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A large international collaboration has started a series of SARS-CoV-2 (COVID) human challenge trials (HCTs). However, critics of previously proposed COVID HCTs deemed them unethical for posing excessive risks to participants. These concerns raise the need to evaluate the risks of the planned COVID HCTs. Systematic analysis reveals that, although the risks of these trials would be high in some populations, they are clearly acceptable for eligible individuals and may even qualify as "minimal" for them. This conclusion is important for evaluating the planned COVID HCTs and highlights the importance of assessing the risks of later COVID HCTs and HCTs in future pandemics systematically. 

Despite their clearly demonstrated safety and effectiveness, approved vaccines against COVID-19 are commonly mistrusted. Nations should find and implement effective ways to boost vaccine confidence. But the implications for ethical vaccine development are less straightforward than some have assumed. Opponents of COVID-19 vaccine challenge trials, in particular, made speculative or empirically implausible warnings on this matter, some of which, if applied consistently, would have ruled out most COVID-19 vaccine trials and many non-pharmaceutical responses. 

Abstract The world’s first coronavirus disease 2019 human challenge trial using the D614G strain of severe acute respiratory syndrome 2 (SARS-CoV-2) is underway in the United Kingdom. The Wellcome Trust is funding challenge stock preparation of the Beta and Delta variant for a follow-up human challenge trial, and researchers at hVIVO are considering conducting these trials. However, little has been written thus far about the ethical justifiability of human challenge trials with SARS-CoV-2 variants of concern. We explore 2 specific characteristics of some variants that may initially be thought to make such trials unethical and conclude that SARS-CoV-2 variant challenge trials can remain ethical. 

In human challenge trials (HCTs), volunteers are deliberately infected with an infectious agent. Such trials can be used to accelerate vaccine development and answer important scientific questions. Starting early in the COVID-19 pandemic, ethical concerns were raised about using HCTs to accelerate development and approval of a vaccine. Some of those concerns pertained to potential exploitation of and/or lack of truly informed consent from volunteers. Specific areas of concern arose around individuals who may be unusually risk-seeking or too economically vulnerable to refuse the payments these trials provide, as opposed to being motivated primarily by altruistic goals. This pre-registered study is the first large-scale survey to characterize people who, early in the pandemic, expressed interest and intention to volunteer to participate in COVID-19 HCTs. We found that individuals expressing interest in SARS-CoV-2 HCTs exhibit consistently altruistic motivations without any special indication of poor risk perception or economic vulnerability. In finding that, early in the pandemic, COVID-19 HCTs were able to attract volunteers whose values align with the nature of these trials, and who are not unusually vulnerable to exploitation, this study may allay some ethical concerns about the volunteers interested in participating in such trials. 

In spring, summer and autumn 2020, one abiding argument against controlled human infection (CHI) studies of SARS-CoV-2 vaccines has been their impact on local communities. Leading scientists and bioethicists expressed concern about undue usage of local residents’ direly needed scarce resources at a time of great need and even about their unintended infection. They recommended either avoiding CHI trials or engaging local communities before conducting any CHIs. Similar recommendations were not made for the alternative—standard phase III field trials of these same vaccines. We argue that the health effects of CHI studies on local residents not participating in the study tend to be smaller and more positive than those of field trials. That is all the more so now that tested vaccines are being rolled out. Whether or not local community engagement is necessary for urgent vaccine studies in the pandemic, the case for its engagement is stronger prior to field trials than prior to CHI studies. 

Notwithstanding the success of conventional field trials for vaccines against COVID-19, human challenge trials (HCTs) that could obtain more information about these and about other vaccines and further strategies against it are about to start in the UK. One critique of COVID-19 HCTs is their distinct paucity of information on crucial population groups. For safety reasons, these HCTs will exclude candidate participants of advanced age or with comorbidities that worsen COVID-19, yet a vaccine should (perhaps especially) protect such populations. We turn this cliché on its head. The truth is that either an HCT or a field trial has intrinsic generalisability limitations, that an HCT can expedite protection of high-risk participants even without challenging them with the virus, and that an important route to obtaining results generalisable to high-risk groups under either strategy is facilitated by HCTs.