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. 2022 Apr 7;386(14):1314-1326.
doi: 10.1056/NEJMoa2119658. Epub 2022 Feb 23.

Population Immunity and Covid-19 Severity with Omicron Variant in South Africa

Shabir A Madhi  1 , Gaurav Kwatra  1 , Jonathan E Myers  1 , Waasila Jassat  1 , Nisha Dhar  1 , Christian K Mukendi  1 , Amit J Nana  1 , Lucille Blumberg  1 , Richard Welch  1 , Nicoletta Ngorima-Mabhena  1 , Portia C Mutevedzi  1
Affiliations

Population Immunity and Covid-19 Severity with Omicron Variant in South Africa

Shabir A Madhi et al. N Engl J Med. .

Abstract

Background: The B.1.1.529 (omicron) variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was first identified on November 25, 2021, in Gauteng province, South Africa. Data regarding the seroprevalence of SARS-CoV-2 IgG in Gauteng before the fourth wave of coronavirus disease 2019 (Covid-19), in which the omicron variant was dominant, are needed.

Methods: We conducted a seroepidemiologic survey from October 22 to December 9, 2021, in Gauteng to determine the seroprevalence of SARS-CoV-2 IgG. Households included in a previous seroepidemiologic survey (conducted from November 2020 to January 2021) were contacted; to account for changes in the survey population, there was a 10% increase in the households contacted, with the use of the same sampling framework. Dried-blood-spot samples were tested for IgG against SARS-CoV-2 spike protein and nucleocapsid protein with the use of quantitative assays. We also evaluated Covid-19 epidemiologic trends in Gauteng, including cases, hospitalizations, recorded deaths, and excess deaths from the start of the pandemic through January 12, 2022.

Results: Samples were obtained from 7010 participants, of whom 1319 (18.8%) had received a Covid-19 vaccine. The seroprevalence of SARS-CoV-2 IgG ranged from 56.2% (95% confidence interval [CI], 52.6 to 59.7) among children younger than 12 years of age to 79.7% (95% CI, 77.6 to 81.5) among adults older than 50 years of age. Vaccinated participants were more likely to be seropositive for SARS-CoV-2 than unvaccinated participants (93.1% vs. 68.4%). Epidemiologic data showed that the incidence of SARS-CoV-2 infection increased and subsequently declined more rapidly during the fourth wave than it had during the three previous waves. The incidence of infection was decoupled from the incidences of hospitalization, recorded death, and excess death during the fourth wave, as compared with the proportions seen during previous waves.

Conclusions: Widespread underlying SARS-CoV-2 seropositivity was observed in Gauteng before the omicron-dominant wave of Covid-19. Epidemiologic data showed a decoupling of hospitalizations and deaths from infections while omicron was circulating. (Funded by the Bill and Melinda Gates Foundation.).

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Figures

Figure 1
Figure 1. Survey Participants.
This survey (conducted from October 22 to December 9, 2021) included the same households that were sampled during our previous survey (conducted from November 4, 2020, to January 22, 2021). To account for possible nonparticipation, out-migration, and death since the previous survey, there was a 10% increase in the households that were sampled; the additional households were sampled in the same clusters used previously.
Figure 2
Figure 2. Cases, Hospitalizations, Recorded Deaths, and Excess Deaths Attributable to Covid-19 in Gauteng, South Africa, from the Start of the Pandemic through January 12, 2022.
Shown are incidences of daily cases, weekly hospitalizations, daily recorded deaths, and weekly excess deaths attributable to coronavirus disease 2019 (Covid-19). The inset shows the incidence of daily recorded deaths on an enlarged y axis. The horizontal dashed line indicates an incidence of zero. The data were sourced from the National Institute for Communicable Diseases daily databases through January 12, 2022, except for the data regarding weekly excess deaths attributable to Covid-19, which were defined by and sourced from the South African Medical Research Council through January 8, 2022. The B.1.1.529 (omicron) variant was first identified on November 25, 2021. Cases included asymptomatic and symptomatic infections with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) confirmed by either a nucleic acid amplification assay or a rapid antigen test. Changes in the frequency of testing limit direct comparisons of case numbers; in particular, the lower frequency of testing during the first wave, which was due to constraints in laboratory capacity and prioritization of testing for hospitalized persons, prevents the direct comparison of cases from the first wave with those from subsequent waves. Hospitalizations included admissions for SARS-CoV-2 infection, as well as admissions for other illnesses in which SARS-CoV-2 infection was incidentally identified on routine screening at the time of admission. The DATCOV system was developed during the first wave, with gradual onboarding of facilities; thus, hospitalizations from the first wave may be underestimated. Definitions of recorded death and excess death attributable to Covid-19 are provided in the Supplementary Appendix.
Figure 3
Figure 3. Covid-19 Cases, Hospitalizations, and Recorded Deaths in Gauteng, South Africa, According to Age Group.
Shown are 7-day moving averages of the incidences of daily cases, hospitalizations, and recorded deaths among participants 4 years of age or younger (Panel A), 5 to 17 years of age (Panel B), 18 to 44 years of age (Panel C), 45 to 59 years of age (Panel D), and 60 years of age or older (Panel E). The horizontal dashed line indicates an incidence of zero. Because the incidences differ across age groups, different y-axis scales are used for each age group to provide clarity and aid in the visual interpretation of the trends in each group.
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