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JOHANNESBURG (AP) -- The omicron variant is offering more hints about what it may have in store as it spreads around the globe: A highly transmissible virus that may cause less severe disease, and one that can be slowed -- but not stopped -- by today's vaccines.
An analysis Tuesday of data from South Africa, where the new variant is driving a surge in infections, suggests the Pfizer vaccine offers less defense against infection from omicron and reduced, but still good, protection from hospitalization.
The findings are preliminary and have not been peer-reviewed -- the gold standard in scientific research -- but they line up with other early data about omicron's behavior, including that it seems to be more easily spread from person to person.
Still, some experts cautioned that it's too soon to draw conclusions about the outcomes from omicron since the variant is still quite new and hospitalizations can lag weeks behind infections.
U.S. health officials estimate that a small, but growing proportion of new COVID-19 infections are due to the omicron variant, and that the rise is particularly dramatic in some places.
Two weeks ago, the omicron variant accounted for less than 0.5% of the coronaviruses that were genetically sequenced in the U.S. That rose to about 3% last week, the Centers for Disease Control and Prevention reported Tuesday.
But it varies from place to place, and is as high as 13% in the New York/New Jersey area, according to the agency.
The CDC is tracking how fast the percentage doubles, and as more cases come in, may be better able to predict whether -- or when -- the omicron variant becomes the dominant version of the coronavirus in the U.S, health officials say.
In the new South Africa findings, people who received two doses of the Pfizer-BioNTech vaccine appeared to have just 33% protection against infection, compared to those who were unvaccinated, during the country's current omicron-fueled surge, but 70% protection against hospitalization. The analysis was conducted by Discovery Health, South Africa's largest private health insurer, and the South African Medical Research Council.
The study did not look at booster shots, which are not yet prevalent in South Africa but which data from elsewhere has indicated improves protection.
The Pfizer vaccine's 70% protection against hospital admission during the omicron surge compares to a 93% protection level seen in South Africa's delta-driven wave, according to the new analysis.
That's a big drop in vaccine protection from severe illness requiring hospitalization, said Dr. Eric Topol, head of the Scripps Research Translational Institute.
"What we don't know yet is whether the booster will restore that back to greater than 90% and for how long," Topol said.
The analysis in South Africa was based on examining more than 211,000 COVID-19 test results that date from Sept. 1 to Dec. 7 -- 41% of which were for adults who had received two doses of the Pfizer vaccine, which is the most commonly used one in South Africa.