NEW YORK — A new analysis of blood samples from 24,000 Americans taken early last year is the latest and largest study to suggest that the new coronavirus popped up in the U.S. in December 2019 — weeks before cases were first recognized by health officials.

Vaccine maker Novavax said Monday its COVID-19 shot was highly effective against the disease and also protected against variants in a large study in the U.S. and Mexico, potentially offering the world yet another weapon against the virus at a time when developing countries are desperate for doses.

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JACKSON, Miss. — New COVID-19 cases are declining across most of the country, even in some states with vaccine-hesitant populations. But almost all states bucking that trend have lower-than-average vaccination rates, and experts warn that relief from the pandemic could be fleeting in regions…