Pfizer’s vaccine for the coronavirus disease 2019 (Covid-19) provides surprisingly robust protection against the highly infectious Delta variant –  but only if you’ve gotten both doses.

Fresh data out of England shows that two doses of the messenger RNA, or mRNA, vaccine is 88-percent effective in protecting people against the Delta variant, compared with about 94-percent effectiveness against the original strain of Covid-19.

However, effectiveness among folks who only got one dose was drastically lower, around 31 percent, researchers reported this week in the New England Journal of Medicine.

“The data is mounting that the mRNA vaccines are the solution to the Delta variant,” said Dr. Amesh Adalja, a senior scholar with the Johns Hopkins Center for Health Security in Baltimore.

“It is also apparent that the second dose of these vaccines is needed to augment the immunity from the first dose, for a person to be resilient to the variant,” Adalja said.

The Delta variant has developed seven different mutations in the coronavirus’ “spike” protein, compared to the original Alpha strain, said Richard Kennedy, co-director of the Mayo Clinic Vaccine Research Group.

“Some of these mutations help the virus infect cells, produce more virus in infected cells, or spread from person to person more easily,” Kennedy said. “Several of these mutations occur at specific regions of the spike protein where antibodies bind and prevent the virus from entering the cell.”

This new study, which was led by Jamie Lopez Bernal from Public Health England, evaluated all Covid-19 cases in the United Kingdom through May, with researchers checking to see if the infected folks have been previously vaccinated.

“Vaccine effectiveness against hospitalization for the Delta variant is uniformly high for the Pfizer vaccine. It’s all good news,” said Dr. Kathleen Neuzil, director of the Center for Vaccine Development and Global Health at the University of Maryland School of Medicine.

“These mRNA vaccines are really exceeding all of our expectations. It’s terrific news that the effectiveness is this high despite these variants. It absolutely supports that our emphasis needs to be on getting the unvaccinated vaccinated,” Neuzil continued.

These results are even more impressive because they’re occurring in real-world situations, rather than in carefully controlled clinical trials, she added.