Cheap and easy to use, face masks may be the single most important tool in our arsenal against the COVID-19 pandemic. According to a new study, after face masks were made mandatory in Germany, the number of new infections dropped by 45% about 20 days after their introduction. The study provides some of the strongest evidence published thus far on the effectiveness of face masks for preventing coronavirus infections in a population.

Mask-wearing is a surprisingly contentious subject, especially among conservatives. Numerous studies have found that both masks and mask mandates reduce the risk of transmission. As a result, both the CDC and the World Health Organization (WHO) recommend the use of face masks in their guidelines in order to protect both the wearer and those around us while out in public. The WHO has even gone a step further, advising face masks both outdoor and indoor if proper ventilation cannot be guaranteed.

What’s more, face masks not only reduce the risk of infection, they can also dramatically reduce the viral dose of infection and, consequently, how ill the wearer may become.

In November, the CDC reported that the average number of daily cases decreases in 24 Kansas counties after mask mandates were introduced over the summer, while 81 countries that did not introduce mask mandates saw an increase in cases.

Now, a new study published in the journal Proceedings of the National Academy of Sciences (PNAS) adds further weight to the body of evidence certifying the effectiveness of face masks against the pandemic.

Although clinical trials produce the strongest evidence of an effect, the study conducted by researchers in Germany on the population of various regions in the country is still robust and offers a rare overview of mask-wearing effectiveness in the real world.

The researchers found that face masks lead to a drop in the number of new COVID-19 infections 20 days after their mandatory introduction anywhere from 15% to 75%, depending on the region. For instance, after face masks became mandatory between 1 April and 10 April 2020 the number of new infections fell almost to zero — it was this highly publicized success that eventually led to the introduction of compulsory mask-wearing at the federal state level.

But perhaps the most encouraging result was that the drop in new infections was greatest among the elderly — new infections dropped by more than 90% for those aged 60 or older.

When the pandemic first hit the world, the original messaging on masks and their effectiveness was muddled by somewhat contradictive viewpoints even among experts. Now virtually all health organizations advise the use of face masks both indoor and outdoor in order to protect the wearers as well as those exposed to them. When a sick person wears a mask, we are protected from contaminated particles, and when healthy people also use a mask, there is additional protection. Simple as that.

“As economic costs are close to zero compared to other public health measures, masks seem to be a cost-effective means to combat COVID-19,” the authors of the study wrote in their study.