The probability of hospitalization in omicron cases decreased by 80% but the risk of severe illness after hospitalization was similar
Patients infected with omicron virus during the fourth wave of the South African epidemic were significantly less likely to end up in hospital, while there was no significant difference in the likelihood that hospitalized patients would develop severe illness, according to a newly published preprint study from the National Institute of Infectious Diseases.
(Source: South African National Institute of Infectious Diseases)
In South Africa, a total of 161,328 COVID-19 patients were reported between October 1 and December 6, and PCR testing (Taqpath) was performed in 38,000 cases, of which 31,000 lost at least one S-gene target, the study noted. Of those 31,000, 29,700 were eventually identified as S gene loss (SGTF) infections. All SGTF samples with genomic data as of 14 December were confirmed as omicron virus infection.
This means that between October and December this year, the proportion of Omicron infections in South Africa jumped from 3% to 98%.
(Source: South African National Institute of Infectious Diseases)
After adjusting for several factors, only 2.5 percent of sgTF-infected people were hospitalized in October and November of this year, compared with 12.8 percent of non-SGTF-infected people, the researchers said. In addition to regional factors, younger (less than 5 years old) and older (more than 60 years old) patients were more likely to be admitted to hospital than younger and female patients. In addition, the severity of the Omicron outbreak dropped by 70 percent compared with the April Delta outbreak. Researchers say this may be partly due to higher immunization rates among local populations. But the study also noted that omicron patients may have a higher viral load in general.
Paul Hunter, a professor of medicine at the University of East Anglia, told the press that although the so-called historical control studies using April and November data meant the results could be skewed over time, they were still important. So even if omicron’s cases are less likely to be hospitalized than Delta’s, it’s hard to say whether it’s the difference in virulence itself or whether herd immunity is stronger this November than it was earlier in the year.
Cheryl Cohen, a professor at South Africa’s National Institute of Infectious Diseases, also told a media briefing that it was unclear whether a similar situation would occur in countries with high vaccination rates but low cumulative infection rates. Cohen points out that an estimated 60-70% of south Africans have already been infected with COVID-19 earlier.