The different therapies they received reflect the evolving course of the virus and medical countermeasures against it.
The more likely scenario is that vulnerable people will need annual boosters for strains that are similar to Omicron, he said.
Of 43 U.S. omicron infections analyzed by the CDC, four-fifths were in fully vaccinated people.
The results mean there are now two promising candidates for treating Covid-19 patients early in the course of the disease.
It may be months or even years before vaccinated Americans need a booster shot, top scientists say.
Pfizer plans to ask the FDA to authorize a third booster shot for vaccine recipients.
Patient advocacy groups have supported the drug's clearance despite skepticism from some scientists.
The appearance of severe blood clotting events in two leading vaccines is a setback in the world's vaccination effort.
The pandemic caused more than 377,000 deaths, and only heart disease and cancer were more deadly.
The vaccine is authorized in the U.S. for people ages 16 and up, but the company will seek approvals for younger ages.
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