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care and the rise of vaccine hesitancy.32,33 Measles has experienced a global resurgence as well, with a tripling of reported cases worldwide in the first quarter of 2019 compared to last year.34 Its persistence can be explained in part by its high transmissibility. Each person infected with measles is capable of infecting between 12 and 18 additional people.27 Comparatively, each Ebola case in the 2014 outbreak was only capable of infecting 1.5 to 2.5 additional persons.35 With proper medical care and vaccination coverage, mortality from measles is approximately 1% in developed countries, but it can range from 10% to 30% in countries lacking sufficient healthcare infrastructure or where access to health services is limited.28 These mortality rates are often higher in vulnerable populations, including children, the elderly, and nonvaccinated or immunocompromised individuals.31 Measles infection rates and mortality rates were much higher before the introduction of the measles vaccine in 1963, with more than 130 million cases and 7 million deaths annually worldwide.31 Despite the effectiveness of the measles vaccine, the world continues to experience a global resurgence of cases. It is also important to remember that there is a certain level of baseline immunity to measles, particularly among older generations for whom measles was a practically ubiquitous childhood disease.28 What if a disease as transmissible as measles had a case fatality as high as SARS or Ebola, for which there was no effective vaccine and no population level immunity? Analyzing what it would take to control this kind of event helps to crystallize what kind of national and global systems would need to be in place to respond novel, high-impact respiratory pathogens. Box 3: Anticipating Challenges During the Deliberate Release of a High-Impact Respiratory Pathogen Previous epidemics and pandemics caused by high-impact respiratory pathogens, such as the 1918 influenza, showed how such pathogens can lead to widespread health, social, and economic damages.36 Now, with advances in biology, a high-impact respiratory pathogen could be engineered to create transmissibility and lethality. The deliberate release of such a pathogen could substantially add to the already extraordinary consequences that would follow a naturally occurring pandemic event. A key difference between deliberate release scenarios and those in which a high-impact respiratory pathogen emerges and spreads via natural mechanisms would be the possibility for there to be multiple attacks, or “reload,” in a deliberate event. A sophisticated assailant could use a bioweapon to target areas of public health vulnerability or to deliberately inflict harm on the population or particular segments of it. The ability to effectively respond to such a deliberate event would depend, in part, on an understanding of the risk of subsequent attacks. Activities in the nonhealth domain, such as attribution and interdiction, would be critical capabilities alongside medical and public health response. Currently, there is no clarity on which agency would lead the response to a deliberate event and what responsibilities this would entail (ie, operating the public health response or the investigation and attribution).37 22 Severe epidemics and pandemics have demonstrated how contagious disease emergencies can exacerbate societal divisions, by fomenting social and political tensions and generating stigma against vulnerable groups who may be blamed.38 The potential for such negative societal consequences could be even worse in a deliberate event. Groups who are perceived as aligned with the perpetrator of an attack may experience backlash.39 Public fear and uncertainty could be high in the aftermath of a deliberate event, requiring highly effective risk communication and public outreach. Geopolitical ramifications should be expected if countries or terrorist groups were behind such an attack and likely addressed at very high levels, such as the United Nations Security Council.40 The involvement of the security and intelligence communities may lead to altered or centrally controlled decision-making structures, with implications for the World Health Organization (WHO) and the wider public health response. Currently, no designated position or entity exists in the United Nations (UN) system with a dedicated mandate to coordinate the response to a deliberate event. In such an event, public health officials would need to interface with law enforcement and/or military personnel, even if their operational goals did not fully align; public health would be attempting to contain the outbreak and identify the pathogenic strain, while national security and law enforcement may consider certain materials or locations to be vital evidence and might think it necessary to limit access to them. Public health and humanitarian workers may be hesitant to operate alongside security personnel in the response, given the possibility or perception that data may be used to further investigations or somehow compromise UN humanitarian principles of independence and neutrality. Security officials may argue that attribution should take priority over the public health response in order to stop a “reload” scenario that could harm others. Communication, data sharing, outreach, and coordination are key capabilities for managing a response to any severe outbreak, particularly in a pandemic, when many sectors from multiple countries would need to interact openly and honestly with each other to mount an effective response.41 Data sharing would also need to take place between health and security officials, between international organizations, and among national governments. However, the investigative and intelligence communities may be hesitant to share information with the public health sector due to security issues. Especially if there were no available medical countermeasures to prevent a disease or treat people after a deliberate event, and if spread could not be readily controlled, national governments may focus on protecting their own citizens rather than cooperating with other countries. The implementation of border closures and travel bans may give governments more credibility among their