Four major United States airports are now on high alert for COVID-19 cases caused by the troubling omicron variant.Dr. Helen Boucher, an infectious disease specialist at Tufts Medical Center, said there is a certain type of PCR test that can have a type of proxy for the presence of the omicron COVID-19 variant, which is called the S gene dropout.”That’s not definitive proof, but it’s a sign. We really need sequencing to know that this particular mutation is present,” said Boucher, who is also the interim dean of the Tufts University School of Medicine.There are no reports of the omicron variant in the U.S., but 20 countries, including Canada, are reporting cases of the coronavirus variant.”As many have said, it’s likely that this variant is already present here in the U.S.,” Boucher said. “We found out (Tuesday) that it was in Europe before South Africa — not surprising. It’s all around the globe, so I think it’s just a matter of time before we identify it here.” The four airports that the U.S. Centers for Disease Control and Prevention is now targeting for expanded omicron variant testing are John F. Kennedy International Airport in New York, Newark-Liberty International Airport in New Jersey, San Francisco International Airport and Hartsfield-Jackson Atlanta International Airport.”These are major points of entry into our country, so it’s likely that people who’ve traveled to other countries where we already know this variant is present will come through those places,” Boucher said. “So I think the odds of finding this variant are higher at those locations.”Boucher said she is concerned that COVID-19 case numbers are rising as the U.S. enters winter, and that there are still people who are unvaccinated and “too many people” who are vaccinated but have yet to receive a booster shot.