People who had previously been infected with COVID-19 were better protected against the Delta variant than those who were vaccinated alone, suggesting that natural immunity was a more potent shield than vaccines against that variant, California and New York health officials reported on Wednesday.

Protection against Delta was highest, however, among people who were both vaccinated and had survived a previous COVID infection, and lowest among those who had never been infected or vaccinated, the study found.

Nevertheless, vaccination remains the safest strategy against COVID-19, according to the report published in U.S. Centers for Disease Control and Prevention (CDC) Morbidity and Mortality Weekly Report.