CHICAGO, Dec 9 (Reuters) - Laboratory studies released this week suggest that the Omicron variant of the coronavirus will blunt the power of Pfizer (PFE.N) and BioNTech"s vaccine to prevent COVID-19 infection after two doses, although a third dose may restore that protection. Data is still needed on how protective COVID-19 vaccines remain in real-world use against the highly mutated Omicron variant. The following is what we still need to know:DOES OMICRON CAUSE MORE SEVERE, OR MILDER, DISEASE? Although some preliminary data suggest that this new version of the coronavirus causes milder disease than prior variants, the jury is still out, said John Moore, a professor of microbiology and immunology at Weill Cornell Medical College.In order to help assess the variant"s severity, scientists will track how many vaccinated people still get infected with Omicron, and whether they require hospitalization or intensive care. Real-world data are needed on people who are unvaccinated, those who have gotten two doses of vaccine and those who have gotten a booster. Such evidence may be needed from multiple countries because the experience with the variant can vary in different regions, said Shane Crotty, a virologist at the La Jolla Institute for Immunology in San Diego. Scientists expect answers to those questions over the next few weeks. WILL OMICRON REPLACE DELTA? Deadly and easily transmitted Delta remains the predominant SARS-CoV-2 variant, representing 99.8% of global infections as of Dec. 7, according to the World Health Organization. For Omicron to be a significant problem, it will need to change that balance to become dominant. "If we suddenly start to see 10% of new infections being Omicron, and then it goes up the next week to 20%, that would tell us that we"re in a replacement wave such as we saw when Delta replaced Alpha," Moore said. Otherwise, it could behave be more like the Beta variant, which demonstrated an ability to reduce vaccine effectiveness, but never became a global threat. "It"s going to be interesting to see ... how those two variants fare and how well they are able to compete for victims" in a highly vaccinated country, said Dr. Amesh Adalja, an infectious disease expert at the Johns Hopkins Center for Health Security. WHAT ABOUT THE OTHER VACCINES? Lab studies of other COVID-19 vaccines from Moderna Inc (MRNA.O), AstraZeneca Plc (AZN.L) and Johnson & Johnson (JNJ.N) are also underway. Those studies analyze the impact of the Omicron variant on blood samples from previously infected and vaccinated people. Many scientists expect these shots also will show a reduction in their ability to neutralize Omicron compared with earlier coronavirus variants. Moderna"s shot uses similar technology as Pfizer/BioNTech"s, but has been shown to offer more durable protection against infection with previous variants, a benefit believed to be due to its higher dose and longer interval between shots. Moderna may see less of a drop in neutralizing antibodies compared with Pfizer, Adalja said. China"s Sinovac Biotech (SVA.O) has said it is conducting studies to determine whether its inactivated virus vaccine works against Omicron or whether the company needs to develop new ones. It could be another three weeks to know how well Russia"s Sputnik V holds up against Omicron, Kirill Dmitriev, head of the Russian Direct Investment Fund, told CNBC. "I suspect you"ll see a diminution (in efficacy) in all of them," Adalja said.