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"Unfortunately, despite long-term efforts, effective vaccines to prevent enteric CoV infections remain elusive, and generally live, but not killed vaccines, have induced the most consistent protection against animal CoVs. Confirmation of the pathogenesis of SARS in humans or animals models that mimic SARS may further aid in vaccine design and evaluation."
https://www.ncbi.nlm.nih.gov/pubmed/15742624
"Since the 1960s, tests of vaccine candidates for diseases such as dengue, respiratory syncytial virus (RSV), and severe acute respiratory syndrome (SARS) have shown a paradoxical phenomenon: Some animals or people who received the vaccine and were later exposed to the virus developed more severe disease than those who had not been vaccinated (
1). The vaccine-primed immune system, in certain cases, seemed to launch a shoddy response to the natural infection."
"This immune backfiring, or so-called immune enhancement, may manifest in different ways such as antibody-dependent enhancement (ADE), a process in which a virus leverages antibodies to aid infection; or cell-based enhancement, a category that includes allergic inflammation caused by Th2 immunopathology."
"“There is the potential for ADE, but the bigger problem is probably Th2 immunopathology,” says Ralph Baric, an epidemiologist and expert in coronaviruses—named for the crown-shaped spike they use to enter human cells—at the University of North Carolina at Chapel Hill. In previous studies of SARS, aged mice were found to have particularly high risks of
life-threatening Th2 immunopathology (
2). Baric expresses his concern about what that might mean for use of a COVID-19 vaccine in elderly people. “Of course, the elderly are our most vulnerable population,” he adds."
https://www.pnas.org/content/117/15/8218