One dose of the Pfizer or AstraZeneca coronavirus vaccine "barely" protects against the Delta variant of the virus, because of mutations the variant has developed, according to a new study published in the journal Nature Thursday.

Why it matters: The study found that two doses of those vaccines generated a neutralizing response to the variant in 95% of people, highlighting the importance of full vaccination against COVID-19.

• The findings may also offer an explanation for why the Delta variant is rapidly spreading in multiple countries around the world — including the U.S., where it is now the most dominant version of the virus.

How it works: The team of French researchers tested how efficiently antibodies produced through natural infections or vaccines neutralized variants of the coronavirus, including Delta, which was first identified in India.

• They discovered that the Delta variant has developed mutations to cell entry mechanisms that allow it to evade certain antibodies.
• While one dose of Pfizer's or AstraZeneca's vaccines was less effective at neutralizing the variant compared to two, it still performed better than the antibodies produced through natural infection, suggesting that people who contracted the virus may still need vaccine to help defend against variants.

Flashback:A study in The Lancet published in June found that the Delta variant is primarily a threat to people who have not been vaccinated.

• Unvaccinated people were twice as likely to be hospitalized if they were infected by the Delta variant compared to infections from the Alpha variant, which was first detected in the United Kingdom.

What they're saying: "In parts of the Midwest and upper mountain states, [the Centers for Disease Control and Prevention's] early sequence data suggests the Delta variant accounts for approximately 80% of cases," said CDC Director Rochelle Walensky during a briefing on Thursday.

• "To be clear, there will likely continue to be an increase in cases among unvaccinated Americans, and in communities with low vaccination rates, particularly given the spread of the more transmissible Delta variant," Jeffrey Zients, the White House coronavirus response coordinator, said.

Go deeper:Delta variant threatens masking guidance