NEW YORK - Temperature checks are based on a simple premise: that fever is a primary symptom of coronavirus.

But what if it’s not?

A recent study by Northwell Health looked at 5,700 COVID-19 patients at its New York-area hospitals, and found just 30 percent had fever when they arrived.

“It has dramatic implications,” said Dr. Thomas McGinn, Deputy Physician-In-Chief at Northwell Health and one of the study’s authors.

Northwell's findings diverge from other studies, including two here in New York, that found 70 percent or more of hospitalized patients showed fever. But it highlights that coronavirus can cause a wide range of symptoms, from rashes to nausea to hallucinations.

“Screening for fevers at different airports and things like that may still be a legitimate thing to do,” McGinn said. “But it’s certainly not what we thought it was, and does not give me a sense of security that that’s going to be an effective way to know whether someone’s COVID or not.”

Temperature checks come with logistical hurdles: Thermometers vary in their reliability. Those administering the checks may need protective equipment. And temperature checks could miss those on fever-reducing medications like Tylenol - not to mention the population of asymptomatic or pre-symptomatic COVID carriers.

Even the World Health Organization says temperature screening is not an effective way to stop international spread of the virus. “Such measures require substantial investments for what may bear little benefits,” reads a notice on the WHO website.

“We know not everybody who has the virus is going to have a fever,” said Dr. Charles Powell, chief of the Division of Pulmonary, Critical Care and Sleep Medicine at the Mount Sinai Health System and CEO of the Mount Sinai-National Jewish Health Respiratory Institute. "And we also know that not everybody who has the virus is going to have a cough or a change in respiratory symptoms. But yet they may have the virus, and they may be contagious.”

Officials say temperature checks will be part of the protocol as New York begins to reopen, but details remain unclear.

"I think this will be an important part of the equation,” Mayor Bill de Blasio said at a briefing Wednesday. “But how are we going to use them? Where are we going to use them? Making sure we have enough thermometers — these are all things that we're planning on right now."

Even skeptics say temperature checks may be better than nothing, especially if they serve as a reminder to the public to stay vigilant.

“I don’t think they’re a bad idea. I don’t think they’re counterproductive,” Powell said. “I think we just have to be cognizant of how we interpret it.”

Experts also note that when a temperature check identifies someone with coronavirus, that person would likely have been infected days earlier -- and could have already spread the virus to others.