Past infection from Omicron does little to boost immunity or offer increased protection against subsequent infection, a study has warned.
Health experts say that even people who have had three doses of a Covid vaccine are not immune from catching the virus again, although the symptoms should be less severe.
The finding could explain why many people have been infected with Omicron more than once and why reinfections have been so frequent over a short period of time.
Professor Rosemary Boyton, an infectious disease immunologist at Imperial College London who led the study, said: “If you were infected during the first wave, then you can’t boost your immune response if you have an Omicron infection.
“When Omicron started flying around the country, people kept saying that’s OK, that will improve people’s immunity. What we’re saying is it’s not a good booster of immunity.”
What did researchers find?
Researchers in the study, published in the journal Science, followed the vaccination and infection experiences of 731 UK healthcare workers from March 2020 to January 2022.
All participants had received three doses of a Covid vaccine but had different histories of infection.
The team collected blood samples from participants in the weeks after their third dose to assess their antibody and T-cell responses towards the BA.1 Omicron variant.
Results showed that T-cells against Omicron proteins were poor among participants after their third vaccine, regardless of their previous infection history, and antibody levels against Omicron proteins were lower than against other variants.
Researchers suggest that protection against infection with subsequent variants of Covid varies depending on a person’s infection and vaccination history - a process described as "immune imprinting".
They found that those who never had Covid, but became infected with Omicron after three vaccine doses had good B and T cell immunity in lab tests against previous variants like Alpha and Delta, but less protection against Omicron.
Past infection with other variants also had little impact on immunity to Omicron. Those who were infected with Alpha earlier in the pandemic had less of an antibody response against Omicron, and their Omicron infection did not then boost their overall immune response.
The finding was a surprise to researchers as it was assumed that a prior Covid infection, even with a different variant, would help to boost a person’s immune response.
The study suggests that people who have already had Omicron will not necessarily have increased protection against the BA.4 and BA.5 variants that are currently on the increase in the UK.
Professor Danny Altmann, co-investigator in the study and also at Imperial College London, said that Omicron "flies under the radar, so the immune system is unable to remember it".
He said: “We have found that Omicron is far from a benign natural booster of vaccine immunity, as we might have thought, but it is an especially stealthy immune evader.”
“We’re not getting herd immunity, we’re not building up protective immunity to Omicron,” he added. “So we face not coming out the other end of infections and re-infections and breakthrough infections.”
The research only looked at the risk of infection from Covid, but Dr Altmann stressed that, despite high levels of infection in the UK, the continued low levels of hospitalisations from Covid suggest that the vaccines do protect against severe disease and death.
Although with new variants continuing to evolve, and Omicron’s ability to evade immunity, it is not a guarantee that past infection or vaccination will continue to protect against severe Covid.
Prof Boyton added: "A concern is that Omicron could potentially mutate further into a more pathogenic strain, or become better able to overcome vaccine protection.
"In this scenario, people who have had Omicron infection would be poorly boosted against future infection depending on their immune imprinting."