Omicron reinfection likely, but will immunity from past infection, vaccines hold up for severe Covid?

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  • SA has high rates of natural immunity from past infection over the previous three waves.
  • In the face of the Omicron variant, however, this immunity is unlikely to protect against reinfection, according to latest data.
  • Natural and vaccine-induced immunity is expected to protect against severe disease, but experts say getting the shot will provide even greater protection.

Around 60-70% of South Africans have already had a Covid-19 infection since the start of the epidemic in March 2020. 

But, according to the latest evidence, the protection (immunity) derived from this infection is unlikely to protect against reinfection with the Omicron variant.

The study was conducted by researchers at the National Institute for Communicable Diseases (NICD) and was published on a preprint server this week, AFP reported.

The team wrote that reinfection was 2.4 times more likely with Omicron than with the Beta and Delta variants which dominated SA’s second and third waves, respectively.  

"These findings suggest that Omicron's selection advantage is at least partially driven by an increased ability to infect previously infected individuals," one of the study's co-authors, Dr Juliet Pulliam, wrote on Twitter. Pulliam is Director of the South African DST-NRF Centre of Excellence in Epidemiological Modelling and Analysis (SACEMA).

Pulliam said that recent reinfections were seen in people whose primary infections occurred across all three waves, with the most having their primary infection in the (third) Delta wave.

Vaccination status unknown

The researchers did not have information about the individuals' vaccination status, and therefore couldn't determine whether Omicron also escaped vaccine-induced immunity, said Pulliam.

The scientists will next look at the extent to which the new variant escapes protection from both natural infection and the vaccines, as well as how transmissible it is compared to the other variants of concern.

Pulliam added that research looking into whether natural- and vaccine-induced immunity is protective against severe disease – associated with Omicron infection – is urgently needed, although several scientists believe the vaccines will continue to hold up in this regard.

Reinfection with Omicron was expected

Epidemiologist Professor Salim Abdool Karim recently explained that some of the mutations seen in Omicron may escape antibodies (one part of the immune system responsible for blocking infection) derived through prior infection and vaccination. 

"And so we can expect ... more reinfection among those who've had previous infection and have not been vaccinated, because this variant will bypass some of that immunity," said Abdool Karim.

A study conducted by the US Centers for Disease Control and Prevention (CDC) this year found that unvaccinated people with past infection were five times more likely to be reinfected, compared to those who were vaccinated, Health24 reported.

Natural immunity against particular variant

Another expert believed that natural immunity protects best against the particular variant that was responsible for that initial infection. 

Professor Thomas Scriba, the deputy director of immunology and laboratory at the SA Tuberculosis Vaccine Initiative at the University of Cape Town, told Health24 this week:

It is clear that people who were infected during the first wave, for example, were more susceptible to the Delta variant because the immunity from the original virus was not fully protective against the Delta variant.

It is expected that protection derived from infection in previous waves would, therefore, not be sufficiently protective against Omicron, he said.

"The many mutations in the spike protein of the Omicron variant further suggest that natural immunity from prior infections will afford lower levels of protection against the Omicron virus,” he said, adding: 

"Again, the best way to protect yourself against Omicron is to ensure that you are fully vaccinated, irrespective of whether you have had a previous infection or not."

Omicron likely won’t affect protection against severe disease

Based on what is known about the other variants, Abdool Karim added that the vaccines will likely continue to offer strong protection against severe disease, hospitalisation and death, caused by Omicron.

While the different Covid-19 vaccines may have different levels of protection, particularly against mild infection, they all remain highly effective against hospitalisation, he said.

Dr Anne von Gottberg, a clinical microbiologist and the head of respiratory diseases at the NICD, on Thursday said that while previous infections do not appear to provide protection against reinfection with Omicron, vaccination will still prevent serious illness.

Commenting on the current study, Professor Paul Hunter, of The Norwich School of Medicine, University of East Anglia, told Sky News:

"With previous variants, epidemiological studies showed that protection against severe disease from other variants was better maintained than protection against infection."

Prior infection, vaccination still needed?

On whether vaccination is still needed for people with past infection, Professor Shabir Madhi recently told Health24 that even a single dose of any Covid vaccine will likely provide the person with significant protection against reinfection, as well as severe disease.

"The vaccine will boost the antibody (and T-cell) responses, and heighten protection against reinfection, and in the case of Omicron, it could also 'overcome' some of the reduced susceptibility of the variant to antibody activity," said Madhi, who is professor of vaccinology at the University of the Witwatersrand.

T cells are one part of the immune system that are known to protect against severe disease and death. Experts suspect that they may be less affected by Omicron’s many concerning mutations than are antibody responses.

Madhi previously said that natural immunity cannot be overlooked and that it offered “really good protection against severe disease and death”, but that this type of immunity was less predictable than vaccine-induced immunity.

Cases increasing fast

On Friday, health minister Joe Phaahla said that "indications are that this [Omicron] variant is highly transmittable” and that it has spread to all the provinces.

Dr Wassila Jassat, Public Health Specialist at the NICD, said hospital admissions were also on the rise. In Gauteng, the centre of the outbreak, admission trends have been higher than in the previous waves, she said. 

Phaahla said: "The evidence is very clear thus far, like all previous variants protection lies with vaccination. We must not hesitate to take the vaccine. It offers protection from serious illness and hospitalisation.”

While SA has high rates of past infection, vaccination rates are relatively low, with about only a quarter of the population being fully vaccinated.

Additional reporting by Tebogo Monama

*For more Covid-19 research, science and news, click here. You can also sign up for our Daily Dose newsletter here.

READ | Omicron variant: Why everyone still needs to be vaccinated, even if they've had Covid-19

READ | More reports that Omicron appears to cause mild Covid - but too early to say for sure, experts warn

READ | Covid-19: Reinfections three times more likely with Omicron variant, early research finds

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