Steven Johnson is the author of the new book Extra Life: A Short History of Living Longer, which is also running as a series on PBS (watch online here). The rapid advance in vaccine technology, which is bringing an end to the COVID-19 pandemic, is best understood in the context of a series of innovations that more than doubled the life expectancy of the human race over the last 100 years. Extra Life explores how innovations in epidemiological statistics, artificial fertilizer, toilets, and sanitation systems, along with vaccines and other measures, have allowed billions of people to flourish until old age. By 2016, global life expectancy at birth had reached 72 years, according to the World Health Organization (WHO).