Will a vaccination that you might already have protect against COVID-19? Here’s what we know

Good news: Preliminary research indicates that a common vaccine, called the BCG vaccine, may offer significant protection against COVID-19. Here’s what you need to know:

• What’s the vaccine? The Bacillus Calmette–Guérin vaccine, which prevents tuberculosis, has been around for a century. It boosts the body’s immune system, and has been associated with reduced risk of a variety of bacterial, parasitic, and viral infections.
• Why the optimism? A preliminary study shows that countries with universal BCG vaccinations, such as Japan and South Korea, have infection and disease rates of COVID-19 as much as 100-fold lower than countries without universal vaccination policies, such as Italy, the Netherlands, and the United States. The elderly, who were vaccinated decades ago, may particularly benefit.
• What don’t we know? Correlation or causality. We don’t know why the infection rates differ. For example, it is possible that the differing infection rates are because of different mask-wearing and social distancing behaviors. (The term for this is “ecological fallacy.”) There is also no information on developing countries, which were not included due to less reliable COVID-19 counts.
• Have I been vaccinated? If you were born outside of the U.S. or previously traveled to developing countries and received vaccinations beforehand, possibly. This Wiki page has a partial list of country practices under “Usage.” Many countries have altered their policies over time.
• When will we know the answer? It is complicated. Studies just started in a half dozen countries, including the Netherlands, Australia, and the U.S., administering the vaccine to thousands of healthcare workers. Those results will be available in 3-4 months. However, fresh vaccinations may have differing effects than vaccinations administered decades ago.