Omicron may have higher rate of asymptomatic carriers, early research suggests

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  • Two trials looked at the possible reason why Omicron can spread fast.
  • The asymptomatic rate increased six times more with Omicron than with Beta and Delta.
  • Non-pharmaceutical measures still play an important part in curbing the spread of Omicron.

Two South African-based studies have found that Omicron has a much higher asymptomatic carriage rate than other variants of concern.

This may be why there is a high prevalence of asymptomatic infection, which is probably a significant factor in the rapid response transmission of the variant globally.

The study findings, published in the preprint server Medrxiv, came from two South African trials – the Ubuntu and Sisonke trials. Ubuntu was launched last month to evaluate the effectiveness of Moderna’s mRNA-based Covid-19 vaccine in people living with HIV who were not previously vaccinated.

The other study forms part of Sisonke, which investigated the effectiveness of the J&J vaccine in healthcare workers.

The cohorts

The Ubuntu trial enrolled 330 people from seven provinces. At the start of the trial, 230 people’s nasal samples were taken from the participants to test before they received their first Moderna dose.

The participants’ HIV viral load was also tested to ensure that they were clinically well.

In the sample from the Sisonke trial, 1 200 healthcare workers formed part of this evaluation, which included those who are HIV infected and health care workers who are pregnant or breastfeeding.

Omicron’s high prevalence

The study findings show that Omicron is more asymptomatic when compared to other variants of concern, which could explain the fast rate at which the variant spreads.  

The study found that 31% of the Unbuntu participants tested positive for Covid-19 with an Omicron linage at the start of the trial. Of these positive cases, 48% did not have any symptoms.

While in the Sisonke group, researchers compared asymptomatic carriage during the Beta and Delta outbreak to the Omicron wave, and found that asymptomatic carriage increased from 2.6% to 16% in both people infected with HIV and those without the virus. The results also found a high carriage rate in healthcare workers that were vaccinated.

"We know that vaccination, testing and treatment are critical for those who face the dual threat of HIV and Covid-19, as they remain at high risk of acquisition and transmission.

"These preliminary study findings add to our understanding of how Omicron is spreading and provide important clues about the amount of asymptomatic transmission," said Dr Lawrence Corey,  a senior author of the study, in a statement.

Corey says that the findings showed the importance of preventive measures.

"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 said. 

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