The global scramble to produce enough Covid-19 vaccine for 7 billion people is about to get even tougher, as drugmakers and countries ready a second round of shots to combat the growing threat of virus variants.

Finding vaccines that can ward off more contagious or virulent strains is only half the battle. The United States has virtually no capacity to manufacture revised vaccines or booster shots alongside the original versions, according to a half-dozen vaccine experts and Biden administration health officials. Setting up additional facilities could take months or even years.

Vaccine makers like Pfizer, Moderna and Johnson & Johnson coped with intense global demand for their original shots by manufacturing millions of doses while the vaccines were still in clinical trials. But that is not an option now, because the companies are still racing to fulfill orders for their existing Covid-19 vaccines — and some, including Johnson & Johnson and AstraZeneca, are struggling with major production setbacks. Pumping out second-generation shots would require factories to switch over manufacturing lines now used for the first wave of vaccines, and in some cases fire up new production processes….

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