Omicron has a much higher rate of asymptomatic ‘carriage’ than other variants of concern

In evaluating the asymptomatic carriage rate of SARS-CoV-2 in persons with no signs or symptoms of COVID-19 on the day of sampling in the Sisonke study, the asymptomatic SARS-CoV-2 carriage rate was 16%, using polymerase chain reaction testing from mid-November to Dec. 7, 2021. Our study, Ubuntu, showed 31% asymptomatic carriage (71 of 230 samples). Prior studies using similar sampling techniques during ancestral, beta and delta variants had rates between 1% and 2.6% – 7 to 12 times less.
Preliminary findings from two clinical trials in South Africa suggest that the omicron variant of the virus that causes COVID-19 may have a much higher rate of asymptomatic ‘carriage’ than earlier variants. This higher asymptomatic carriage rate is likely a major factor in the rapid and widespread dissemination of the variant, even among populations with high prior rates of coronavirus infection.
“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 and principal investigator of the Fred Hutchinson Cancer Research Center-based operations centre of the COVID-19 Prevention Network, or CoVPN. “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 Organization (WHO) in mid-November, has quickly spread to many countries around the world.
Dr Glenda Gray, president of the South African Medical Research Council (SAMRC), said the findings are preliminary – part of a paper posted at medrxiv.org prior to 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 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,” she said.
A study called Ubuntu, launched in early December in sub-Saharan Africa, aims to evaluate the effectiveness of Moderna’s mRNA-based COVID-19 vaccine in people living with HIV. The other study is a substudy of the Sisonke large phase 3B implementation study that evaluated the effectiveness of the single-dose Ad26.SARs.CoV.2 vaccine. The Sisonke substudy evaluates the immunogenicity and breakthrough infections in 1,200 health care workers, included those who are HIV infected, and health care workers who are pregnant or breastfeeding.
Dr Nigel Garrett, head of Vaccine and HIV Pathogenesis Research at the Center for the AIDS Program of Research in South Africa (CAPRISA), 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 undergoing screening for the Ubuntu study between Dec. 2 and Dec. 17 with available swab samples, 31% tested positive for the coronavirus, and all samples available for sequencing analysis (56) were subsequently verified to be omicron. The researchers said this is in stark contrast to the positivity rate pre-omicron, which ranged from less than 1% to 2.4%. The substudy 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 Linda-Gail Bekker, MBChB, professor of medicine and director of the Desmond Tutu HIV Centre, 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 the acquisition.”
“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,” Corey said. “These preliminary study findings add to our understanding of how omicron is spreading and provide important clues about the amount of asymptomatic transmission.”
Vaccination guidelines and availability vary from country to country and even within regions, with many people in African countries and others still needing a first vaccine dose. Corey encourages everyone to get two doses and a booster as soon as they’re eligible.
Edited by: Stephen Morton