Scientists want to find out if vaccines already in existence might provide protection against COVID-19.

The idea does not seem to make sense. Vaccines are designed to target a single virus or bacterium. But vaccines made using active strains of bacteria or viruses seem to help the body's natural defenses, called the immune system, fight non-targeted diseases as well.

There is no evidence yet that such vaccines could improve the immune system enough to resist the new coronavirus. But, a COVID-19 vaccine is expected to take 12 to 18 months. So, some researchers say it is time to test a tuberculosis vaccine for possible effects on the new coronavirus.

FILE - This March 1931 file photo shows ampules of the BCG vaccine against tuberculosis in a laboratory at the Institute Pasteur in Paris, France. Dec. 2, 1947 file photo.

Dr. Mihai Netea is with Radboud University Medical Center in the Netherlands. He says if the TB vaccine works, "it could be a very important tool to bridge this dangerous period" until a COVID-19 vaccine is available.

The World Health Organization on Monday strongly warned against the use of the tuberculosis vaccine against COVID-19 until studies prove it works. Netea's team is leading such a study already involving almost 1,500 Dutch health care workers as subjects. The researchers are treating some of the workers with the TB vaccine called BCG. It is made of a live but weakened bacterium related to TB bacteria. The other study subjects are treated with a placebo, an inactive substance.

Another study of BCG is taking place with 4,000 hospital workers in Australia. Similar research is being planned in other countries, including the United States.

Another possibility for treatment is a polio vaccine made of live but weakened polio viruses. The Global Virus Network in Baltimore, Maryland wants to begin studies with that vaccine, which is taken by mouth. Dr. Robert Gallo, a founder of the group, told The Associated Press that he is discussing the idea with health officials.

Jennifer Routh of the U.S. National Institutes of Health also said researchers there are discussing TB and polio vaccines as a possible COVID-19 defense.

Live vaccines, however, are risky for people with weakened immune systems. They should not be tried against COVID-19 outside of a research study, said Dr. Denise Faustman. She is immunobiology chief at Massachusetts General Hospital and is planning a TB vaccine study.

She added that her study provides a good chance "to prove or disprove this off-target effect."

Early evidence

Years ago, scientists began noticing that people vaccinated with some live vaccines had improved immunity to non-targeted sicknesses.

The TB vaccine, for example, is given mostly to babies in developing countries. It offers only partial protection against bacterial TB. But studies suggested that the vaccinated babies had better survival rates and resisted lung infections better.

In 2018, Netea's team published a more direct test. It showed that BCG vaccine increases immunity overall. In fact, it was good enough to at least partly block another virus given experimentally a month later.

There has been a report that researchers in the former Soviet Union found that influenza cases dropped after polio vaccinations in the 1970s.

And in 2015, Danish researchers found similar results with polio vaccinations given by liquid drops through the mouth. These are still used in developing countries, while the U.S. and other areas use the inactivated shot for childhood vaccines.

Immunity from live vaccines

The goal of a vaccine is to prepare the body to recognize a targeted health threat. Vaccines cause the body to make antibodies, products in the blood that fight bacteria and viruses. BCG appears to influence immune cells so they can more readily remove germs in general, said Netea, the Dutch researcher.

Scientists not involved in the effort to try these vaccines against COVID-19 told the AP that such studies are worthwhile.

Lauran Neergaard reported this story for the Associated Press. Mario Ritter Jr. adapted it for VOA Learning English. Caty Weaver was the editor.

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