A seroprevalence survey in Cape Town found that 40% of 2,700 public healthcare users tested positive for COVID-19, with antibody prevalence higher in more densely populated areas, reports Daily Maverick.
Initial results from a seroprevalence survey conducted in the Cape Town metro indicate that 37% of pregnant women and 42% of people living with HIV have tested positive for COVID-19 antibodies. Prevalence varied from 30% to 46% across different sub-districts in the metro. Prevalence for different age groups (all between 20 and 60) fell into a relatively narrow range of 36% to 43%.
The presence of antibodies in their blood samples shows they had been infected by SARS-CoV-2, the virus that causes COVID-19, and while this indicates some degree of immunity, it does not necessarily mean they will be protected from the virus if re-infected. Although clinical studies are ongoing, there is no certainty how much protection antibodies offer, or for how long.
The prevalence of antibodies was higher in more densely populated Cape Town metro sub-districts such as Khayelitsha and Klipfontein, which researchers say suggests a high proportion of people in poorer communities have been exposed to and infected with SARS-CoV-2. This is consistent with previously reported infection and mortality rates.
Professor Mary-Ann Davies, a public health medicine specialist with the Centre for Infectious Disease Epidemiology and Research at the University of Cape Town, said during a provincial press briefing yesterday that although there is still a lot of uncertainty about antibody protection, it’s unlikely that in the short term there will be “explosive outbreaks” in previous hot spots – or high density, vulnerable communities – which have experienced high morbidity and mortality.
She says the results suggest there is some level of immunity, but there are people in these communities, like the elderly and people with comorbidities, who have shielded themselves during the last five months and who are still susceptible to infection.
“There is also a lot of difference across the city and across the province, and there will still be population subgroups where people will be vulnerable to future outbreaks. So, vigilance is critical to make sure we are able to detect a second wave,” Davies said.
Davies cautioned that the study only gave a snapshot of the dynamics of the epidemic, and could not be generalised as far as the broader population was concerned.
The pharmaceutical company, Roche, devised the antibody test used in the study.
Davies said the test, recently authorised by the South African Health Products Regulatory Authority (Sahpra) for COVID-19 surveillance, has a sensitivity of 90% and a specificity of 99%. This means that out of every 100 people who have had COVID-19, it will correctly diagnose 90 people – and for every 100 people who have not had COVID-19, it will produce one false positive. The findings presented on 3 September were not adjusted to account for these test performance issues.
Questioned on the test’s accuracy, Davies said the “low sensitivity of the test may mean we are slightly under-detecting the proportion of people who have been infected”. She also pointed out that people with asymptomatic infection may mount less of an antibody response, while people who had been infected a long time ago may not be detected because antibody response wanes over time.
“In the confirmed COVID-19 patients we tested, however, we found that the antibody responses were quite well maintained… up to about three months after infection,” Davies said.
The Western Cape Health Department intends to have more community serological surveys as part of its surveillance and containment strategy, which seeks to detect any upsurges in new cases and to contain them until a vaccine becomes available.
A national seroprevalence survey run by the National Institute for Communicable Diseases and the Human Sciences Research Council is also expected to start soon.
[link url="https://www.dailymaverick.co.za/article/2020-09-04-covid-19-high-prevalence-found-in-cape-town-antibody-study/"]Full Daily Maverick report[/link]