  The Bacille Calmette-Gu\'erin (BCG) tuberculosis vaccine has immunity
benefits against respiratory infections. Accordingly, it has been hypothesized
that it may have a protective effect against COVID-19. Recent research found
that countries with universal Bacillus Calmette-Gu\'erin (BCG) childhood
vaccination policies tend to be less affected by the COVID-19 pandemic.
However, such ecological studies are biased by numerous confounders. Instead,
this paper takes advantage of a rare nationwide natural experiment that took
place in Sweden in 1975, where discontinuation of newborns BCG vaccination led
to a dramatic fall of the BCG coverage rate from 92% to 2% , thus allowing us
to estimate the BCG's effect without all the biases associated with
cross-country comparisons. Numbers of COVID-19 cases and hospitalizations were
recorded for birth cohorts born just before and just after that change,
representing 1,026,304 and 1,018,544 individuals, respectively. We used
regression discontinuity to assess the effect of BCG vaccination on Covid-19
related outcomes. This method used on such a large population allows for a high
precision that would be hard to achieve using a classical randomized controlled
trial. The odds ratio for Covid-19 cases and Covid-19 related hospitalizations
were 0.9997 (CI95: [0.8002-1.1992]) and 1.1931 (CI95: [0.7558-1.6304]),
respectively. We can thus reject with 95\% confidence that universal BCG
vaccination reduces the number of cases by more than 20% and the number of
hospitalizations by more than 24%. While the effect of a recent vaccination
must be evaluated, we provide strong evidence that receiving the BCG vaccine at
birth does not have a protective effect against COVID-19.
