Vaccines to prevent Covid-19 infection are hurtling through development at speeds never before seen. But mounting promises that some vaccine may be available for emergency use as early as the autumn are fueling expectations that are simply unrealistic, experts warn.

Even if the stages of vaccine development could be compressed and supplies could be rapidly manufactured and deployed, it could take many more months or longer before most Americans would be able to roll up their sleeves. And in many countries around the world, the wait could be far longer still — perpetuating the worldwide risk the new coronavirus poses for several years to come.

That reality is being obscured by reports that some of the earliest vaccine candidates — including one from the biotechnology company Moderna and another from University of Oxford — may within months have enough evidence behind them to be administered on an emergency use basis….

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