Omicron has Higher Asymptomatic Carriage, Says South African Studies

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According to two South African clinical trials, the Omicron variant has a higher asymptomatic carriage than previous variants.  

The Omicron variant was first discovered in Botswana, South Africa. Since its discovery in late November, the variant has spread to more than 70 countries worldwide. Moreover, in such a short period it has managed to become the dominant strain in various countries, such as the U.S. It is unclear as to what makes the variant so transmissible. Multiple studies are currently underway to further assess the variant’s transmissibility and virulence. Preliminary findings from two South African studies suggest that a higher asymptomatic carriage rate might be to blame for the surge in Omicron cases. The two clinical trials, Ubuntu and Sisonke, found a greater number of asymptomatic people testing positive for COVID-19 than previous studies.

The Ubuntu trial began in early December and recruited over 200 people living with HIV in sub-Saharan Africa. Researchers assessed effectiveness of the Moderna COVID-19 vaccine among them. Of the 230 study participants undergoing routine screening, 31% (71) tested positive for coronavirus. Moreover, genetic sequencing confirmed all as an Omicron infection. However, this is a sharp increase from pre-Omicron time when the positivity rate ranged from less than 1% to 2.4%.

Since so many people may be asymptomatic, we can’t always know who is carrying the virus, but we do know what we can do to protect ourselves and to help prevent further spread: Wear a mask; wash your hands; avoid large, indoor gatherings; and get fully vaccinated as soon as possible.

Dr. Lawrence Corey, senior study author of Ubuntu Study

Greater Positive Results Compare to Previous Trials

On the other hand, a subgroup of the Sisonke trial looked at the efficacy of Johnson & Johnson’s COVID-19 vaccine. The study included 1200 health care workers, some of whom had HIV. The preliminary findings reported a 2.6% asymptomatic carriage among South African healthcare workers during the Beta and Delta outbreaks. However, this number rose to 16% during the Omicron period. Furthermore, the results showed a high asymptomatic carriage rate even among vaccinated healthcare workers.

According to Linda-Gail Bekker at the Desmond Tutu HIV Centre at the University of Cape Town, it is unclear how vaccines affect symptomatic infection and spread.

We further need to devise strategies for rapid detection of asymptomatic carriage, particularly in long-term care facilities and hospitals, where transmission to high-risk populations may occur.

Professor Linda-Gail Bekker, University of Cape Town

Despite experiencing a surge in cases during Omicron’s discovery, South Africa is now seeing a fall in COVID-19 cases. Thus, suggesting that the variant likely causes less severe infection.


Garrett, N., Tapley, A., Andriesen, J., Seocharan, I., Fisher, L. H., Bunts, L., . . . Corey, L. (2021). High rate of asymptomatic carriage associated with variant strain OMICRON. doi:10.1101/2021.12.20.21268130


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