JOHANNESBURG — An early study of coronavirus cases in South Africa suggests that, so far, Omicron seems to cause lower rates of hospitalization than previous versions of the virus.
The study — which was released on Tuesday and is based on only three weeks of data — points to less severe illness from the new variant, but also shows waning vaccine efficacy and a higher risk of breakthrough infections for Omicron. Epidemiologists have cautioned that data from a few more weeks would be needed to draw firmer conclusions, in part because the variant has not yet spread widely and because only a small percentage of infected people become ill enough to be hospitalized.
The study, by a private health insurance company, offers a preliminary look at the effect of the Omicron variant, but there are other possible explanations for the trends that were observed. For example, infections may appear to be milder overall because more people in this wave have some protection from prior infection or immunization.
Also, the mean age of the people in the study was 34, and young people generally tend to have mild symptoms. That may also make Omicron appear milder than it is.
The findings echo existing data that show that while new coronavirus cases have increased exponentially, the trajectory of hospital admissions has been much flatter.
A snapshot of the first three weeks of each of the four waves of infection shows that hospital admissions are significantly lower during the Omicron-driven fourth wave — 38 admissions per 1,000 compared with 101 during the Delta-driven wave, and 131 per 1,000 when the Beta variant was dominant, the study showed.
Anecdotally, those who were admitted to hospitals had much milder illness and shorter hospital stays. A majority of patients who needed oxygen were unvaccinated, and only 16 percent of I.C.U. admissions were vaccinated.
The efficacy of the Pfizer-BioNTech vaccine seemed to wane, decreasing to just 33 percent from 80 percent during the Omicron wave. Few boosters are available in South Africa right now.
“Omicron has materially reduced vaccine effectiveness against new infections, potentially compounded by waning durability,” the study showed.
Still, the Pfizer-BioNTech vaccine provided 70 percent of protection against severe illness and hospital admission.
The study was conducted by Discovery Health, which tracked data from 211,000 positive cases from across the country and drew on anecdotal evidence from private hospital and primary health care groups in South Africa. That means that the findings draw only on a slice of society that can afford private health care, and a vast majority of South Africans rely on state-run hospitals and public clinics.
Although the study found low Covid-19 hospital admissions for children in absolute terms, it said children seemed to be at greater risk of hospitalization during the Omicron-driven wave. But some of the cases in this wave were seen in children admitted for other reasons, possibly making it seem as if the risk is higher than it is.
The South African experience is limited because of existing immunity and vaccinations. With more than 40 percent of adults vaccinated, the study offers little understanding of how the Omicron variant would behave in countries with low vaccination rates, said Glenda Gray, who led the trial.