There are eugenicist implications
Wanting to save just Americans, and leave other countries to suffer, incurring millions of avoidable foreign deaths? This action smacked troublingly of a 'master race' vision for the future.
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Similar to the way in which some pharmaceutical companies may push the price for drugs that are in high demand to astronomically lofty prices to promote exclusivity, by allowing only Americans to have access to a COVID-19 vaccine the preventable deaths of countless innocent will be incurred; specifically, those of low socioeconomic statuses like minorities, the disabled, and the elderly. The idea that Americans alone will be saved has eugenic implications on its own as it suggests that the life of an American is somehow more valuable. However, it is extremely poignant when looking to the groups that will be most affected within both the United States and countries outside of the US. That is to say, these groups (minorities, the disabled, and the elderly) have experienced deep ties to eugenic policies long before as well as long after the rise of the Nazis. For example, in looking to the COVID-19 pandemic alone, conversations are not free from statements explaining that a now-deceased individual had a preexisting condition/disability, or that they were elderly.  By implying that the death of an individual is somehow less emotionally impactful due to the aforementioned reasons also suggests that such persons are expendable during this pandemic as they hold less value to society— a clear eugenicist idea that is reflected in disabled individuals being denied ventilators in favor of able-bodied persons within the United States. All of which is playing into an effort to develop a 'master race.'
There are no eugenic implications in regards to Trump attempting to buy the rights to a COVID-19 vaccine, instead, the situation is far too over analyzed. For starters, the eugenic ideology entails that President Trump, and in turn, the American people believe that they are superior to other nations, and are actively attempting to purge the world of international populations. However, that is not the case. Instead, Trump is simply looking to put the American people first, a sentiment that all citizens should hold their own leaders too. Thus, this does not translate into a vision of a 'master race.'  It is just an example of a good world leader.