but potentially serious side effects linked to johnson & johnson s vaccine, a cdc panel recommended adults not get a j&j vaccine if pfizer or moderna is available. the j&j vaccine has a rare side effect, but real blood clotting in the brain which can cause fatalities and that is not a problem that the mrna vaccines have. reporter: as our nation enters a new twin threat from dueling variants, tonight more schools are returning to remote learning. in san diego, students are headed out for winter break with take-home covid tests. we don t want another outbreak here. reporter: at cornell university, where there s been over a thousand positive cases on the nearly fully vaccinated campus, officials now fear the nation s first known omicron cluster. in new york city, broadway is once again shuttering some shows, as the city s positivity rate has doubled in three days.