WASHINGTON — President Biden’s coronavirus czar said Wednesday that the United States was woefully behind other nations in tracking potentially dangerous variants of the virus, and used the first White House public health briefing to issue a stark warning that Americans will remain vulnerable to the deadly pandemic unless Congress acts.

“We are 43rd in the world in genomic sequencing — totally unacceptable,” said Mr. Biden’s Covid-19 response coordinator, Jeffrey D. Zients, citing December data from the GISAID Initiative, which provides a global database of coronavirus genomes. In a brief interview later, he corrected himself, saying he had since learned that the United States was now behind 31 other nations.