Panelists said that even as more people get vaccinated, continued vigilance will be important. How long vaccine-derived immunity will last remains a question mark but should become apparent in the months ahead. Another question is whether the vaccine reduces transmission as well as preventing contraction of serious illness. Marc Lipsitch, director of the Harvard Chan School’s Center for Communicable Disease Dynamics, said some data from the Moderna vaccine trial showed that levels of virus in the nose were lower in those arriving to get their second dose, which likely translates to lower transmissibility. Those results are preliminary and need to be confirmed, but Lipsitch said that if the vaccines are shown to reduce transmission but not eliminate it, the need to use masks and distancing measures may remain for some time.

“If you say to someone ‘you’re 95 percent less likely to get sick [when you get vaccinated], that’s great for the individual, but if you say ‘you’re 50 or 60 percent less likely to infect your grandmother’ — if that’s the number — that’s not that reassuring,” Lipsitch said. “You probably still want to take precautions.”