What Happened: More than 5 million people in Israel received at least one of the two Pfizer/BioNTech vaccine doses, representing more than half the population.

The data on the ground that Pfizer gathered from the vaccination effort shows the vaccine is 97% effective in preventing mild disease, hospitalization and death, Bourla said. 

The vaccine is 94% effective in preventing asymptomatic cases, Pfizer announced Wednesday. The new study coincides with the one-year anniversary of the COVID-19 being declared a pandemic.

"This is the first time we are coming with confirmation of a real-world evidence study of that magnitude, and of course we never expected that high number," the CEO said.

Related Link: CDC Issues Guidelines For Americans Vaccinated Against COVID-19: What You Need To Know

Why It's Important: The data collected by Pfizer coincided with the rapid spread of the B.1.1.7 variant across Israel, so the standard vaccine offers "promising" protection against the variants, Bourla told CNBC. 

What's Next: Pfizer continues to explore if it can improve the vaccine against unknown variants, the CEO said.

The company is exploring the benefits of a third vaccine shot that will be administered six months after the first, he said. 

It is possible that a third shot may not be needed, as data from Israel shows the vaccine in its current form offers "very good protection already," he said.

PFE Price Action: Pfizer shares were down 0.21% at $34.86 at last check Thursday.