evidence: Scientific evidence for the effectiveness of large-scale vaccination campaigns is well established.[31] Two to three million deaths are prevented each year worldwide by vaccination, and an additional 1.5 million deaths could be prevented each year if all recommended vaccines were used.[32] Vaccination campaigns helped eradicate smallpox, which once killed as many as one in seven children in Europe,[33] and have nearly eradicated polio.[34] As a more modest example, infections caused by Haemophilus influenzae (Hib), a major cause of bacterial meningitis and other serious diseases in children, have decreased by over 99% in the US since the introduction of a vaccine in 1988.[35] It is estimated that full vaccination, from birth to adolescence, of all US children born in a given year would save 33,000 lives and prevent 14 million infections.[36] [...] In several countries, reductions in the use of some vaccines were followed by increases in the diseases' morbidity and mortality.[301][302] According to the Centers for Disease Control and Prevention, continued high levels of vaccine coverage are necessary to prevent a resurgence of diseases that have been nearly eliminated.[303] Pertussis remains a major health problem in developing countries, where mass vaccination is not practiced; the World Health Organization estimates it caused 294,000 deaths in 2002.[304] Vaccine hesitancy has contributed to the resurgence of preventable disease. For example, in 2019, the number of measles cases increased by thirty percent worldwide and many cases occurred in countries that had nearly eliminated measles.[32]