▎ WuXi AppTec content team editor
In the context of more than two years of COVID-19 and the push for COVID-19 vaccination in the past year, what impact will the Omicron epidemic bring? Although previous studies have focused on vaccine efficacy or analyzing the characteristics of Omicron infection in patients, there is a lack of perspective under the herd immune barrier.
Recently, the New England Journal of Medicine (NEJM) recently published a real-world study from South Africa, an Omicron endemic area, based on serological studies and case data, analyzing the results of the Omicron epidemic in large populations with immunity.
截图来源：The New England Journal of Medicine
Studies have shown that before the Omicron variant became epidemic, 73.1% of the sampled population was positive for antibodies. However, Omicron also brought a large number of infections compared to the first three local outbreaks, suggesting Omicron's high transmissibility and breakthrough infection.
However, this spike in infections subsided more quickly, and there were no significant hospitalizations and deaths. The paper notes that this may be due to the widespread presence of previous natural infections and vaccination-induced cellular immunity in the population.
Representative data before the Omicron epidemic
More than a year ago, a population-wide serological survey was conducted in Gauteng Province, South Africa (completed on 22 January 2021). The results showed that 19.1% of the population had a positive serum covid-19 antibody (by detecting IgG antibodies against the monologue receptor binding domain of the new coronavirus spike).
Subsequently, south Africa faced a third wave of COVID-19 due to the emergence of the delta variant, which lasted from April 7 to November 1.
On 9 December 2021, gauteng, South Africa, renewed its serum findings. Of these, 83% of the samples were obtained before the first detection of the Omicron variant on 25 November, and by December 2021, 98.4% of new cases sequenced in South Africa were Omicron infected.
Therefore, this latest serological survey data can be regarded as the level of population immunity before the fourth wave of the epidemic. In addition, Gauteng is the epicenter of the recent outbreak in South Africa and has 26% of the country's population, so the data in the region are quite representative.
Population immunity levels: more than seven adults have antibodies
A total of 7010 people were sampled in the survey, of which 1319 (18.8%) have been vaccinated against COVID-19.
Among all participants, the overall positive rate of serum IgG antibodies (against covid-19 spike protein or nucleocapsid protein) was 73.1%.
Comparing the antibody positivity rate 1 year ago, combined with the vaccination rate, this indicates that a large number of people have had a previous infection. In the previously clearly detected infected populations, the antibody positivity rate was indeed higher than that of the uninfected (88.2% vs 71.7%, 1.23 times).
However, the antibody positivity rate was higher among vaccinated people (93.1% vs 68.4%), and the probability of antibody positivity was 1.36 times higher than that of unvaccinated people.
Antibody positivity rates also vary in different age groups:
Children under 12 years of age had the lowest antibody positivity rate of 56.2 percent; none of them were vaccinated.
Antibody positivity rates were significantly higher in the 12-17 age group, reaching 73.8 percent; vaccination rates were 3.0 percent.
The antibody positivity rate was 73.6% in the 18-50 age group and 17.1% in the vaccination rate.
Adults over the age of 50 had the highest antibody positivity rate at 79.7%. This group also had the highest vaccination rate at 38.4 percent.
Trends in COVID-19 cases: more incidence and less severe disease
The research team also analysed trends in COVID-19 infections in South Africa's Gauteng province, including the number of cases, hospitalizations, recorded deaths and over-deaths from the beginning of the pandemic until 12 January 2022.
From the beginning of the pandemic until 12 January 2022, there were daily cases (yellow), weekly hospitalizations (blue lines), daily deaths recorded (gray lines) and weekly excess deaths (red lines) due to COVID-19 in Gauteng Province, South Africa. (Image source: References[1])
The data shows that during the fourth wave of the epidemic, which is dominated by the Omicron variant, the incidence increased faster than any previous wave, but the decline also seems to be faster. The fourth wave of the outbreak lasted 1 month from emergence to peak, compared with 2 months for the third wave (dominated by delta).
Specifically, the total number of COVID-19 cases in the fourth wave (226,932) was higher than that in the second wave (182,564) and lower than in the third wave (511,638), while the total number of hospitalizations, recorded deaths and excess deaths was lower than that of the first three waves. Although as of 12 January 2022, it has not fully recovered to pre-fourth wave levels, the fourth wave is nearing its end. At this point, deaths attributed to death or overdose are rarely recorded.
A similar trend has been observed in all regions of Gauteng. Reductions in COVID-19 hospitalization and mortality rates have been observed in all age groups over the age of 17, as well as in men and women; ≤, hospitalization and mortality rates for children and adolescents aged 17 years have been significantly lower than in adults, but the data in the fourth wave of outbreaks were generally flat with earlier outbreaks.
In addition to antibodies, the protective power of cellular immunity
The research team pointed out that the high incidence under the epidemic of Omicron variant confirmed the high transmissibility of the Omicron variant and the immune evasion ability of antibodies.
At the same time, the low severity rate of this wave is likely to be due to cellular immunity from previous natural infections and vaccinations. Although there is evidence that spike protein-based vaccines and neutralizing antibodies brought on by previous infections with other variants inactivate the Omicron variant, two recent preprint studies support that most T cell responses induced by vaccination or natural infection can cross-identify the Omicron variant, potentially helping to prevent serious disease. Through the effector function of Fc-mediated, non-neutralizing antibodies, antibody-mediated phagocytosis, complement deposition, and natural killer cell activation can be induced.
In addition, the research team speculates that the Omicron variant itself may also be less "toxic" than other variants. Studies suggest that the Omicron variant may be more susceptible to infection of the upper respiratory tract than the lower respiratory tract.
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However, the research team is also cautious to say that due to the lack of population-level antibody data from previous waves of the epidemic, it is difficult to fully determine the extent of the impact of Omicron's "toxicity" changes and immune enhancements for the source of protection for severe disease after infection. In addition, although the antibody positivity rate was higher among vaccinators, considering that the antibody positivity rate in the South African population was mainly from previous infections, and in areas where immunization protection was mainly derived from vaccination, it remains to be determined whether the conclusions are universal.