Virtually everyone in SA had antibodies to Covid-19 by the end of the fourth wave in March, but this was not enough to stop another surge in coronavirus cases less than two months later, according to a new survey of blood donors.
SA’s fourth wave was driven by the BA.1 and BA.2 Omicron sub-variants, and was swiftly followed by another wave powered by Omicron sub-variants BA.4 and BA.5.
Analysis of almost 3,400 blood samples provided by donors from all nine provinces in mid-March showed 98% of donors had anti-nucleocapsid antibodies, generated in response to infection or antispike antibodies triggered by vaccination or both. Just more than one in 10 (10.8%) of the blood samples had only antispike antibodies, indicating their donors had been vaccinated and avoided infection.
“The timing shows even having almost universal antibody coverage has not prevented an outbreak from happening.
“That is a really sobering message about the rate of immune escape this virus is capable of,” said Alex Welte from the SA Centre of Excellence in Epidemiological Modelling and Analysis at Stellenbosch University and co-author of the study, which has yet to be peer-reviewed. It was published as a preprint by Research Square on May 26.
The coronavirus will continue to mutate, and it would be a mistake to say it will necessarily evolve into a more transmissible and milder form, said Welte. Many serious diseases, such as polio and rabies, have remained extremely dangerous, he said.
“It looks like not a single unvaccinated person has avoided infection. That is quite striking,” he added. While vaccination does not provide complete protection from infection, the study clearly shows it helps reduce that risk, he said.
Lower risk
The researchers analysed blood samples provided by the Western Cape Blood Service and the SA National Blood Service, which between them offer services in all nine provinces. They found no statistically significant variation by age or sex but did find substantial variation across racial groups, which they said is likely to be due to varying ability to reduce risk.
The proportion of donors who had antispike antibodies but no anti-nucleocapsid antibodies — in other words those who had been vaccinated but not infected — was highest among white and Asian donors, and lowest among black and coloured donors.
This is the third seroprevalence study the researchers have conducted among blood donors. Their first analysis, conducted in May 2021 after SA’s second, Beta-driven wave, showed 47% of adults had antibodies. A second study, using samples from November 2021 — which was after the third Delta-drive surge but before the fourth Omicron wave — showed 71% of the donors had antibodies.
“Our previous analyses noted a modest increase in seroprevalence during the Delta wave. The present analysis ... suggests that the infectious pressure of the Omicron variant was extraordinarily high to have produced such a significant bump in prevalence at this relatively mature stage of the epidemic.
“It is hardly possible to imagine much higher prevalence values, given that there is antibody waning and not all individuals develop, upon infection, antibodies detectable by any given assay,” said the researchers.
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