(iStock/Getty Images Plus/wildpixel) While the world struggles with two-shot vaccine production as a handful of pharma companies try to make enough for almost the entire global population, a single-dose option from Johnson & Johnson has become a major hope in the fight against COVID-19. Candidates from AstraZeneca-University of Oxford, Moderna and Pfizer-BioNTech all require two full doses, but with so many countries demanding more, supply is struggling to keep up. Many governments are now faced with tough choices, such as either delaying a second shot—required for the full immune response needed according to data from clinical trials—or getting one dose out to as many people as possible in the hope of partly protecting most rather than almost fully protecting all.