Plague is an age-old disease that can still be deadly today, but now researchers are developing new vaccines that could potentially protect against plague infection, early research in animals suggests.

In a new study, researchers tested three vaccines that were designed to protect people against infection from the bacteria that cause plague, known as Yersinia pestis. To create the vaccines, the researchers modified several genes of the bacteria so that they couldn't cause disease, but would likely trigger an immune response in an animal. Specifically, the vaccines were designed to protect people against the bacteria that cause pneumonic plague, the most serious form of plague and the only type that spreads through airborne transmission.

Mice and rats were given two doses of each of the three vaccines. The researchers then infected the animals with pneumonic plague up to four months (120 days) after they were vaccinated. In the different experiments, between 80 and 100 percent of the animals that were vaccinated survived the plague.

"It is crucial that a potential vaccine candidate … [against plague] demonstrates long-term immune responses and protection," the researchers wrote in the Oct. 13 issue of the journal npj Vaccines. The new study showed that all three vaccines stimulated an immune response in the animals that was capable of protecting them from developing a pneumonic plague infection, they said. [Pictures of a Killer: A Plague Gallery]

Although vaccines against plague have been developed in the past, there is currently no plague vaccine that's approved by the U.S. Food and Drug Administration. There was previously a vaccine that protected against bubonic plague (another form of plague that causes swollen lymph nodes, called buboes), but this older vaccine did not prevent pneumonic plague, and was discontinued by its manufacturer, according to information about the vaccine from the U.S. Navy.

Plague is best known for killing millions of people in Europe in the 1300s, in a pandemic called the Black Death. Today, there are an average of seven human plague cases reported every year in the United States, according to the Centers for Disease Control and Prevention. The plague can be cured with antibiotics if the drugs are started soon after infection.

But without prompt treatment, plague is nearly 100 percent fatal, the researchers said.

Because of the high fatality rate without treatment, "the optimal strategy for protecting people … against this deadly disease would be through vaccination," Ashok Chopra, a professor of microbiology and immunology at The University of Texas Medical Branch at Galveston, said in a statement. Government officials are also concerned that plague bacteria could be used as a biological weapon.

The researchers plan to conduct more studies in animals to test the safety of their vaccines, as well as better understand the way that the vaccines protect against plague. Eventually, the researchers plan to test the effectiveness of the vaccines in nonhuman primates (such as monkeys), which is an important step in testing vaccines before they are used in people.

Original article on Live Science.