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Omicron has a 7-12 times higher asymptomatic ‘carriage’ rate than other variants- study

South Africa. Cape Town. A 6-inch long swab used to be inserted into the nose cavity to test for COVID-19. Picture: Ian Landsberg/African News Agency (ANA).

South Africa. Cape Town. A 6-inch long swab used to be inserted into the nose cavity to test for COVID-19. Picture: Ian Landsberg/African News Agency (ANA).

Published Jan 11, 2022

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The Covid-19 Omicron variant has been found to have a higher rate of asymptomatic “carriage” than other variants of concern, which could explain its rapid spread.

Preliminary findings based on two clinical trials in South Africa found that people who are asymptomatic or show no symptoms of Covid-19 could still transmit the virus, but those with the Omicron variant had 7-12 times more asymptomatic carriage.

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The data was drawn from the Ubuntu clinical trial and the Sisonke vaccine trial.

Head of Vaccine and HIV Pathogenesis Research at the Center for the AIDS Program of Research in South Africa (CAPRISA), and lead author of the Ubuntu study, Dr Nigel Garrett, said a higher asymptomatic carriage rate means that more people are infected with the virus but don’t show symptoms and are possibly able to transmit to others.

“There are a lot more asymptomatic people within the Omicron wave compared to the Delta wave. We feel that Omicron is a milder disease, but symptoms alone doesn’t seem to determine anymore whether someone tests positive,” he said.

The Ubuntu trial, which took place in December in sub-Saharan Africa, evaluated the effectiveness of Moderna’s mRNA-based vaccine in people living with HIV.

Of the 230 unvaccinated and asymptomatic participants in the study, 31% tested positive for the Covid-19 Omicron variant, which is a much higher percentage compared to pre-omicron studies that ranged from less than 1-2.4%.

Data was also drawn from a sub-study of the Sisonke large phase 3B implementation study that evaluated breakthrough infections in 1,200 health care workers, including those who are HIV infected and health care workers who are pregnant or breastfeeding.

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The Sisonke sub-study, which examined 577 participants vaccinated with the Johnson & Johnson vaccine, found the average asymptomatic carriage rate of 2.6% during the Beta and Delta outbreaks rose to 16% during the Omicron period.

The results suggest a high carriage rate even in those who have been vaccinated.

President of the South African Medical Research Council (SAMRC), Dr Glenda Gray, said the findings of the study are preliminary and are awaiting peer review. However, they appear to be in line with the bigger picture about Omicron’s high transmissibility.

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“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.

Garrett, said the studies were initiated because sub-Saharan Africa has been hit hard by both HIV and the Covid-19 pandemic.

“Ubuntu and Sisonke will provide important data on safety, dosage and effectiveness of vaccines, but they already are helping us better understand the way this virus can change and how those changes affect transmission and severity. It is critical that we know how omicron and other variants spread among those who are immuno-compromised as well as those who are not,” he said.

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