BOSTON (CBS) – Coronavirus cases are rising in many parts of the country, with the Delta variant now the most dominant strain in the United States. Dr. Mallika Marshall is here to answer questions about the COVID-19 Delta variant.

Q: Doctor, just a week ago the Delta variant accounted for 1 in 4 infections in the U.S. Now it’s more than half. Why is it spreading so fast?

READ MORE: Norton Man On Scooter Dies After Collision With Another Car

A: The Delta variant is estimated to be about 50-percent more contagious and possibly more severe than the Alpha strain of the coronavirus. So for example, a Yale epidemiologist said in an environment where no one is vaccinated or wearing masks, someone infected with the Alpha variant might infect 2-3 people around them whereas, with the Delta variant, it might be 3-4 people. So the virus can spread like wildfire in communities with low vaccination rates. And those same areas could become breeding grounds for other variants to emerge.

Q: There is reporting out of Israel that the Pfizer vaccine may not be as effective against the Delta variant as originally thought? What can you tell us?

READ MORE: Man Allegedly Pulls Knife On Fellow Red Line Rider After Mocking COVID Mask

Researchers in Israel are reporting that the Pfizer vaccine is only about 64% effective against the Delta variant. It’s not clear why that number is considerably lower than several other studies that have found the vaccine is anywhere from 79% to 88% effective at preventing infection with the Delta variant. One possibility is that Israel does more widespread testing and therefore may be picking up more asymptomatic cases. But most studies, including the one out of Israel, have found the Pfizer vaccine and others remain highly effective against severe disease and hospitalization, which is what really matters most.

Q: What advice would you give to people who are understandably concerned about the Delta variant?

MORE NEWS: COVID Vaccine Passport Program Could Be Coming To Massachusetts Soon, Baker Says

I would say if you’re vaccinated and in an area with high vaccination rates, don’t worry. You should still be well protected. However, if you’re vaccinated but traveling to a region of the country with rising coronavirus cases, be careful. Consider wearing your mask when in crowds, for example. And if you’re unvaccinated, get vaccinated as soon as possible, continue to wear your mask in public, and socially distance. You’re at high risk of getting sick.

Dr. Mallika Marshall