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You’ve seen the picture: exhausted travelers waiting to have a thermometer pointed at their heads.

There are temperature checks at airports, screenings outside shops, people being stopped and examined while they walk down the street.

Two months into this pandemic, with anxiety running high, many people are wondering: Shouldn’t we all be getting our temperature checked?

The answer, according to experts, is no.

Temperature checks are an important tool that can be effective in many contexts, including doctors offices and hospitals. But when it comes to curbing the global spread of the coronavirus that causes covid-19, temperature screening has serious limitations — and comes with costs.

What can be missed

The biggest problem is that temperature screening can miss cases, unwittingly sending sick people through. It can also deliver false positives, potentially sending healthy people into spaces where others are seriously ill.

For some jurisdictions, it is difficult and expensive to acquire enough screening equipment and to train the personnel to use it. Because of this, some governments are choosing to focus on other measures, especially if resources are scarce.

“A thermometer, if calibrated and if used appropriately, can detect a fever. Great. We are all happy about that,” said Isaac Bogoch, an infectious disease specialist at Toronto General Hospital. “But is it an effective method for screening people for covid-19 infection? The answer is no.”

Colorado has opened one of the country's first drive-through coronavirus testing centers, where patients with a doctor's note can get tested for free. (Lance Murphey/The Washington Post)

The use of temperature screening in the response to this coronavirus has been spotty so far, with some countries going all-out and others steering clear.

China, for instance, has made extensive use of temperature screening setting up checkpoints not just at airports, but outside apartment buildings and even at stores. India is testing all international arrivals at airports. South Korea, Russia and others are screening arrivals from certain places.

Many variables on readings

Other countries don’t bother. Even with an outbreak raging in Italy, German health officials have argued that airport temperature screenings are not effective.

There are several reasons that temperature screening is not effective on its own.

To start, in crisis situations, people may use inappropriate instruments, or use the right instruments incorrectly, leading to inaccuracies.

“We are seeing photographs of people using industrial grade infrared thermometers to measure the temperature of people, but these instruments are not meant to measure human body temperature,” said Jim Seffrin, an expert on infrared devices at the Infraspection Institute in New Jersey.

Seffrin said industrial grade instruments are calibrated to measure things such as pipes. Using them to measure human skin temperature can lead to numbers that are off by up to 7 degrees Fahrenheit, he said.

It also matters where and how you use them. People have been using infrared thermometers on people standing outside in the winter cold. Since this type of instrument measures surface temperature, not core body temperature, that could mean missing a fever.

False positives are also possible. “A person can show up with a higher than average surface temperature because they just went out for a jog, or they may have been standing near a heating lamp, causing them to be warm,” Seffrin said.

Another problem: people taking medicine to mask their fever.

Even if done correctly, temperature checks are not necessarily enough to pinpoint the right cases.

‘Partially effective’

Yanzhong Huang, a senior fellow for global health at the Council on Foreign Relations, said temperature checks might catch someone with a fever caused by influenza, while possibly missing somebody who has been infected with the covid-19 but is not yet showing symptoms.

“Screening can be partially effective, but won’t be effective for asymptomatic carriers,” he said.

Huang pointed to research that found border screening during the H1N1 epidemic identified less than one-third of cases.

More recent studies also call the efficacy of airport-type screening into question.

Researchers at the European Centre for Disease Prevention and Control found that three-quarters of passengers leaving Chinese cities with covid-19 would not be detected by entry screening.

Entry and exit screening “is unlikely to prevent passage of infected travelers into new countries or regions where they may seed local transmission,” according to a study by a group at the London School of Hygiene & Tropical Medicine.

The World Health Organization’s guidelines suggest that thermometers can be part of a broader strategy, including interviews and data collection about recent travel and contacts.

For countries with the resources and equipment, temperature checks can also be a starting point for educating the general public about what is going on, how to stay safe and what to do if they get sick.

Part of what screening does is signal that, yes, the authorities are doing something — and that matters, experts said.

“If the community thinks you're doing nothing, they are not going to buy into your other public health policies like the social distancing measures that everyone should be employing on a mass scale right now,” said Bogoch, the infectious disease specialist.

“The optics are really important.”

Min Joo Kim in Seoul, Joanna Slater in New Delhi and Loveday Morris in Berlin contributed to this report.