The recently identified Omicron variant of SARS-CoV-2 is associated with a substantial ability to evade immunity from prior Covid-19 infection, according to a study based on population-level evidence.
The team of researchers in South Africa noted that, in contrast, there is no population-wide epidemiological evidence of immune escape associated with the Beta or Delta variants.
The B.1.1.529 Covid variant, first detected in South Africa, was designated by the World Health Organisation as a Variant of Concern (VOC), the health body's top category for worrying coronavirus variants.
Experts believe the variant carries around 50 mutations. Of these, 32 are in spike proteins, which the virus uses to enter the human cells, and 10 are mutations of high relevance.
This is the first data-based study to suggest that Omicron may evade immunity from prior infection.
"Population-level evidence suggests that the Omicron variant is associated with substantial ability to evade immunity from prior infection. In contrast, there is no population-wide epidemiological evidence of immune escape associated with the Beta or Delta variants," the authors of the study noted.
In the yet-to-be peer-reviewed study posted on the pre-print repository medRxiv last week, the researchers examined whether SARS-CoV-2 reinfection risk has changed through time in South Africa, in the context of the emergence of the Beta, Delta, and Omicron variants.
They used surveillance data on SARS-CoV-2, the virus that causes Covid-19, between March 4, 2020 and November 27, 2021, collected through South Africa's National Notifiable Medical Conditions Surveillance System.
The study participants included 2,796,982 individuals with laboratory-confirmed SARS-CoV-2 who had a positive test result at least 90 days prior to November 27, 2021.
Individuals having sequential positive tests at least 90 days apart were considered to have suspected reinfections.
The study identified 35,670 suspected reinfections among 2,796,982 individuals with laboratory-confirmed SARS-CoV-2.
“These findings suggest that Omicron’s selection advantage is at least partially driven by an increased ability to infect previously infected individuals," said Juliet R.C. Pulliam, from South African DSI-NRF Centre of Excellence in Epidemiological Modelling and Analysis (SACEMA), Stellenbosch University, South Africa.
"Immune escape from prior infection, whether or not Omicron can also evade vaccine derived immunity, has important implications for public health globally, but there is still a lot we don't know," Pulliam, corresponding author of the study, tweeted.
The study also suggests that the spread of the Beta and Delta variants of Covid-19 was due to increased transmissibility, rather than ability to evade immunity.
"We find evidence of a substantial and ongoing increase in the risk of reinfection that is temporally consistent with the timing of the emergence of the Omicron variant in South Africa, suggesting that its selection advantage is at least partially driven by an increased ability to infect previously infected individuals," the authors said.
The finding has important implications for public health planning, particularly in countries like South Africa with high rates of immunity from prior infection, they noted.
Urgent questions remain regarding whether Omicron is also able to evade vaccine-induced immunity and the potential implications of reduced immunity to infection on protection against severe disease and death.
"We do not have information about the vaccination status of individuals in our data set and therefore cannot make any assessment of whether Omicron also evades vaccine-derived immunity," Pulliam said her in tweet.
"Next steps include quantifying the extent of Omicron's immune escape for both natural and vaccine-derived immunity, as well as its transmissibility relative to other variants," she added.