Pre-Omicron Natural Immunity Doesn’t Protect Well Against Omicron Subvariant: Study

The protection from prior COVID-19 infection does not protect well against reinfection against an Omicron subvariant, according to a new study.

The protection, known as natural immunity, was estimated at just 6 percent against reinfection with BA.2.75, the Omicron subvariant, by researchers in Qatar, where the subvariant is the dominant virus strain.


Natural immunity protected better if a person was infected during the Omicron era, which started in late 2021.

People who contracted BA.1 or BA.2 had 49.9 percent protection against reinfection. People who contracted BA.4 or BA.5 had 80.6 percent protection against reinfection.

Further, people who were initially infected during the pre-Omicron era and then infected again with BA.1 or BA.2 had 56.4 percent protection and people who were initially infected during the pre-Omicron era and then infected again with BA.4 or BA.5 had 91.6 percent infection.

“A combination of pre-omicron and omicron immunity is most protective against BA.2.75 reinfection,” the researchers said.

They utilized national COVID-19 databases, which include results of all tests conducted at healthcare facilities in Qatar.

The study was published ahead of peer review on medRxiv.

Researchers were unable to estimate protection against severe disease because there were not enough severe cases of COVID-19 among the population. An earlier study by the Qatari researchers found that natural immunity remained 97 percent effective after more than one year, “with no evidence of waning.”

Pre-Omicron infection also does not protect well against reinfection against BA.4 and BA.5, the researchers said in a separate study published in October.

Pre-Omicron infection effectiveness against symptomatic BA.4 or BA.5 reinfection was just 35.5 percent, and the effectiveness against any BA.4 or BA.5 reinfection regardless of symptoms was 27.7 percent.

On the other hand, the effectiveness of Omicron infection against BA.4 or BA.5 reinfection was 76.2 percent (symptomatic) and 78 percent (regardless of symptoms).

Protection from a previous SARS-CoV-2 infection against BA.4 or BA.5 reinfection was modest when the previous infection had been caused by a pre-omicron variant but strong when it had been caused by a post-omicron subvariant (including BA.1 or BA.2),” the researchers said in the paper, which was published by the New England Journal of Medicine.

In another recent paper, published in The Lancet, researchers in Denmark found that vaccinated people were more likely to be infected with the BA.2 or BA.5 subvariants than unvaccinated people, with the triple vaccinated the most at risk.

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