n The Conjurer, a 16th-century painting by Dutch artist Hieronymus Bosch, a spectator stands in rapt attention as a magic trick is performed. He bends forward, wholly absorbed by the spectacle, blissfully unaware of the man behind him casually picking his pocket. As we mark World Immunisation Week 2020, all eyes are on Covid-19. Across the globe, researchers have responded with remarkable speed and ingenuity. More than 100 vaccine candidates are in development, and several human clinical trials have begun. Though significant hurdles lie ahead, we can be optimistic that a vaccine will emerge from the fray. But what about the hazards to which we aren’t paying attention? Existing vaccines keep a litany of life-threatening diseases at bay. Young children, who thankfully escape the worst of Covid-19, are most at risk of many of these vaccine-preventable illnesses. Without systematic efforts to maintain immunisation programmes, the virus’s legacy could include a disastrous surge in childhood deaths. Recent history cautions us of the consequences of disrupted vaccination efforts. Explosive outbreaks of measles have consistently followed humanitarian crises, the war in Syria, the Ethiopian famine in 2000 and the Boxing Day tsunami in 2004. Last year, while buckling under the strain of its largest ever Ebola outbreak, the Democratic Republic of the Congo was hit by a measles epidemic. So far, this has caused at least 310,000 cases and more than 6,000 deaths – more than double the number of fatalities from Ebola. Epidemics are ongoing, and were joined by Covid-19 in early March. One trauma compounded by another. It does not even take a crisis to derail decades of immunisation work; all that is needed is fear. In 2002, the eradication of polio in Africa was achingly close – just 202 cases were recorded across the entire continent. Rumours over the safety of the oral polio vaccine began to spread across Nigeria in 2003, leading five states to temporarily boycott it. The effects would ripple across the continent for several years. Nigeria suffered 782 polio cases in 2004, and the virus was exported to six previously polio-free neighbouring countries. It took the best part of a decade to bring the numbers back down to 2002 levels. Wealthier countries in Europe and north America are not exempt from these threats. Vaccination rates of at least 95% are needed to suppress measles, and far too many countries fall short. Last year there were 1,282 cases of the disease in the US – the highest number since 1992. The UK had more than 800 cases and lost its measles-free status in August. Wherever there are gaps in vaccination, the threat of measles will remain. Immunisation guidelines issued by the World Health Organization (WHO) on 26 March put the situation in stark relief: “Disruption of immunisation services, even for brief periods, will result in increased numbers of susceptible individuals and raise the likelihood of outbreak-prone vaccine preventable diseases”. Lockdowns, essential to protect public health, have the potential to cause such disruption. In the UK, routine childhood inoculations are designated “high priority” activities that must continue regardless of the scale or duration of the pandemic. Yet despite such guidelines, concerns remain over the effect on childhood vaccination rates. While we must heed advice to stay at home wherever possible, vaccination visits are among the essential trips we must keep making. Many places face far steeper challenges. Across low- and middle-income countries, mass vaccination campaigns form a crucial supplement to routine immunisation in clinics. These campaigns involve door-to-door visits alongside immunisation in public hubs such as schools, churches, mosques and markets – activities clearly at odds with social distancing. As Seth Berkley, head of the vaccine alliance Gavi, put it, we face “a devil’s choice”. Do we continue mass campaigns and risk fanning the flames of a pandemic, or halt these programmes and leave millions of children at risk of life-threatening infections? For now, the potentially catastrophic toll of an unmitigated Covid-19 pandemic has been deemed the greater threat. The WHO recommended the temporary suspension of mass campaigns “where there is no active outbreak of a vaccine-preventable disease”. Countries must do what they can to maintain routine vaccination programmes, while preparing to respond swiftly with mass campaigns if outbreaks do ignite. The challenges of maintaining vaccination programmes amid a pandemic should not be understated. Countries must contend with disruption of the vaccine supply chain, deficits in protective equipment for healthcare workers and the risk of coronavirus exposure in overcrowded clinics. If outbreaks of diseases such as measles do occur, they will be compounded by issues that beset the world’s most vulnerable populations, such as malnutrition. As we marvel at the efforts of researchers racing to develop a coronavirus vaccine, we must match this with ingenuity in delivering the vaccines that are already available. We may be faced with a new foe, but old ones lie in wait. We must find ways to keep vaccinating. • Dr Edward Parker is a research fellow at the Vaccine Centre, London School of Hygiene & Tropical Medicine