By Keita Noguchi, Naoyuki Himeno, and Shuichi Doi

As Japan eyes easing travel restrictions on business people and other travelers, airports are introducing saliva-based quantitative antigen tests because the results are available quickly. The antigen test, however, is less accurate than the PCR test, causing some experts to worry that travelers could spread the virus in Japan after passing the test at the airport with a false negative.

On July 17, the government approved use of the saliva-based antigen test, and the test is now administered at Narita, Haneda, and Kansai airports. While it takes several hours before results are available with PCR tests, the results of these antigen tests only take about 30 minutes. Although the antigen test using saliva requires special equipment, the accuracy of the test exceeds that of simpler test kits.

At a press conference on July 17, Health Minister Katsunobu Kato said, “We have confirmed that the results of the saliva-based test match those of the PCR test in a high percentage of cases.”

However, documents distributed two days earlier among members of the health ministry’s infectious disease team said that, of the 37 patients who were asymptomatic but tested positive in a PCR test, only 28 tested positive in the saliva-based antigen test, marking a positive agreement rate of 76%. This means that one out of four positive cases may go undetected by the saliva-based antigen tests.

The health ministry explained: “Those cases were undetected because the amount of virus in the samples was smaller. The small amount of virus would make the risk of transmission low.”

On most days, well over 1,000 PCR and antigen tests are conducted at domestic airports. As use of the antigen test increases, the risk of overlooking positive cases also grows.

The government is currently in negotiations with 16 countries and regions, including China, South Korea, and Taiwan, about easing entry restrictions on business travelers and others. Japan has already reached an agreement with the government of Singapore to reopen travel between their nations starting in mid-September. Short-term business travelers would be exempt from the two-week at-home/hotel quarantine period on the condition that they test negative prior to the trip, restrict their movement to travel between their business sites and hotel, and do not use public transportation for two weeks after arrival.

The antigen tests using saliva will be administered to most of these new arrivals. A government source stressed, “Those travelers will already have taken PCR tests at home to confirm that they don’t have the virus. Conducting the antigen tests is just to double-check the results.”

What do the experts say? Professor Atsuro Hamada of Tokyo Medical University (travel medicine) points out, “The PCR test doesn’t always accurately detect positive cases either. But the government should accurately convey to the public that there is indeed a risk of false negatives.” Professor Katsunori Yanagihara at Nagasaki University says, “From an academic point of view, I cannot recommend use of the antigen tests for screening purposes. However, considering that we need to conduct a large number of tests, we may have to accept the risk of having a few more false negatives among these travelers.”