“Theoretically, a vaccine should stop both the infection as well as the transmission and spread,” said Dr. Purvi Parikh, an immunologist with the nonprofit Allergy & Asthma Network and a co-investigator on the Pfizer vaccine trials.

But we don’t know yet if that is true of the COVID-19 vaccines, she told me. That’s because the focus of the clinical trials was narrow. It had to be because of the time constraints. Scientists wanted to know whether these things prevented illness. They wanted to know whether the drugs were safe. And they got those answers.

But getting those questions answered fast came at the expense of answering other questions — like whether vaccinated people can still spread the virus. “With a lot of other vaccines, you have years of data to analyze that,” Parikh said.

So, experts are being careful — balancing their excitement and relief with caution that you can’t just switch off 2020 Mode and return to a normal state of being. They need a little more time to know for sure.