Countries that had compulsory bacillus Calmette-Guérin (BCG) vaccination at least until 2000 appear to have built up a degree of “herd immunity” against COVID-19, a new study suggests.
There would have been approximately 80% fewer COVID-19 deaths in the United States by March 29, 2020, if the country had employed mandatory BCG vaccination since at least 2000, according to the new research.
The number of these deaths in the U.S. by March 29 was 2,467, but it would have been 468 with mandatory vaccination, say the scientists.
The researchers, all at the University of Michigan, in Ann Arbor, have published their findings in the journal Science Advances.
BCG vaccination is usually given at birth or during childhood to protect against tuberculosis (TB). But there is also evidence that it protects against other respiratory infections and lung cancer. In addition, it is an effective treatment for some forms of bladder cancer.
A study in 2018 found that the vaccine appeared to “reprogram” immune cells to produce more of a particular immune signaling molecule. This, in turn, boosted immunity against a viral infection.
Reporting biases
Previous studies have found a link between BCG vaccination and a slower spread of COVID-19 in populations around the world, but they failed to account for national differences in test availability and data reporting.
The new model attempts to get around this limitation by using the initial rate of increase in the number of reported cases and deaths, rather than the absolute numbers.
For each country, the researchers focused on a 30-day period after the first 100 confirmed cases or the first death.
Provided any national differences in reporting remained stable during this period, say the scientists, this should eliminate the effect of this bias on the data.
In their analysis, the scientists also tried to account for other factors that could have affected the number of COVID-19 cases and deaths in different countries. These included the median age of populations, population size and density, and gross domestic product, or GDP, per capita.
They found that having mandatory BCG vaccination significantly “flattened the curve” of the initial spread of COVID-19 throughout the populations studied.
Herd immunity
BCG vaccination was first used around a hundred years ago, when TB posed a major threat to public health.
Many countries, such as China, Ireland, and France, have had universal BCG vaccination, at least until very recently, whereas others never made it mandatory, including the U.S., Italy, and Lebanon.
Interestingly, the new study found that COVID-19 spread just as rapidly in countries that had only made BCG compulsory in the past 20 years as it did in countries where it was never compulsory.
This suggests that a large proportion of a population must be protected by the vaccination for it to prevent COVID-19 from spreading easily, the researchers say. This effect is known as herd immunity.
However, the latest findings add to evidence that widespread BCG vaccination of adults and children could protect populations from future COVID-19 outbreaks.
In Australia, a clinical trial is underway to find out whether the vaccine protects medical staff in hospitals from the infection that causes this disease.
Assuming that trials like this prove the vaccine’s safety and efficacy against COVID-19, the question remains whether vaccination of the wider population should be compulsory.
The authors of the new study note that individuals may be tempted to “free ride” and not get vaccinated if they believe that they will be protected through herd immunity.
In the absence of compulsory vaccination, they write, achieving herd immunity will depend on enough individuals choosing to get vaccinated for the wider public good.
Open questions
The authors emphasize that several other uncertainties remain. It is unclear, for example, whether BCG vaccination protects adults against COVID-19, and if so, how long the protection lasts.
In addition, scientists have yet to investigate whether BCG might have adverse effects on someone who already has the infection responsible for COVID-19.
Finally, even among countries with mandatory BCG vaccination, the new research found large variations in the rates of COVID-19 spread.
“Hence, BCG is by no means a magic bullet that assures safety against COVID-19,” the researchers write. “In all likelihood, there are some societal variables that moderate this effect. This variation must be addressed in future work.”
“All these limitations notwithstanding, the current evidence is the first to show a significant advantage of universal BCG policies in reducing the spread of COVID-19, thereby justifying a thorough investigation of the merit of the mandatory BCG vaccination in the fight against COVID-19.”
Additional caveats are worth noting. Although the vaccine was first administered to humans in the 1920s, a review of the evidence in 1994 found that it is not reliably effective in the treatment of lung-based TB. Since this is the most common and infectious form of the disease, many countries choose not to include the vaccine in routine immunizations.
Furthermore, as the authors of the present study suggest, authorities will need more reliable data than the study’s correlations before they can consider distributing the BCG vaccine en masse to curb COVID-19.
More comprehensive studies into the link between BCG policy and COVID-19 will have to account for complex variables.
For instance, the vaccine is highly effective and sorely needed as a treatment for severe forms of TB in children. The fact that a vast number of children around the world rely on this vaccine should be accounted for when considering its repurposing.
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