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Seropositivity was widespread in South Africa prior to the Omicron wave, and higher infection rates when Omicron arrived did not lead to a corresponding rise in hospitalizations and deaths, researchers found.

There was 73.1% seropositivity in Gauteng province from Oct. 22 to Dec. 9, 2021 (95% CI 72.0-74.1), including 68.4% seroprevalence among unvaccinated individuals and 93.1% among those who received a COVID vaccination (RR 1.36, 95% CI 1.33-1.39), reported Shabir Madhi, PhD, of University of the Witwatersrand in Gauteng, South Africa, and colleagues.

Moreover, epidemiologic data indicated that the Omicron wave contributed to a minority of hospitalizations, recorded deaths, and excess deaths due to COVID since the start of the pandemic (11.2%, 3.9%, and 3.3%, respectively), the authors wrote in the .

"We think that the evolution of cell-mediated immunity from previous natural infection and vaccination has resulted in the decoupling of the high case incidence seen with the Omicron variant from the incidence of severe disease (hospitalizations and deaths)," Madhi's team wrote.

The findings came from a follow-up seroepidemiologic survey that captured epidemiologic trends from the start of the pandemic through Jan. 12, 2022. Households who participated in a previous survey from November 2020 to January 2021 were contacted, with a 10% increase in sampled households to account for "possible nonparticipation, out-migration, and death since the previous survey," the authors noted. Dried-blood-spot samples were collected and analyzed.

Overall, Madhi's group looked at samples from 7,010 participants across 3,047 households. About 58% of participants were female and 58% were ages 18 to 50. The large majority of participants (81.2%) were unvaccinated, and 85% had never been tested for COVID. Over two-thirds had no preexisting conditions.

They found that female participants were more likely to be seropositive than male participants (76.9% vs 67.9%, respectively; RR 1.13, 95% CI 1.10-1.17). Not surprisingly, seroprevalence was highest among adults ages 50 and up (79.7%) and lowest among children ages 12 and under (56.2%).

Among the total vaccine-eligible population of Gauteng as of Nov. 26, 2021 (ages 12 and up), Madhi's team noted that 36% had received one dose of either Johnson & Johnson or Pfizer's vaccine, while 20% had received two doses. Vaccination rates were higher in those over 50 years of age, where 44.2% had received two doses of Pfizer's vaccine.

Madhi's group also examined epidemiologic data from the South African National Institute for Communicable Diseases daily databases. Hospitalizations included both admission for COVID infection as well as admission for other illnesses where COVID was identified during routine screening.

As noted in prior research, cases during the Omicron wave increased more rapidly, but also declined more quickly, with a time to onset of 1 month to the peak of the wave (versus 2 months for the prior Delta wave).

Interestingly, while the Omicron wave (fourth wave) had more cases than Beta, the second wave (226,932 vs 182,564, respectively), it fell below the 511,638 cases from the third wave, Delta. And hospitalizations, recorded deaths, and excess deaths in the Delta wave comprised a much higher percentage of the pandemic overall, at 43.6%, 49.3%, and 52.7%, respectively.

Limitations to the data include that publicly available data may have changed over time, which could affect the comparisons of one wave to another. The frequency of testing over time may also have changed and the Omicron wave had not entirely subsided at the time this analysis was conducted, which could affect the accuracy of the numbers.

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