Researchers at Oxford University found significant differences in the levels of immune response among their healthcare workers who contracted COVID-19. They found that previous infection with the virus did not guarantee long-term protection in everyone.

According to The Guardian, the scientists measured markers of immunity up to six months after members of their medical staff contracted COVID-19, and found that some people were far better equipped to thwart infections than others. The experts said that their findings reinforced the advice that everyone should get vaccinated, especially with new, aggressive variants emerging.

“If you look at the trajectory of the immune response after infection, mostly it is still detectable six months later, but it’s highly variable between people,” said Professor Eleanor Barnes, who was one of the senior authors of the Oxford study. “This is quite different to vaccination. If you vaccinate you get a really robust response, but with natural infection there’s much more diversity in responses.”

The researchers analyzed blood samples monthly from 78 healthcare workers who had COVID-19 and measured the levels of antibodies that target the virus, B cells that produce antibodies that remember the enemy, and T cells that kill off infected cells thus reducing the severity of the disease, says The Guardian.

People who mounted a weak immune response at one month had no antibodies against the Alpha or Beta variants of the virus at the six-month mark. The researchers did not test for a response to the currently worrisome Delta variant that is threatening to become the global dominant virus.

Most of the healthcare workers who had symptomatic disease did have antibodies six months later, but 25% of them did not. Those who did not have any COVID-19 symptoms during their infection had no measurable immune response six months later.

“In our view, previous infection does not necessarily protect you long-term from SARS-CoV-2, particularly variants of concern,” said Barnes. “You shouldn’t depend on it to protect you from subsequent disease, you should be vaccinated.”

A recent study by the Cleveland Clinic found that its employees who were infected by COVID-19 remained healthy up to five months later even if they did not get vaccinated. The researchers warned that although natural immunity seems to prevail, we do not know how long it lasts.

“What we don’t know is what’s the duration of protection? And also remember that our population of healthcare workers is younger in general, it’s healthier,” said Dr. Steve Gordon, of the Cleveland Clinic, one of the study authors. “We’re not saying don’t get the vaccine.”

Gordon pointed out that antibody response differs from person to person, so there is no way to accurately gauge someone’s immunity to COVID-19 based on previous infection. And infectious disease experts warn that natural immunity may not protect against the more aggressive variants emerging.

“Even though you’ve had COVID-19, it’s still very important for you to get the vaccine,” says Dr. Kristin Englund, an infectious disease expert,  according to Health Essentials. “We know a small number of people can get COVID-19 a second time.”

Englund adds that the vaccine can boost your immune system so that even if you do encounter the virus again you are much less likely to become seriously ill.

“It’s much better to get yourself vaccinated,’ she says. “Then you don’t have to worry about moving forward until we learn more about whether we need booster shots or not.”

And Professor Danny Altmann, an immunologist at Imperial College London, who was not involved in the Oxford study, said the new study illustrates the diversity of immune response among individuals.

“People show rather diverse trajectories after infection, but immunity seems to hold up at six months, he said, according to The Guardian. “Most of all, studies such as this remind us that policy decisions on ‘boosting’ need to be evidence based in the context of a strong program of immune monitoring.”