It's been more than 20 years since that now-discredited study linking autism and vaccinations was first published. Since then, numerous studies have proven again and again that immunizations are safe but 30 per cent of parents still express concerns. Vaccine hesitancy is now considered to be a major threat to global health. In Part 2 of a special series, Heather Yourex-West looks at how we got here and why there is still such an absence of trust.