A two-dose Pfizer/BioNTech vaccination provides just 33% protection against infection by the Omicron variant of coronavirus, but 70% protection against hospital admission, according to major analysis in South Africa.
The first large-scale analysis of vaccine effectiveness in the region where the new variant was discovered appears to support early indications that Omicron is more easily transmissible and that the Pfizer jab is not as effective in protecting against infection as it was against the Delta variant.
The analysis was based on more than 211,000 positive Covid-19 test results, 41% from adults who had received two doses of the Pfizer vaccine.
About 78,000 of the positive test results between November 15 and December 7 were attributed to Omicron infection. The study was carried out by Discovery Health, South Africa's largest private health insurer, and the South African Medical Research Council.
The study was carried out in the weeks since Omicron was first announced in November by scientists in South Africa and Botswana. The researchers emphasised that its findings are preliminary and not peer reviewed.
The data comes from the first three weeks of South Africa's Omicron-driven wave and may change as time passes. South Africa was the first country to experience a surge in Covid-19 driven by the variant.
South Africa has experienced rapid community spread - concentrated in its most populous province, Gauteng - dominated by the Omicron variant.
The seven-day rolling average of daily new cases in South Africa has risen over the past two weeks from 8.07 new cases per 100,000 people on November 29 to 34.37 on December 13, according to Johns Hopkins University.
The death rate has not increased during that period.
"Superb genetic surveillance by the Network for Genomic Surveillance in South Africa identified that Omicron infection accounts for over 90% of new infections in South Africa, and has displaced the formerly dominant Delta variant," Discovery Health chief executive Dr Ryan Noach said.
"The Omicron-driven fourth wave has a significantly steeper trajectory of new infections relative to prior waves. National data show an exponential increase in both new infections and test positivity rates during the first three weeks of this wave, indicating a highly transmissible variant with rapid community spread of infection," he added.
The result shows vaccinated individuals who received two doses of the Pfizer vaccine have 33% protection against infection from Omicron, relative to those who were unvaccinated in the first weeks of South Africa's Omicron-driven wave.
This represents a significant drop from the 80% protection against infection during the earlier period, probably on the basis of lower antibody susceptibility, following the extensive spike protein mutations in the Omicron variant.
Encouragingly, the result shows the same vaccinated individuals who received two doses of the Pfizer vaccine have 70% protection against hospital admission in this same time period. This is reduced from 93% with South Africa's Delta-driven variant.
The study shows that protection against hospital admission is maintained across all ages, in people from 18 to 79 years old, with slightly lower levels of protection for older people, with 67% in people aged 60-69 and 60% for people aged 70-79.
Protection against admission is also consistent across a range of chronic illnesses including diabetes, hypertension, hypercholesterolemia and other cardiovascular diseases.
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