COVID-19 scan February 24, 2022
COVID-19 scan February 24, 2022

COVID-19 scan February 24, 2022

High immunity, low vaccination rate in South Africa point to previous infections

A seroepidemiological study of 7,010 people in Gauteng province, South Africa, before the SARS-CoV-2 Omicron variant outperformed the Delta strain, shows that 80% of those over 50 had antibodies to the virus, and most seropositivity is likely generated by previous infection.

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

Of the 7,010 participants, 18.8% were 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 over 50 years. Vaccinated participants had higher rates of IgG seropositivity than their unvaccinated peers (93.1% vs.%).

A survey 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 epidemiological data, COVID-19 cases increased and then decreased more rapidly during the study period than in previous increases, and the cases were also milder with lower rates of hospitalization, deaths, and excessive death.

“We believe that the decoupling of the incidence of Covid-19 cases from the incidence of hospitalization and death during the omicron-dominant wave in South Africa heralds a turning point in the Covid-19 pandemic if the primary objective is protection against serious illness 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 the samples had antibodies to the nucleocapsid protein expressed by the virus, while 70% had antibodies to the virus’ tip protein generated by vaccines. With only a third of the population vaccinated against COVID-19, the result suggests that nucleocapsid antibody is not a very sensitive marker of previous infection, he said, but using both types of antibodies may be.

“So in this part of South Africa, immunity levels were very high, even though vaccination rates were low, suggesting that there had been a significant amount of infection during previous outbreaks.”
Feb 23 NEJM examination and audio interview


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

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

A team led by the University of Birmingham in England led the study, which used related empirical data on the accuracy of the test and the likelihood of a positive viral culture or viral transmission, as well as viral load 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 airway samples from 176 patients and 2,474,066 close contacts. Viral load was based on 70 infected patients in Liverpool and 62 in Birmingham.

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

In contrast, two mathematical models underestimated lost infections by 8%, 10%, and 32%, respectively, in each of the three settings, while the assumptions from another model estimated that no one would be missed.

The researchers said the percentage of unanswered infections is large enough to have clinical implications. “The proportion missing missed between settings due to different viral load distributions and is likely to be highest in those without symptoms,” they wrote. “Key models have significantly overestimated the sensitivity of LFTs compared to empirical data.”

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

“The clear implication is that any positive impact from the detection of asymptomatic people can be offset by the effects of infectious people being falsely sedated and therefore interfering with others, perhaps even ignoring their symptoms,” they wrote. “Not surprisingly, therefore, testing is chaotic and wasteful.”
Feb 23 BMJ examination and comment

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