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" ... How does SARS-CoV-2 engineer this disappearing trick, and what implications would the mechanism have for the development of vaccines? One possibility is that the coronavirus proteins have evolved to become weak immunostimulants. If this is so, it will be a problem for Covid-19 vaccine candidates that use natural variants of the virus proteins as antigens. To be successful, their makers will have to engineer proteins that differ from those in the virus itself—not an impossible task, but a difficult one that would take time. We won’t know the answer until we learn how well the vaccines protect against infection, and how long protection lasts. ... "