What are vaccines, how do they work and why are people sceptical?

Vaccines have saved tens of millions of lives in the past century, yet in many countries health experts have identified a trend towards “vaccine hesitancy” – an increasing refusal to use vaccination. The World Health Organization estimates that vaccines prevented at least 10 million deaths in just five years between 2010 and 2015. It says vaccines have been one of the biggest success stories of modern medicine.

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