However, adults ages 65 and older who had been previously infected had only about 47.1 percent protection against a repeat infection, compared to a protection rate of 80.5 percent among younger people, the study showed.

The difference can likely be explained by natural changes that weaken your immune system as you age, the study’s authors said.

“We know that as we get older, the robustness of our immune systems wanes,” explains C. Buddy Creech, M.D., an infectious disease specialist and director of the Vanderbilt Vaccine Research Program in Nashville, Tennessee. “It’s the reason why we give older adults shingles boosters and high-dose influenza vaccine, or influenza vaccine that has a special immune stimulant in it called an adjuvant.”

Natural immunity is not enough

The study underscores the importance of face masks, social distancing and getting the COVID-19 vaccine, even for those who have already had the coronavirus, especially if they’re older. “Natural protection, especially among older people, cannot be relied on,” the study’s authors wrote.

The coronavirus vaccines that have been authorized in the U.S. offer significantly better protection than natural immunity, Creech says. “If you take 100 individuals who have all had COVID, their immune response might be all over the map,” he says. “It often correlates with a variety of factors, including severity of their initial disease. But when you look at immune response [to the vaccines], they are far stronger and more consistent.”

In a piece of good news, the study found “no evidence” that a person’s immunity declines within a six-month period of testing positive for the virus.