ATLANTA (CNN) -- As routinely as some people might drop in for an allergy shot, Jennifer Clarke visits Atlanta's Northside Hospital once a month for a cancer vaccine.

Clarke doesn't mind getting stuck by a needle. Just six months ago, she was undergoing debilitating chemotherapy treatment for breast cancer that had spread to her liver.

Then she was selected as one of a handful of women -- whose cancer could not be easily controlled by conventional means -- to receive an experimental cancer vaccine.

"The strategy here is to immunize patients with this tumor vaccine against breast cancer, to actually get the person's own immune system to attack their disease as opposed to just giving them medication," explained Dr. Kent Holland.

The vaccine is designed to prime the immune system, so the body might recognize and destroy any new cancer cells before they take hold.

"To date, about 15 women nationwide have been entered in this trial, and what we do know is that a significant number of the patients are responding well to the vaccine," Holland said.

However, the long-term effectiveness of the vaccine is still not known.

X-ray of a cancerous liver

In Clarke's case, her liver has recovered significantly and now appears almost normal. Doctors say the chemotherapy and other conventional treatments she underwent likely cleared up the tumors. The test of the vaccine will be whether it can prevent them from growing back.

Clarke will continue to receive the monthly shots for the next two years as a participant in the vaccine trials. The worst side effect so far, she said, is a sore arm.

"Anything that's going to benefit someone else in the future, I'm really glad to be part of that," Clarke said. "And (benefit) myself in the meantime, hopefully."