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Omicron has higher asymptomatic carriage rate – Ubuntu study

Preliminary findings from SA clinical trials suggest the Omicron coronavirus variant has a much higher rate of asymptomatic carriage than earlier variants.

The studies, one of which was carried out when Omicron infections were surging in SA last month and another which resampled participants around the same time, found a far greater number of people tested positive for the coronavirus but were not showing symptoms compared with previous trials.

In the Ubuntu study (posted at medRXiv) evaluating the efficacy of Moderna’s COVID-19 vaccine in people living with HIV, 31% of 230 participants undergoing screening tested positive, with all 56 samples available for sequencing analysis verified to be Omicron.

“This is in stark contrast to the positivity rate pre-Omicron, which ranged from less than 1% to 2.4%,” the researchers said on the South African Medical Research Council (SAMRC) website.

Dr Glenda Gray, president of the SAMRC, said the findings are preliminary – part of a paper posted at before peer review and publication – but they appear to be in line with the bigger picture coming together about omicron’s high transmissibility.

“The larger studies were designed to analyse data at the intersection of COVID-19, vaccines, and people living with HIV, but they also are giving us useful information about Omicron and how its spread differs from those of previous variants of concern,” she said.

In a subgroup of the Sisonke trial evaluating the efficacy of Johnson & Johnson’s vaccine, the mean asymptomatic carriage rate rose to 16% during the Omicron period from the 2.6% of the Beta and Delta outbreaks.

“The Sisonke study included 577 subjects previously vaccinated … with results suggesting a high carriage rate even in those known to be vaccinated,” the researchers said. The “higher asymptomatic carriage rate is likely a major factor in the rapid and widespread dissemination of the variant, even among populations with high prior rates of coronavirus infection”.

SA experienced a surge in COVID-19 infections from late November, around the time its scientists alerted the world to Omicron. But new cases have since fallen back and early indications are that the wave has been marked by less intense disease than earlier variant waves.

Study details

High Rate of Asymptomatic Carriage Associated with Variant Strain Omicron

Nigel Garrett, Asa Tapley, Jessica Andriesen, Ishen Seocharan, Leigh H. Fisher, Lisa Bunts, Nicole Espy, Carole Wallis, April Kaur Randhawa, Ameena Goga, Linda-Gail Bekker, Glenda E. Gray, Lawrence Corey

Posted at medRXiv on 20 December 2021

The early widespread dissemination of Omicron indicates the urgent need to better understand the transmission dynamics of this variant, including asymptomatic spread among immunocompetent and immunosuppressed populations. In early December 2021, the Ubuntu clinical trial, designed to evaluate efficacy of the mRNA-1273 vaccine (Moderna) among persons living with HIV (PLWH), began enrolling participants. Nasal swabs are routinely obtained at the initial vaccination visit, which requires participants to be clinically well to receive their initial jab. Of the initial 230 participants enrolled between December 2 and December 17, 2021, 71 (31%) were PCR positive for SARS-CoV-2: all of whom were subsequently confirmed by S gene dropout to be Omicron; 48% of the tested samples had cycle threshold (CT) values <25 and 18% less than 20, indicative of high titers of asymptomatic shedding.

Asymptomatic carriage rates were similar in SARS-CoV-2 seropositive and seronegative persons (27% respectively). These data are in stark contrast to COVID-19 vaccine studies conducted pre-Omicron, where the SARS-CoV-2 PCR positivity rate at the first vaccination visit ranged from <1%-2.4%, including a cohort of over 1,200 PLWH largely enrolled in South Africa during the Beta outbreak.

We also evaluated asymptomatic carriage in a sub study of the Sisonke vaccine trial conducted in South African health care workers, which indicated 2.6% asymptomatic carriage during the Beta and Delta outbreaks and subsequently rose to 16% in both PLWH and PHLWH during the Omicron period.

These findings strongly suggest that Omicron has a much higher rate of asymptomatic carriage than other VOC and this high prevalence of asymptomatic infection is likely a major factor in the widespread, rapid dissemination of the variant globally, even among populations with high prior rates of SARS-COV-2 infection.


SAMRC article – Preliminary findings from studies in South Africa indicate that omicron has a much higher rate of asymptomatic ‘carriage’ than other variants of concern (Open access)


medRXiv article – High Rate of Asymptomatic Carriage Associated with Variant Strain Omicron (Open access)


See more from MedicalBrief archives:


Encouraging safety results from Sisonke trial of J&J vaccine in SA


J&J booster 84% effective against hospitalisation — Sisonke 2


Omicron variant: Clinical severity and hospitalisation profile in South Africa


Sisonke: Only 2% of breakthrough infections in J&J vaccinated health workers are severe



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