The U.S will soon have another 200 million doses of the Pfizer BioNTech COVID-19 vaccine on hand, bringing the total number of doses to 500 million, of which roughly 208 million have already been delivered.

On the heels of this news, a new study out of Israel says the vaccine is only 39% effective at protecting against infection. The same study found the vaccine to be 91% effective in preventing serious illness.

A new study from the New England Journal of Medicine in the United Kingdom has similar findings -- it found the vaccine to be 88% effective in preventing serious illness.

"By vaccinating people, we can prevent severe illness, prevent people requiring hospitalization and also reducing transmission to others. That's really the main value of these vaccines," says Dr. David Hirschwerk, an infectious disease specialist at Northwell Health.

Now the country is seeing breakthrough cases, from a White House official to Olympic athletes to the New York Yankees. People who are vaccinated are still testing positive for COVID-19.

"There are going to be breakthrough infections. The vast majority of infections tend to be very mild. There will also be people who get infected but have zero symptoms, and they just test positive because they might've had to have been tested for some other reason and they're found to have it," Hirschwerk said.

Pfizer representatives did not comment on either of the studies but did maintain their vaccine is highly effective.

They also said once they make the decision to do so, they can develop a new vaccine specifically made to prevent against the delta variant, within 100 days.

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