A bite from a tick carrying Lyme disease often leaves a telltale bull’s-eye rash. But according to new research from Dustin Brisson, an assistant professor in the Department of Biology in Penn Arts & Sciences, an infectious bite can also lead to immunity that lasts for several years—with one important catch.

The immune system only learns to protect against the specific strain of the Lyme-causing bacteria, Borrelia burgdorferi, that caused the original infection, leaving a person vulnerable to the other 15 or so strains that are known in the United States.

Nevertheless, Brisson says the finding does have a positive implication: If the body can be trained to protect against one strain of the bacteria, it may be possible to design a vaccine that imitates that same protection against many different bacterial strains.

“If you could make a vaccine that covers several of these strains,” he says, “you could substantially reduce the probability of infection in vaccinated people.”

Brisson and colleagues made this insight into the immune response to Lyme disease by examining data from 17 people who had contracted the disease more than once over a period of several years. A previous study had showed that the subsequent infections in all but one of the patients were of different strains of B. burgdorferi from the original infection. And the lone patient who had a repeat infection of the same strain had, in fact, been infected with Lyme four different times, with the same strain causing the disease in the first and third instances, five years apart.

“In the present study,” Brisson says, “we wanted to see if so few patients were infected by the same strain multiple times because they were protected against subsequent infections with the same strain.” 

Brisson’s team used various statistical tests to address this hypothesis, finding that it would be nearly impossible to arrive at the results seen in the 17 patients unless the immune system was defending against re-infection to the original bacterial strain.

“If there was no strain-specific immunity, then there should be a random distribution of strains in patients, and you would expect several of the patients to be affected by the same strain twice,” Brisson says. “But only one patient was.”

Another statistical model showed that this immunity likely lasted between six and nine years. Brisson says this duration mimics some commonly available vaccines, like tetanus. A vaccine against Lyme might have some of the same properties.

“The vaccine could last several years, perhaps requiring a booster once every several years,” he says.