JOHANNESBURG—It is still too early to tell whether Covid-19 caused by the new Omicron variant is milder or more severe than that from other strains of the coronavirus, doctors tracking a rapidly growing outbreak in South Africa said Friday.

The country, which has more known Omicron cases than any other, is likely to be the first to deliver answers to the questions doctors and scientists have been grappling with since the World Health Organization declared the new strain a “variant of concern” a week ago. Key among those are...

JOHANNESBURG—It is still too early to tell whether Covid-19 caused by the new Omicron variant is milder or more severe than that from other strains of the coronavirus, doctors tracking a rapidly growing outbreak in South Africa said Friday.

The country, which has more known Omicron cases than any other, is likely to be the first to deliver answers to the questions doctors and scientists have been grappling with since the World Health Organization declared the new strain a “variant of concern” a week ago. Key among those are whether Omicron makes those infected sicker, whether it is more transmissible and in how far current Covid-19 vaccines or a past infection protect against the variant.

Michelle Groome, who heads the public-health division at South Africa’s National Institute for Communicable Diseases, said the latest data is starting to offer some preliminary answers to the latter two questions.

“There is definitely evidence that [Omicron] is more transmissible and that there is some immune escape,” she said in a new conference organized by the South African health ministry.

Scientists and vaccine makers are investigating Omicron, a Covid-19 variant with around 50 mutations, that has been detected in many countries after spreading in southern Africa. Here’s what we know as the U.S. and others implement travel restrictions. Photo: Fazry Ismail/EPA-EFE/Shutterstock The Wall Street Journal Interactive Edition

Dr. Groome said those early conclusions were based on the rapid increase in cases in South Africa over the past weeks and research showing that among those infected there was a disproportionately large number of people who had already had Covid-19.

The number of newly recorded infections jumped to 11,535 on Thursday, from 2,465 a week earlier, with most of them likely coming from the Omicron variant, according to data from the NICD. While South Africa’s case load is still much lower than the number of per-capita cases reported in much of Europe and the U.S., where infections are driven by the Delta variant, there are signs that many infections in South Africa aren’t being recorded.

Nationwide, nearly one in four tests came back positive on Thursday, while in the hot spot Gauteng province—home to the economic and political capitals of Johannesburg and Pretoria, respectively—one in three tests showed a Covid-19 infection. The current outbreak in South Africa also started at a time when infections were at the lowest level since the start of the pandemic, so even with the current exponential rate of increase, it takes time for overall case numbers to build up.

“The curve [of new infections] is much steeper than any rise or curve we’ve seen in the previous three waves,” said Health Minister Joe Phaahla.

Research published Thursday also indicated that the risk of reinfection for a person who has already recovered from Covid-19 has been around three times as high during the early days of South Africa’s Omicron wave than in the previous Delta-driven one, said Cari van Schalkwyk, a researcher at South Africa’s Centre of Excellence in Epidemiological Modelling and Analysis and one of the authors of the paper.

The research is based on the first few weeks of known Omicron infections and hasn’t been published in a peer-reviewed journal. It didn’t include information on whether those with a previous infection were experiencing milder symptoms.

Covid-19 hospitalizations have also increased sharply, although for now the local health system is still managing, doctors said. In Gauteng province, the average number of people admitted with Covid-19 a day was about four times as high in the two weeks ended Nov. 27 than the previous two weeks, said Wassila Jassat, a public-health specialist at the NICD.

It will take about two to three weeks to find out whether these patients are going to be less severely ill than those hospitalized in previous waves, said Dr. Jassat. Part of that is because many Covid-19 patients only become severely ill in the second week of symptoms and because doctors often admit less severely ill patients early in a wave of infections, when there is still space in hospitals.

“Even if we are seeing a slightly less severe disease right now, it’s too early to say whether that’s a characteristic of this variant, because it’s also a nature of the phase of the wave,” said Dr. Jassat.

Most vaccine and immunology experts say they still expect Covid-19 vaccines to protect against serious illness and death from Omicron—a prediction that lines up with early anecdotal evidence from the new variant. Health authorities outside of South Africa, where Omicron has been detected primarily in travelers who are more likely to be vaccinated, have said that most of the early cases either had mild or no symptoms.

In South Africa, where around a quarter of the population is fully vaccinated, gaps in the data make it difficult to assess the impact of vaccines on hospitalization for now. Among the currently hospitalized patients, the vaccination status is unknown for 75%, while 23% were unvaccinated and 2% vaccinated, said Ntsakisi Maluleke, a public-health specialist at the Gauteng Department of Health.

Write to Gabriele Steinhauser at gabriele.steinhauser@wsj.com