The research, published on Tuesday in the journal Science, assessed how people’s immunity following vaccination offers protection against the Sars-CoV-2 virus as well as the protection offered by prior infection.
While recovery post contracting Covid-19 is thought to provide a natural immunity boost against future infection with the virus, scientists, including those from Imperial College London, say Omicron provides a poor natural boost of Covid-19 immunity against re-infection.
In people who were triple vaccinated and had no prior Sars-CoV-2 infection, Omicron infection provided an immune boost against previous variants, but less so against Omicron itself.
Researchers say individuals who were infected during the first wave of the pandemic in 2020, and then again later with Omicron “lacked any boosting.”
The findings shed light on why breakthrough infections have been common during the Omicron wave.
However, scientists stress that vaccination does provide protection against severe disease, hospitalisation, and death from infection.
“Getting infected with omicron does not provide a potent boost to immunity against re-infection with omicron in the future,” study lead author Rosemary Boyton from Imperial’s Department of Infectious Disease, said in a statement.
Researchers say Omicron is “far from a benign natural booster of vaccine immunity,” but a “stealthy immune evader.”
“Not only can it break through vaccine defenses, it looks to leave very few of the hallmarks we’d expect on the immune system – it’s more stealthy than previous variants and flies under the radar, so the immune system is unable to remember it,” Danny Altmann, another co-author of the study from Imperial’s Department of Immunology and Inflammation, said.
Patterns of immunity against the novel coronavirus are “imprinted” on the immune system by infection history.
“Previous SARS-CoV-2 infection impacts on the ability to boost immunity against subsequent SARS-CoV-2 infection through a process called ‘immune imprinting’, and this may apply to sub-variants of omicron including BA.4 and BA.5,” Dr Boyton explained.
To understand why there were many reported Omicron breakthrough infections, even among those who were triple vaccinated, scientists analysed blood samples from UK healthcare workers who received three doses of mRNA vaccine, and those who had different SARS-CoV-2 infection histories.
Researchers assessed their immune systems’ antibody, T and B cell functions against omicron.
The study found that people with no prior Sars-CoV-2 infection, who then had Omicron, had enhanced cross-reactive immunity to previous variants – including the involvement of B and T cell immunity – against Alpha, Beta, Gamma and Delta variants.
However, they showed a reduced boosting against the Omicron variant’s spike protein – which the virus uses to enter cells and against which most vaccines work.
Those with prior Alpha infection showed a less sustained antibody response against Omicron, scientists found.
The study also revealed that people infected during the first wave of the pandemic and then again with Omicron lacked any immune-boosting in an effect scientists called “hybrid immune damping.”
While vaccination protects against severe disease, scientists say the impact of infection and re-infection on long term health, including long Covid, is unknown.
They highlighted the importance of receiving booster vaccine doses to prevent future infections.
“While our latest findings highlight clear concerns about the nature of omicron infection, vaccination remains effective against severe disease. Those who are eligible to receive a booster should be encouraged to do so,” Dr Altmann said.
Although the study focused only on triple mRNA vaccinated individuals, the findings shed light on the link between a person’s infection history and their response to vaccination.
Since there is a “broad diversity” of infection history across populations, scientists say further exposure to vaccines could have “different implications for different people.”
“Previous infection with different variants impacts both the potency and durability of your immune responses,” Joseph Gibbons from Queen Mary University of London said.
“The effectiveness of current vaccination strategies will depend not only on which variants become dominant in the future, but also on how previous waves of infection have impacted our immunity,” Dr Gibbons said.
In future studies, scientists hope to understand the diverse immune impacts of vaccines, and how these patterns might shape protection against future variants of the coronavirus.