Omicron Infections, Without Vaccinations, Provide Little Immunity

This past winter, the Omicron variant surged through the country, causing a wave of COVID-19 cases. Now, several months later, those affected are left with a nagging question: how protected am I from future variants? This question is particularly germane as another variant, BA.2.12.1, is causing a recent spike in cases. Now, a team of researchers may have an answer.

New research shows that infection with the Omicron variant of SARS-CoV-2 provides little long-term immunity against other variants in unvaccinated people. In experiments using mice and blood samples from donors who were infected with Omicron, the team found that the Omicron variant induces only a weak immune response. In vaccinated individuals, this response—while weak—helped strengthen overall protection against a variety of COVID-19 strains. In those without prior vaccination, however, the immune response failed to confer broad, robust protection against other strains.

“In the unvaccinated population, an infection with Omicron might be roughly equivalent to getting one shot of a vaccine,” said Melanie Ott, Md, PhD, director of the Gladstone Institute of Virology. “It confers a little bit of protection against COVID-19, but it’s not very broad.”

This research is published in Nature in the paper, “Limited cross-variant immunity from SARS-CoV-2 Omicron without vaccination.”

As the Omicron variant of SARS-CoV-2 spread around the globe in late 2021 and early 2022, scientists weren’t initially sure how the variant, which appeared to result in a weaker infection, might impact long-term immunity against COVID-19 .

“When the Omicron variant first emerged, a lot of people wondered whether it could essentially act as a vaccine for people who didn’t want to get vaccinated, eliciting a strong and broad-acting immune response,” said Irene Chen, a graduate student in Ott’s lab.

To find the answer, the team of researchers examined the effect of Omicron in mice. Compared to an ancestral strain of SARS-CoV-2 and the Delta variant, Omicron led to far fewer symptoms in the mice. However, the virus was detected in airway cells, albeit at lower levels. Similarly, Omicron was able to infect isolated human cells but replicated less than other variants.

The team then characterized the immune response generated by Omicron infections. In mice infected with Omicron, despite the milder symptoms, the immune system generated the T cells and antibodies typically seen in response to other viruses.

The study’s first authors, Tongcui Ma (left), Irene Chen (center), and Rahul Suryawanshi (right). [Michael Short/Gladstone Institutes]

“We demonstrated in this study that the lower pathogenicity of Omicron is not because the virus cannot take hold,” says Nadia Roan, PhD, associate investigator at Gladstone.

That leaves other reasons that might explain why Omicron differs from other variants in terms of symptoms and immunity, including the lower replication seen with Omicron or the antibodies generated.

To gauge how the immune response against Omicron fared over time, the researchers collected blood samples from mice infected with the ancestral, Delta, or Omicron variants of SARS-CoV-2 and measured the ability of their immune cells and antibodies to recognize five different viral variants—ancestral (WA1), Alpha, Beta, Delta, and Omicron.

Blood from uninfected animals was unable to neutralize any of the viruses. Samples from WA1-infected animals could neutralize Alpha and, to a lesser degree, the Beta and Delta virus—but not Omicron. Samples from Delta-infected mice could neutralize Delta, Alpha, and, to a lesser degree, the Omicron and Beta virus. However, blood from Omicron-infected mice could only neutralize the Omicron variant.

The team confirmed these results using blood from ten unvaccinated people who had been infected with Omicron—their blood was not able to neutralize other variants. When they tested blood from 11 unvaccinated people who had been infected with Delta, the samples could neutralize Delta and, as had been seen in mice, the other variants to a lesser extent.

When they repeated the experiments with blood from vaccinated people, the results were different: vaccinated individuals with confirmed Omicron or Delta breakthrough infections all showed the ability to neutralize all the tested variants, conferring higher protection.

“When it comes to other variants that might evolve in the future, we can’t predict exactly what would happen, but based on these results, I’d suspect that unvaccinated people who were infected with Omicron will have very little protection,” said Otto. “But on the contrary, vaccinated individuals are likely to be more broadly protected against future variants, especially if they had a breakthrough infection.”

“Our results may be useful not only to inform individuals’ decisions on vaccination, but also for the design of future COVID-19 vaccines that confer broad protection against many variants,” said Charles Chiu, MD, PhD, professor of infectious diseases at UCSF .

“This research underscores the importance of staying current with your vaccinations, even if you have previously been infected with the Omicron variant, as you are still likely vulnerable to re-infection,” said co-senior author Jennifer Doudna, PhD, a senior investigator at Gladstone, a professor at UC Berkeley, and an investigator of the Howard Hughes Medical Institute.

Leave a Comment