Omicron may have a higher rate of asymptomatic carriage, say experts

Preliminary findings from two clinical trials in SA suggest that Omicron may have a much higher rate of asymptomatic “carriage” than earlier variants of Covid-19.

This higher asymptomatic carriage rate is likely a major factor in the rapid and widespread dissemination of the variant, even among populations with high rates of earlier coronavirus infection, the SA Medical Research Council (SAMRC) said in a statement.

“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 Dr Lawrence Corey, senior author of the study.

“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.”

Omicron, first reported to the World Health Organisation (WHO) in mid-November, has quickly spread to many countries around the world.

SAMRC president Prof Glenda Gray said the findings are preliminary — part of a paper posted at medrxiv.org 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.

Dr Nigel Garrett, head of vaccine and HIV pathogenesis research at the Centre for the Aids Programme of Research in SA, 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 immunocompromised as well as those who are not,” he said.

Of the initial 230 participants who underwent screening for the Ubuntu study between December 2 and 17, 31% tested positive for the coronavirus.

The researchers said this is in stark contrast to the positivity rate pre-Omicron, which ranged from less than 1% to 2.4%.

The sub-study of the Sisonke trial found that the mean asymptomatic carriage rate of 2.6% during the Beta and Delta outbreaks rose to 16% during the Omicron period.

The Sisonke study included 577 subjects previously vaccinated with the AD26.SARS.CoV.2 vaccine, with results suggesting a high carriage rate even in those known to be vaccinated.

“We are not yet able to determine how vaccination affects asymptomatic infection and spread,” said Prof Linda-Gail Bekker, professor of medicine and 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.

“Our data also strongly support the need to reach global equity with primary vaccination and to develop second-generation vaccines that may be even more protective against acquisition.”

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