The school reopening debate is presenting an early test of President Joe Biden’s allegiance with powerful teachers unions. As teachers unions in some areas demand vaccines before returning to in-person instruction, the CDC’s director on Wednesday said teachers do not need vaccines to reopen safely. Some believed the CDC’s position would drive a wedge between Biden and teachers unions, but unions say that’s not the case. Even those taking a hard line on vaccines say shots would not be required if schools were taking other steps to make buildings safe. But they say schools have failed to update ventilation systems and take other important steps, making vaccines critical.