The preliminary study posted on medRxiv, a site for unpublished medical research, finds a correlation between countries that require citizens to get the bacillus Calmette-Guerin (BCG) vaccine and those showing fewer number of confirmed cases and deaths from Covid-19. Though only a correlation, clinicians in at least six countries are running trials that involve giving frontline health workers and elderly people the BCG vaccine to see whether it can indeed provide some level of protection against the new coronavirus.

Gonzalo Otazu, assistant professor at the New York Institute of Technology and lead author of the study, started working on the analysis after noticing the low number of cases in Japan. The country had reported some of the earliest confirmed cases of coronavirus outside of China and it hadn’t instituted lockdown measures like so many other countries have done.

Otazu said he knew about studies showing the BCG vaccine provided protection against not just tuberculosis bacteria but also other types of contagions. So his team put together the data on what countries had universal BCG vaccine policies and when they were put in place. They then compared the number of confirmed cases and deaths from Covid-19 to find a strong correlation.

Among high-income countries showing large number of Covid-19 cases, the U.S. and Italy recommend BCG vaccines but only for people who might be at risk, whereas Germany, Spain, France, Iran and the U.K. used to have BCG vaccine policies but ended them years to decades ago. China, where the pandemic began, has a BCG vaccine policy but it wasn’t adhered to very well before 1976, Otazu said. Countries including Japan and South Korea, which have managed to control the disease, have universal BCG vaccine policies. Data on confirmed cases from low-income countries was considered not reliable enough to make a strong judgment.

Caution Urged

With nearly 900,000 cases and 45,000 deaths, the world is struggling to control Covid-19. Any vaccine for the disease is more than a year away from being available and the effectiveness of drugs under trial won’t be known for months to come. That’s why it’s reasonable to look at whether BCG vaccine could provide protection against Covid-19, said Eleanor Fish, professor at the University of Toronto’s immunology department. Otazu’s study is yet to undergo review by peers, a strict criteria for science studies.

“I would read the results of the study with incredible caution,” Fish said.

Otazu, who said he’s already received comments from other experts, is working on a second version of his study that will address some of their concerns. He has also submitted the study for a formal review process with the journal Frontiers in Public Health.

One of the first to conduct the trial of BCG vaccine’s effectiveness against coronavirus is Mihai Netea, an infectious-disease expert at Radboud Universty Medical Center in the Netherlands. Netea’s team has already enrolled 400 health workers in the trial—200 got the BCG vaccine and 200 received a placebo. He doesn’t expect to see any results for at least two months. He’s also about to start a separate trial to study the effectiveness of the BCG vaccine on those older than 60. Other trials are taking place in Australia, Denmark, Germany, the U.K. and the U.S.

Scientists are still working to better understand why the BCG vaccine may be effective against not just tuberculosis but other disease microbes. Netea’s decade-long work shows that BCG vaccine sensitizes the immune system in such a way that, whenever any pathogen that relies on the same attack strategy as the tuberculosis bacteria attacks, it is ready to respond in a better way than the immune system of those who haven’t received the vaccine.

“It’s like the BCG vaccine creates bookmarks for the immune system to use later in life,” Netea said.

Even if BCG vaccine is shown to be effective, that’s no reason to stockpile.

“People should not hoard or try to get BCG vaccine like they did toilet paper,” Otazu said. There is a small chance that the BCG vaccine could increase the risk of coronavirus, but scientists won’t know until after the clinical trials.

In any case, the BCG vaccine shouldn’t be the only tool to fight Covid-19.

“No country in the world has managed to control the disease just because the population was protected by BCG,” Otazu said. Social distancing, testing and isolating cases will need to be implemented to manage the spread of the disease.

More coronavirus coverage from Fortune:

—Hospitals are running low on the most critical supply of all
—More ventilators are coming, but there’s still a shortage of people who can operate them
—China releases data on asymptomatic coronavirus cases—but doubts remain
—”It’ll never be fast enough”: 5 questions for a ventilator manufacturer
—The stock market had its worst quarter since 1987—and its worst Q1 ever
—How to job hunt during the coronavirus pandemic
—PODCAST: Two health care CEOs on why coronavirus tests and vaccines are the ammunition needed to fight COVID-19
—VIDEO: World leaders and health experts on how to stop the spread of COVID-19

Subscribe to Outbreak, a daily newsletter roundup of stories on the coronavirus pandemic and its impact on global business. It’s free to get it in your inbox.