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Last week, a group of researchers from Stanford unveiled a vaccine that cures cancer in mice, and if we can get it to work in humans, it will save a lot of lives. Which sounds amazing, because cancers are terrible.

But it also sounds weird, because when we talk about vaccines, we usually mean something you get before you are sick, and that’s meant to protect against things that you might catch from other people. Cancer vaccines stimulate the immune system in a similar way, except they’re meant to be injected after you get sick and to cure something that’s basically a part of yourself gone rogue. And it’s a cool idea.

But despite what’s being reported in a lot of the articles about this new research, it’s not based on a totally new concept. Instead, it’s an improved version of a technique that’s been tried before — a version that hopefully will not be quite as deadly when we try it in humans. Your immune system is how your body deals with its enemies—whether foreign, like strep throat, or domestic, like a tumor.

So a lot of the same parts that battle bacteria also battle cancers; they’re just … not always very effective. For decades, doctors have been trying to improve the immune system’s cancer-fighting ninja skills, but it’s been much harder to do that than anyone anticipated. The new treatment, published in the journal Science Translational Medicine, is a type of cancer vaccine designed to stimulate T cells, which kickstart the immune system’s response when there’s a threat.

Like with preventative vaccines, the goal here is to program these cells to target a certain chemical signature, so that when it shows up again they can efficiently launch a full-scale assault. With, like, the measles vaccine, they’re programmed to seek out and destroy the measles virus. With this new therapy, they learn to target cancer cells.

But with the measles and other preventative vaccines, you’re basically injecting some form of a virus or bacteria, and that’s gonna activate T cells in a way that’s kind of like handing a bloodhound somebody’s scent. That strategy doesn’t really work for cancer. Researchers have been studying cancerous cells for decades, hoping to find something unique to just those cells that they could create a vaccine against.

But every tumor turned out to be different, so there’s nothing universal that doctors can use as a target for all cancerous cells. The one cancer vaccine that’s been approved so far only works for certain kinds of prostate cancer, and it involves sending the patient’s cells to a lab to custom-design a vaccine for them. So it’s pretty limited and very expensive.

This new research builds on a different strategy. The idea is that instead of finding something within the tumor to target, this vaccine basically gets T cells in the tumor to send out signal flares. When a cancerous tumor first starts to form, the body generally does realize that something is wrong, and the immune system starts trying to get rid of it.

But cancer cells tend to mutate really quickly, and they often find ways to shut off whatever the immune system sees as a danger signal, which stops the attack. By that point, though, some cells have already infiltrated the tumor, and they know how to recognize it. So they could launch an immune assault against it … but only if they get a signal that says “hey! danger! attack!” That’s where comes in.

It’s a special concoction with two main parts: an antibody, a protein meant to target other specific proteins; and a weird, short piece of DNA originally discovered in bacteria that tends to induce a really strong immune response in mammals like us. This DNA is what’s called a CpG oligodeoxynucleotide, which binds to special receptors on T cells. In response, those cells express a protein called OX40, which is basically a signal to other immune cells that it’s found something to attack.

Meanwhile, the antibody part of the injection binds to the OX40, adding fuel to the immunological fire. Each of those ingredients had been looked at separately as an anti-cancer drug, but neither was effective enough on its own. And treatments that used other antibody-DNA combos tended to make people’s immune systems overreact, to the point where they led to a deadly complication known as autoimmune toxicity.

Injecting these types of vaccines into tumors directly usually causes fewer side effects … but it also tends to be less effective. That’s what makes this new vaccine so promising. Similar ideas have been tried before, but this version is injected directly into tumors, and it’s incredibly effective.

At least in mice. First, the researchers took 40 mice and gave them two lymphoma tumors. For each mouse, they injected either the antibody, the DNA, the combo vaccine, or a control solution into one of the tumors.

Then they waited to see what happened to that tumor and the tumor that they didn’t inject. The DNA alone knocked out the tumor it was injected into, but didn’t do much to the other tumor. The antibody alone slowed tumor growth, but that was about it.

The combo, on the other hand, did everything they could have hoped: both tumors disappeared. Then they tried similar experiments with breast, skin, and colon cancers. And for the most part, those tumors didn’t come back either— of the 90 mice in total that received the vaccine, 87 were completely cured, and the others responded well to a second dosing.

But since all of those tests involved artificially giving the mice cancer, the researchers stepped things up a notch by testing the vaccine in mice genetically predisposed to breast cancer. Not only do these types of mice have a strong likelihood of developing breast cancer — their cancers usually metastasize, meaning that cancerous cells break off and start new tumors elsewhere in the body. In those mice, the vaccine shrank the tumors it was injected into, and also seemed to keep other tumors from popping up.

Which all sounds super promising. But! It remains to be seen whether this technique works in humans.

Mouse models can be and have been extremely helpful during drug development, but treatments that work in rodents often aren’t effective in us. The real test for this vaccine will be if it’s just as effective in humans, without causing that deadly autoimmune toxicity. Still, the researchers are hopeful—so hopeful that they’re launching a small 15-patient clinical trial to test the method in people with certain kinds of lymphoma.

If all goes smoothly, they’d then move on to larger trials. So, as exciting as this is, it’s more of a small step than a giant leap in the quest to cure cancer. But if the treatment does work in humans, we will be closer.

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