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Nearly everyone in SA has COVID antibodies – SANBS study

The latest COVID seroprevalence survey shows that nearly every adult in South Africa has some antibodies, either from being vaccinated or having had COVID. For many, it’s both.

The study analysed blood from more than 3,000 blood donors, the results of which were then weighted against national demographics. It was conducted by the South African National Blood Service (SANBS), which is responsible for blood donations in eight provinces, and the Western Cape Blood Service.

The researchers estimated that by March 2022, before the fifth wave, which appears to have peaked in the past few weeks, 98% of adults had some detectable antibodies, whether from COVID or from vaccination. This means that only 2% had neither been vaccinated nor been infected.

Only 10% had been vaccinated but not infected by COVID.

The study appeared as a preprint in Research Square and has not yet been peer-reviewed.

What the survey tested for

Blood samples were collected and tested from 3,395 consenting donors from all provinces in mid-March 2022. While they are not precisely representative of the population, the researchers say the study is representative enough.

This is the first time the blood services researchers have been able to look for two types of antibodies.

One test indicates if a sample has antibodies to the nucleocapsid proteins (anti-nucleocapsid antibodies). These antibodies develop if someone is infected, but won’t develop after they receive a vaccine only (at least not those vaccines currently available in South Africa).

The other test indicates if the sample has antibodies to the spike protein (anti-spike antibodies). These antibodies develop when someone has been infected or has been vaccinated (or both).

Using these two tests together, researchers can, for the first time, evaluate the proportion of the population that has been vaccinated and not infected.

Results

After weighting the results to reflect national demographics, the researchers found that a mere 2% of the population had neither anti-spike nor anti-nucleocapsid antibodies. These are people who have probably never had COVID nor been vaccinated.

A mere 10% had only anti-spike antibodies. These are people who were probably vaccinated, but never infected.

The researchers noted that there is “an increasing incidence of re-infection” with the Omicron wave.

Blood service survey is the best we have

The blood services have been regularly testing blood samples from donors throughout the pandemic, looking at the presence of anti-nucleocapsid antibodies.

While other surveys might be more representative of the population than the blood donor ones, these have been infrequently published or published long after the survey was conducted. By contrast, the blood donor surveys are relatively affordable and quick to publish. Also, as far as we are aware, it is the only survey repeatedly testing the same group of people, so that comparisons across time are possible.

Past blood surveys

The blood services’ survey from samples taken in May 2021 estimated that 47% of the adult population had previously been infected.

The next survey of blood samples was taken in November 2021 after the Delta wave. This was just before the Omicron wave. The researchers estimated that about 70% of people had been infected.

The latest survey indicates that about 87% of people have been infected. The previous surveys found that levels of infection differed by province. Now these differences have “largely disappeared as prevalence appears to have saturated”.

Differences across race

There are significant differences in rates of infection when different races are compared.

The November survey showed that about 80% of black donors and 40% of white donors had been infected with COVID.

In the latest survey the proportion of white and Asian donors who only have anti-spike antibodies (indicating vaccination but no infection) was higher than black and coloured donors.

The researchers suggest that “white donors are both unusually likely to avail themselves of vaccination, and they are unusually able to avoid exposure, for instance by working predominantly from home, (and) living in smaller family units”.

Study details

Estimates of prevalence of anti-SARS-CoV-2 antibodies among blood donors in South Africa in March 2022

Jeremy Bingham, Russel Cable, Charl Coleman, Tanya Nadia Glatt, et al.

Published in Research Square on 26 May 2022

Abstract

In line with previous instalments of analysis from this ongoing study to monitor ‘COVID Seroprevalence’ among blood donors in South Africa, we report on an analysis of 3,395 samples obtained in mid-March 2022 from all provinces of South Africa – a timepoint just after the fourth (primarily Omicron) wave of infections.
As in our previous analyses, we see no evidence of age and sex dependence of prevalence, but significant variation by race. Differences between provinces have largely disappeared, as prevalence appears to have saturated. In contrast to previous estimates from this study, which reported only prevalence of anti-nucleocapsid antibodies, this present work also reports results from testing for anti-spike antibodies.
This addition allows us to categorise those donors whose only antibodies are from vaccination.
Our race-weighted national extrapolation is that 98% of South Africans have some antibodies, noting that 10% have anti-spike antibodies but not anti-nucleocapsid antibodies – a reasonable proxy for having both 1) been vaccinated and 2) avoided infection.

 

Research Square article – Estimates of prevalence of anti-SARS-CoV-2 antibodies among blood donors in South Africa in March 2022 (Open access)

 

GroundUp article – Nearly everyone in South Africa has Covid antibodies, study shows (Creative Commons Licence)

 

See more from MedicalBrief archives:

 

Discovery: Data show 80% of South Africans may have had COVID-19

 

‘Most South Africans have had COVID-19’ — National Blood Service study

 

Donor plasma not successful in treating South Africa variant — SA NICD study

 

 

 

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