The highly infectious Delta variant of SARS-CoV-2 has spread to more than 140 countries and accounts for 98% of U.S. Covid-19 cases. While U.S.-authorized vaccines provide strong protection against severe illness, they aren’t 100% effective and breakthrough infections, though rare and generally mild, do occur.

With this in mind, scientists are working to develop shots that would target the Delta variant specifically. The mRNA platform behind some vaccines might make this process relatively straightforward, according to Matthew...

The highly infectious Delta variant of SARS-CoV-2 has spread to more than 140 countries and accounts for 98% of U.S. Covid-19 cases. While U.S.-authorized vaccines provide strong protection against severe illness, they aren’t 100% effective and breakthrough infections, though rare and generally mild, do occur.

With this in mind, scientists are working to develop shots that would target the Delta variant specifically. The mRNA platform behind some vaccines might make this process relatively straightforward, according to Matthew Johnson, senior director of product development at the Duke Human Vaccine Institute.

Vaccines work by exposing the body to a harmless version or portion of a virus. This teaches our immune systems to recognize and fight the real virus if our bodies become infected with it. The mRNA-based vaccines developed for Covid-19—including the two made by Moderna Inc. and by Pfizer Inc. and partner BioNTech SE —rely on the outer spike proteins of the new coronavirus to prime our immune defenses. Spike proteins stud the outer surface of the virus and help it latch onto and infect healthy cells.

Current vaccines are based on versions of the virus that were circulating early in the pandemic. But the virus has undergone genetic changes, resulting in newer variants including Delta. Some of these mutations have altered the structure of Delta’s spike protein, making it better at infecting cells—one reason Delta is more infectious than other variants.

This model generated by Robert F. Garry, a virologist at Tulane University, shows parts of the protein that are affected by Delta’s mutations.

Delta’s mutations potentially make it harder for antibodies generated by the original vaccines to recognize it, because these antibodies were designed to recognize features on earlier versions of the virus.

However, scientists say that the existing mRNA vaccines can be tweaked to take such mutations into account.

Write to Alberto Cervantes at Alberto.Cervantes@wsj.com