Pfizer announced this week that their COVID-19 vaccine exceeds expectations for effectiveness. So when can Americans expect to see doses in their doctor’s office?

Medical ethicist Arthur Caplan told Boston Public Radio some people could receive the vaccine “as early as the end of December.”

The vaccine itself needs to pass another milestone before it’s authorized for emergency use: current test subjects need to be monitored for safety issues for a few more weeks.

Then, the distribution may be limited by logistics within the supply chain due to refrigeration requirements. Higher risk populations like health care workers and older people will likely be prioritized, but Caplan said the broader population could see the vaccine by spring 2021.

“It’s a two-dose vaccine … even if you start administering it in April, you’re probably not going to start seeing benefits until May and June, because it takes that long to work.”

Caplan is the Drs. William F and Virginia Connolly Mitty Chair, and director of the Division of Medical Ethics at the NYU Grossman School of Medicine.