In an interview during the Thursday NBC Dateline special “Race for a Vaccine”, Pfizer chairman Albert Bourla told journalist Lester Holt that he was “not certain” whether the pharmaceutical company’s COVID-19 vaccine will stop vaccinated people from being able to spread the lethal virus to others.

“This is something that needs to be examined,” Bourla said. ” We are not certain about that right now with what we know.”

This is an important question, as frontline health care workers, nursing home residents and employees will be prioritized to receive the first vaccinations. If they remain contagious, it could endanger other at-risk populations.

Pfizer claims its vaccine is 90 percent effective, but it must be delivered and stored in extreme sub-zero temperatures in order to work: the optimal temperature is minus 70 degrees Celsius, colder than winter temperatures in Antarctica. Once stored at a normal refrigeration temperature, the vaccine must be used within four or five days or else its active mRNA will break apart, making the inoculations useless.

The extreme temperatures required for its storage have raised serious questions about its costs for transport, storage and availability to millions of Americans who’d need to be inoculated in order to help the pandemic end.

A mid-May survey by the Associated Press and Norc Center for Public Affairs Research at the University of Chicago found that 20 percent of American adults say they won’t get a COVID-19 vaccine with an additional 31 percent of people saying they’re not sure if they’ll get vaccinated or not.

Epidemiologists say 70 percent of the population will need immunization in order to end the pandemic, according to Science Magazine.

People who fear receiving vaccines often worry about negative health effects, view medical authorities suspiciously or want more information about them before receiving them. Pro-vaccination experts say that the medical community must use emotional messages and first-person testimonies that appeal to people’s empathy about protecting loved ones rather than fear about killing them.