Blood samples were collected before vaccination, after the first dose, and twice after the second dose, according to the paper published in the journal Science Translational Medicine on Tuesday.

The researchers observed a decline in the neutralising antibodies – a predictor of immune protection – from one month to six months after the two doses were given.

All four variants of concern consistently showed lower neutralising antibodies compared to a strain at the early stage of the pandemic, with the Omicron variant having the most pronounced resistance.

At six months, about 56.3 per cent of health care workers taking part in the study had levels below the detection limit against Delta, while 89.6 per cent were below that limit for Omicron.

Twelve of the health care workers were found to be infected with the coronavirus at different phases of vaccination and their level of neutralising antibodies was about six times higher six months later than the subjects who had not been infected.

However, 30 per cent of the infected people still had no detectable level of neutralising activity, compared to 60 per cent of the uninfected health care workers.

Booster of Chinese mRNA vaccine candidate raises effectiveness against Omicron

The researchers suggested that the ability of Omicron to evade immunity highlighted the need for booster shots.

“We observed a profound escape of the Omicron variant from mRNA vaccine-induced immunity, even at three to four weeks after the second dose … Further, this escape was not rescued in most health care workers by breakthrough infection,” the paper said.

“Although breakthrough infection can boost neutralising antibody responses, it appears largely ineffective for providing protection from Omicron, at least for individuals infected before the Omicron wave.”

In South Africa, meanwhile, a study led by Dr Penny Moore at the National Institute for Communicable Diseases, under the National Health Laboratory Service in Johannesburg, looked at the neutralising effect of Omicron infection in combination with particular vaccines.

Moore’s study found an Omicron infection induced antibodies against that variant in unvaccinated people but neutralising was “significantly compromised” against the Beta and Delta strains.

In contrast, people who had received the Johnson & Johnson or Pfizer vaccines before being infected with Omicron showed greatly improved cross-reactivity, with high antibody titres against a variant from the early stage of the pandemic, as well as the Beta, Delta and Omicron strains.

“In the absence of vaccination, Omicron-elicited humoral responses, while potent against the matched Omicron spike, show significantly less activity against variants of concern. Thus, while highly immunogenic, Omicron does not elicit cross-neutralising responses,” the authors wrote in a paper posted on MedRxiv.org without peer review.

“This may result in risk of reinfection in this unvaccinated group with other variants that continued to circulate and evolve in South Africa at the time of this study, albeit at low levels, including Beta, Delta.”

The research was based on limited samples, with only 20 unvaccinated and seven vaccinated people infected during the Omicron wave. The team said the findings had implications for the design of second-generation vaccines based on Omicron.

Animal studies by Moderna have shown that primates given Omicron-specific vaccines induced antibody titres at levels similar to those given its original mRNA vaccine.

“Overall, these data suggest that boosting individuals with or without immunity with vaccines specific for Omicron is unlikely to be superior to existing regimens,” the South African researchers said.