People who were previously infected with the coronavirus have some level of protection against reinfection, but that doesn't mean they shouldn't get vaccinated.

Why it matters: If you're trying to calculate herd immunity, previous infections count for something. But protection from both vaccines and infections likely decreases over time, and getting at least one shot of a vaccine increases people with prior infections' level of protection against variants.

Driving the news: Evidence is growing that prior COVID infections protect against future ones.

• A study published in Nature earlier this week found that people infected with the coronavirus retain relevant immune responses a year later, regardless of whether they were vaccinated. However, those who were vaccinated had stronger responses.

Yes, but: We don't know how long immunity lasts.

• Just as vaccinated people may eventually need a vaccine booster to strengthen their immune response, people with previous infections can benefit from getting at least one shot.
• “One dose of the mRNA vaccines boosts your antibody response right up," said John Moore, a professor and virologist at Cornell. "So natural infections, those give you a degree of protection, but it’s boosted — in a highly relevant way — by vaccination.”
• “With antibody-resistant variants on the spread, getting the vaccine is an important thing to do," he added.

Between the lines: Americans started getting infected with the coronavirus nearly a year before the vaccination campaign began, meaning that people who were infected a while ago may have a decreased level of protection than those who were infected or vaccinated recently.

• There's also wide variation among individuals in the level of protection generated by vaccines and prior infections.

What they're saying: "Some 80% to 85% of American adults are immune to the virus: More than 64% have received at least one vaccine dose and, of those who haven’t, roughly half have natural immunity from prior infection," Marty Makary, a professor at the Johns Hopkins School of Medicine, Bloomberg School of Public Health, wrote earlier this month in an op-ed in the Wall Street Journal. "There’s ample scientific evidence that natural immunity is effective and durable."

The bottom line: Calculating the level of "herd immunity" in the U.S. is complex.

• "You can make an argument that you can add these numbers up, but it’s not a direct 1 + 1 =2. It’s more complicated than that, and I don’t know how to quantify it," Moore said.