The researchers found that the countries in which many people received the vaccine experienced lower mortality from Covid-19.
While this does not mean that BCG somehow reduces the risk of serious disease from a coronavirus infection, it is consistent with other research that suggests that BCG may boost the immunity of people in general and perhaps help against coronavirus.
The World Health Organization has warned against using the BCG coronavirus vaccine until more is known, but teams around the world are investigating the possibility that it may help.
Luis Escobar of Virginia Polytechnic Institute and his colleagues used existing data to determine whether countries without a national BCG vaccination program have higher coronavirus mortality rates. In order to make a fair comparison, they considered factors such as population density, access to health care, and the response to Covid-19.
Non-specific immunity to the vaccine
The result is "remarkable, but not enough to establish a causal link," the team wrote. It is not enough to show with certainty that the BCG vaccine somehow protects people from the coronavirus.
Over 100 years of age, the BCG vaccine is used in many countries except the United States, and has been associated with reduced overall mortality rates in infants and children. There is strong evidence to suggest that the vaccine offers non-specific immunity - protection beyond tuberculosis. The effects of the vaccine on adults are inconsistent.
Other researchers have suggested that vaccines against polio and measles, mumps and rubella could provide similar protections against lethal infections, including the coronavirus.
Researchers warn that clinical trials are needed to prove the impact of the vaccine on severe Covid-19. The first overseas clinical trials are currently focusing on healthcare workers on the front line of coronavirus control.
A $ 10 million grant from the Bill and Melinda Gates Foundation in May supported clinical trials in Australia, Spain and the Netherlands, which will expand to 10,000 healthcare workers.
Dr. Denise Faustman, director of immunobiology at Massachusetts General Hospital and associate professor of medicine at Harvard Medical School, hopes to start clinical trials with her team in Boston as soon as possible.
Faustman has been studying the untargeted effects of the BCG vaccine for years. "BCG strengthens the innate immune system," said Faustman, "So whenever you see an infectious disease, you can fight it faster."
She says it is one of many recent studies that have supported the potential of the BCG vaccine as a tool to fight coronavirus.
"The signal is really very strong that countries that have already vaccinated BCG have protection against incidence and mortality," said Faustman, who added that there was even a possibility that the BCG vaccine could increase the effectiveness of a specific coronavirus vaccine, once it has been created.
If the protection of the BCG vaccine against the severity of coronaviruses is confirmed with new research, this could have important implications for public health.
Most Asian countries have universal BCG vaccination programs, unlike the United States. According to the CDC, BCG is generally not recommended in the United States, as the risk of infection is low, the vaccine's effectiveness may vary, and it can potentially interfere with tuberculosis testing.
"It is possible that some of the strategies to reduce social distancing adopted by Asian countries, in order to restart their economies, are not effective in North America and in Western European countries, and could lead to a second wave of 'infections,' said Escobar. the team wrote.
In theory, the vaccine could be administered in countries that do not currently have a universal vaccination program.
"The good thing is that it is given only once. It is potentially quite possible to increase the production of the vaccine, if it works and is safe," a said Dr. Wafaa El-Sadr, professor of epidemiology at Columbia University and director of ICAP, a global health program.
Dr. William Schaffner, an infectious disease specialist at Vanderbilt University School of Medicine, who previously described the use of the BCG coronavirus vaccine as "a passage from the Hail Mary", cautioned against putting Too much stock in observational data collected across countries, as not all countries assess Covid-19 cases and deaths in the same way.
"It is certainly an innovative and provocative approach to try to prevent this disease. We need all the help we can get," said Schaffner, who agrees that clinical trials are needed to really assess the potency of the vaccine. BCG in the fight against coronavirus.
"What we have seen are glimpses of upcoming attractions, as they say at the movies," said Schaffner. "Let's wait for the movie."