People who are infected but have no symptoms are still far more likely to infect others than they would have been with earlier variants, the data shows.

"As we witness the quick, global spread of Omicron, it is clear that we urgently need a better understanding of the transmission dynamics of this variant," said senior study author Dr. Lawrence Corey. He is principal investigator of the Fred Hutchinson Cancer Research Center-based operations center of the COVID-19 Prevention Network.

"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," he added in a network news release.

Both of the new studies were done in Africa.

The Sisonke study used PCR testing from mid-November 2021 to Dec. 7, 2021, in asymptomatic people. It found the carriage rate was 16%.

The larger Ubuntu study found 31% asymptomatic carriage, or in 71 out of 230 samples between Dec. 2 and Dec. 17, 2021. All the samples available for sequencing analysis were verified to be Omicron.

Past studies on ancestral, Beta and Delta variants had asymptomatic transmission rates of between 1% and 2.6%, seven to 12 times less than with the Omicron samples, the researchers said.

The Ubuntu study began in early December with the goal of evaluating the effectiveness of Moderna's COVID-19 vaccine in people living with HIV.

The Sisonke research was a sub-study of a larger study that evaluated the effectiveness of a single dose of the Johnson & Johnson COVID-19 vaccine. The sub-study evaluated immune responses and breakthrough infections in 1,200 health care workers, including those who are pregnant or breastfeeding or who have HIV. The study included 577 people vaccinated with Johnson & Johnson's COVID-19 vaccine, with results suggesting a high carriage rate even in those known to be vaccinated.

"The larger studies were designed to analyze 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," Dr. Glenda Gray, president of the South African Medical Research Council (SAMRC), said in an SAMRC news release.

Sub-Saharan Africa has been hit hard by both HIV and the COVID-19 pandemic, said Dr. Nigel Garrett, head of Vaccine and HIV Pathogenesis Research at the Center for the AIDS Program of Research in South Africa.

"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 immunocompromised as well as those who are not," Garrett said.

Preliminary findings on both studies were published on the preprint server medRxiv, and have not been peer-reviewed.

"We are not yet able to determine how vaccination affects asymptomatic infection and spread," said Linda-Gail Bekker, director of the Desmond Tutu HIV Centre at the University of Cape Town. "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."