First Data Emerge on Omicron Strain's Ability to Evade Immunity

Until now this had only been assumed due to the large number of mutations.

South African epidemiologists have obtained the first data on the ability of the Omicron strain to partially overcome the natural immunity to the coronavirus that people who have recovered possess. Until now, there has been no reliable information on this issue. We explain what has become known from the study. 

Until now, it had only been assumed (based on the mutations acquired by the strain) that Omicron may be able to evade immunity acquired by those who have recovered and those who have been vaccinated. 

However, no laboratory studies of Omicron's ability to evade immunity had been conducted, since it was only discovered on November 23. The study was based on an analysis of several databases containing coronavirus test results. 

The data had been collected since the start of the pandemic by various medical institutions in South Africa. The researchers combined different databases and matched the test results that residents of the country had taken at different times. They identified among them probable reinfection cases. 

A repeat infection was considered simply to be a positive test result appearing more than 90 days after the previous positive result. Based on the collected data, scientists were to determine how much the risk of repeat infections changed during different waves of morbidity. 

In South Africa, the waves clearly coincided with the spread of several new strains:

1. In the first wave (summer 2020), the 'Wuhan' SARS-CoV-2 strain predominated. 
2. In the second wave (January 2021), the Beta strain (South African) predominated. 
3. In the third wave (which began in June 2021), Delta displaced Beta. 
4. In mid-November, a new surge in morbidity began, linked to Omicron. 

It is known that virus particles of the Delta and Beta strains are neutralized less effectively by antibodies produced by the original 'Wuhan' strain. Therefore, an increase in the risk of reinfections could have been expected during the second and third waves of morbidity. 

The risk of reinfections associated with the virus's characteristics cannot be calculated based on the frequency of such events, that is, the number of reinfections found per unit of time. This number itself depends on the overall incidence in the given period.

Therefore, scientists analyzed not the frequency of repeat infections, but compared the ratio of primary and secondary infections over the entire course of the epidemic. The results of the analysis:

• Despite the well-established drop in the binding power of antibodies against the Beta and Delta strains, this decline did not affect the risk of repeat infections in any way. In other words, people who had recovered from the 'Wuhan' strain were reinfected with Delta or Beta with about the same probability as with the same 'Wuhan' variant. 
• In the case of Omicron, the risk of repeat infections rose almost twofold relative to the baseline. 

An increase in the frequency of repeat infections leads, in a population that has recovered, to a higher rate of virus spread. This may even look like higher contagiousness. However, the issue is not contagiousness itself, but that such an immune-evading virus simply has a larger 'potential audience' for successful infections. 

This may explain the high rate of virus spread in South Africa, which is indeed observed alongside the emergence of the new strain. For example, in early November about 300 new infections were registered daily, and on December 2, 11,535 cases were recorded. The share of positive results among all tests conducted is also growing, which means the real increase in infections is even greater. 

Two aspects of the new study are important to understand:

1. It concerns infections identified through laboratory tests, not diagnosed diseases. Not every infection leads to an illness that causes any symptoms at all, and in this respect repeat infections may differ greatly from primary ones. There are grounds to believe that repeat infections on average proceed much more easily than primary ones. 
2. The authors did not account for people's vaccination status, as this information was simply not available in the databases they worked with. We do not yet know whether the risk of repeat infection with Omicron differs between vaccinated people and those who previously recovered. This difference may be significant, but establishing it requires additional research.