COVID-19 Scan for Feb 24, 2022

SARS-CoV-2 antibodies in South Africa
COVID rapid antigen test reliability

High immunity, low vaccine rate in South Africa point to prior infections

A seroepidemiologic survey of 7,010 people in Gauteng province, South Africa, before the SARS-CoV-2 Omicron variant outcompeted the Delta strain shows that 80% of those older than 50 years had antibodies against the virus, with most seropositivity likely generated by previous infection.

In the study, published yesterday in the New England Journal of Medicine (NEJM), South African researchers analyzed dried-blood samples from participants from 3,047 households from Oct 22 to Dec 9, 2021. They looked for anti–SARS-CoV-2 immunoglobulin G (IgG) antibodies and assessed COVID-19 cases, hospitalizations, and deaths and excess deaths through Jan 12, 2022. Omicron was first identified in the region on Nov 25, 2021, but didn't become dominant until December.

Of the 7,010 participants, 18.8% had been vaccinated against COVID-19. IgG seroprevalence ranged from 56.2% (95% confidence interval [CI], 52.6% to 59.7%) among children younger than 12 years to 79.7% (95% CI, 77.6% to 81.5%) in adults older than 50. Vaccinated participants had higher rates of IgG seropositivity than their unvaccinated peers (93.1% vs 68.4%).

A study of many of the same participants in January 2021 had found that 19.1% of the population had anti–SARS-CoV-2 IgG.

According to epidemiologic data, COVID-19 cases rose and then declined faster during the study period than in previous surges, and cases were milder as well, with lower rates of hospitalization, death, and excess death.

"We think that the decoupling of the incidence of Covid-19 cases from the incidences of hospitalization and death during the omicron-dominant wave in South Africa heralds a turning point in the Covid-19 pandemic, if the primary goal is protection against severe disease and death rather than prevention of infection," the authors concluded. 

In an audio interview, NEJM Editor-in-Chief Eric Rubin, MD, PhD, noted that 40% of samples had antibodies against the nucleocapsid protein expressed by the virus, while 70% had antibodies against the virus's spike protein generated by vaccines. With only a third of the population vaccinated against COVID-19, the finding suggests that nucleocapsid antibody isn't a very sensitive marker for previous infection, he said, but using both types of antibodies may be.

"Thus, in this part of South Africa, immunity levels were very high, even though vaccination rates were low, suggesting that there had been a considerable amount of infection during earlier outbreaks."
Feb 23 NEJM study and audio interview


Study: Innova COVID-19 rapid test may miss many asymptomatic cases

The Innova lateral-flow (rapid antigen) test (LFT) may miss 20% of COVID-19 infections at UK testing centers, 29% of asymptomatic cases at mass-testing centers, and 81% of asymptomatic infections at university screenings, predicts an analysis published yesterday in BMJ. Innova tests are not sold in the United States.

A team led by University of Birmingham in England led the study, which used linked empirical data on the accuracy of the test and the probability of a positive viral culture or viral transmission, as well as viral loads in COVID-19 patients in the three settings.

Estimated sensitivity of the Innova test was based on 70 COVID-19 patients, while infectiousness was based on viral cultures of 246 respiratory samples from 176 patients and 2,474,066 close contacts. Viral loads were based on 70 infected patients in Liverpool and 62 in Birmingham.

The researchers predicted that the test would miss 20% of people tested at a National Health Service Test-and-Trace center, 29% of asymptomatic patients at a municipal mass testing site, and 81% asymptomatic attendees at a university screening. The test, they found, would also miss 38%, 47%, and 90% of sources of secondary infections, respectively.

In contrast, two mathematical models underestimated missed infections at 8%, 10%, and 32% in each of the three settings, respectively, while the assumptions from another model estimated that none would be missed.

The researchers said that the percentage of missed infections is large enough to have clinical implications. "The proportion missed varied between settings because of different viral load distributions and is likely to be highest in those without symptoms," they wrote. "Key models have substantially overestimated the sensitivity of LFTs compared with empirical data."

In a related commentary, Angela Raffle, MBChB, of the University of Bristol, and Mike Gill, MBBS, a former regional director of public health, said that the lack of hard evidence of the utility of lateral flow tests has been striking in light of the more than £7 billion ($9.4 billion US) Britain has spent on these tests.

"The clear implication is that any positive impact from the detection of asymptomatic people may be outweighed by the effects of infectious people being falsely reassured and therefore mixing with others, perhaps even ignoring their symptoms," they wrote. "Unsurprisingly, testing is therefore chaotic and wasteful."
Feb 23 BMJ study and commentary

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