they suggest, highlights the effect of supplementing vaccination campaigns with continued public health and social measures (e.g., masking) on reducing disease transmission.
