The availability of the Moderna and Pfizer mRNA vaccines, shown to be 95 percent effective in preventing severe disease, have raised hopes of reaching herd immunity. Some political and public health leaders have argued that if we vaccinate 70 percent or 80 percent of the global population we can effectively end further transmission. But even countries fortunate enough to procure sufficient quantities of vaccine may never reach herd immunity, in which case COVID-19 could become a seasonal affliction that comes each year. Many factors will be determinative – some already well understood and others not. While it's possible to reach herd immunity by next winter, it seems increasingly unlikely we will do so, and in light of that we all need to shift our expectations.
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