of the virus have been detected in at least 33 states across america. researchers say these variants could prolong the pandemic and possibly create another surge. on the vaccine front, the united kingdom launched a new trial to test the safety and efficacy of different vaccines one dose from astrazeneca, followed by one from pfizer. if that's effective, health experts say it could speed up the sluggish vaccine rollout dr. celine gounder now, she's an infectious disease specialist and epidemiologist at nyu. what do you make of mixing the shots, is that a realistic vaccine distribution of these issues >> we're essentially seeing science play out in action the scientific method is about asking a question, saying will this work, and testing it, and so instead of mixing and matching vaccines without science, without data, they are doing the right thing here of actually studying this and seeing if that will work. >> dr. gounder, there is a dip in cases and hospitalizations as we reported. why do you think that is >> well, over thanksgiving, over christmas, over new year's, people got together with family and friends. they did so, at least some of them, without masks, without social