After the winter break, many schools across the United States have returned to virtual learning or closed due to a shortage of coronavirus testing, which has made schools incapable of screening returning students amid infection surges, according to a report by The New York Times on Tuesday.

The report attributed the difficulties of conducting effective testing programs to a combination of the ultra-contagious Omicron variant, political factions, conflicting federal guidance and a national shortage of rapid-test kits.

More than 5,400 schools have reverted to virtual learning since January 3, the newspaper said, citing data from Burbio, a company that audits how schools have operated through the pandemic.

One reason for the issue is that "many districts are bungling the execution or failing to muster the resources necessary to test properly," the report said, while confirming that virus testing should have been an effective measure particularly in combination with vaccination, face masks and other precautions.

"A lot of schools are just testing parts of their population once a week, or not using the tests strategically, or confusing surveillance with testing to suppress outbreaks," Dr. Michael J. Mina, a former Harvard University epidemiologist and a leading expert on rapid testing, was cited as saying.

The expert compared the present results of failed testing to "an army going to battle without knowing how to use its weapons or understanding its objectives."

Public health experts say few districts are testing enough, or strategically enough -- particularly in the wake of Omicron, The New York Times said.

Mina said that the lack of clear federal guidance on rapid tests has also been an issue.

In some health experts' view, the result at many schools is "a hodgepodge of half-measures." For example, in some Republican-led states, state administrations have de-emphasized school testing or lagged behind in distributing stockpiles.