Why is China giving travelers anal tests for COVID-19?

Travelers to some Chinese cities have been required to take anal swab tests for COVID-19, a measure that has sparked outcries from other countries, according to news reports.

This week, officials in Japan complained that some Japanese citizens arriving in China had been subjected to the tests, which "caused great psychological pain," according to the BBC. 

And in February, some U.S. diplomats said they were required to take the tests, which prompted a complaint from the U.S. State Department, Vice reported. 

"The State Department never agreed to this kind of testing and protested directly to the Ministry of Foreign Affairs when we learned that some staff were subject to it," a spokesperson for the State Department told Vice. (Chinese officials denied that U.S. diplomats had been asked to undergo anal testing, Vice reported.)

It is unclear how many international travelers have been required to take the anal tests, but Beijing and Shanghai have reportedly required the tests for some arrivals, according to The New York Times.

Some Chinese doctors say the tests are performed to catch silent carriers of the virus — those who may not show symptoms or who develop mild symptoms but recover quickly — because the novel coronavirus can be detected in feces for longer than in the nose and throat.

"Some asymptomatic patients or those with minor symptoms recover fast [from COVID-19], and it's possible that throat tests won't be effective for these people," Li Tongzeng, an infectious disease doctor in China, told CNN.

"Researchers have shown that for some infected people, the duration time of positive nucleic results lasted longer on their excrement and anal swab test [samples] than those on the upper respiratory tract. Therefore, adding anal swab tests can improve the positive detection rate of the infected," he said, referring to the so-called PCR diagnostic tests for the virus.

Some Chinese citizens have also been required to take anal COIVD-19 tests. In January, more than 1,000 students and teachers in a school district in Beijing received anal swab tests, as well as nasal swab tests, for COVID-19 after a 9-year-old student in the district tested positive, according to Vice. Other people staying at quarantine hotels have been asked to take the tests. One person told Vice that he was asked to take an anal test in September 2020 while at a quarantine hotel after he returned from Australia. He said the test was administered by nurses and that it "felt like he was having diarrhea," Vice reported.

The tests have been controversial among experts, even in China. Yang Zhanqiu, a deputy director of the pathogen biology department at Wuhan University, told the state-controlled Chinese newspaper the Global Times in January that nasal and throat tests were still more effective than anal tests, because the virus is known to spread through respiratory droplets rather than through feces. If the point of the tests is to keep an infected person from spreading the virus, the argument goes, then the nasal/throat tests would work best.

"There have been cases concerning the coronavirus testing positive in a patient's excrement, but no evidence has suggested it had been transmitted through one's digestive system," Yang said.

Experts outside of China also question the practice, since those who test positive for COVID-19 on an anal test but not on a nasal or throat test would likely not be very contagious, according to The New York Times.

"The value of detecting people with the virus is to stop transmission," Benjamin Cowling, a public health professor at the University of Hong Kong, told the Times. "If someone has got an infection but they're not contagious to anyone else, we didn't need to detect that person."

Originally published on Live Science.