As coronavirus cases continue to rise around the world, many countries are doubling their testing guidelines that allow or prohibit travelers trying to move across international borders.
But an unusual new testing policy announced by China in late October baffled health experts. Incoming travelers must present negative results from an antibody test that cannot reliably rule out infection or prove that one person does not transmit the virus to others.
"I don't see why they would do that," said Dr. Krutika Kuppalli, an infectious disease doctor at the Medical University of South Carolina. "It seems that this is their method of safety theater."
The strange guidelines, experts say, reflected an outdated understanding of how the virus and the immune system interact. In the spring, several companies tried to market antibody tests as a potential diagnostic tool for active infections. Some (but not all) later toned down or rolled back those ads as researchers gathered more information about the timing of the antibody response to the virus, which doesn't kick off until the pathogen's levels wear off.
"I thought we passed this stage," said Elitza Theel, clinical microbiologist and expert on antibody testing at the Mayo Clinic in Rochester, Minnesota. The Centers for Disease Control and Prevention guidelines describe antibody positives as a generally poor proxy for the presence of the active virus in the body.
Previous iterations of Chinese politics stipulated that travelers only had to test negative through a "nucleic acid test," a tool that looks for the genetic material of the coronavirus. Most of the tests available that meet this requirement are based on a laboratory technique called the polymerase chain reaction, or P.C.R., which allows the virus to invade even if it is present in the body in very small amounts.
But as the coronavirus continued to spread in countries like the United States, the United Kingdom, Ireland and Russia, Chinese health officials became concerned that some foreign travelers could slip through the diagnostic rifts. The officers decided to call P.C.R. A spokesman for the Chinese embassy in the US wrote in an email with a second test to ensure that the Covid-19 screening is "more accurate".
In theory, this might be a reasonable strategy, said Deepta Bhattacharya, an immunologist at the University of Arizona, "We're fans of being able to look up with two independent types of tests." But the approach falls apart quickly, he added, when officials choose the wrong test as a safety net, as seems to be the case with China.
In addition to demanding negative nucleic acid test results, China is now asking travelers to provide evidence that they had a negative IgM antibody test, which was done within 48 hours of boarding. These tests detect disease-fighting molecules called antibodies, especially one called immunoglobulin M or IgM. This is usually the first type of antibody raised against infectious invaders. The presence of IgM is short-lived. Finally, two other types of antibodies that live much longer take over, IgG and IgA.
It's possible for an IgM test to go positive when the virus is on its way out of the body, said Dr. Theel. However, these products look for a delayed response to the virus rather than the virus itself. Therefore, the absence of IgM does not guarantee that a person is virus-free.
Many tests to detect IgM have bothered researchers with false positive results and incorrectly identified antibodies in people who are actually lacking them. "Some of these tests are kind of junk," said Dr. Kuppalli.
In the spring, defective antibody tests flooded the market after being hastily turned green by the Food and Drug Administration. The alarm about their poor accuracy led the agency to revoke the release of several products. The quality of testing has improved significantly since then, but IgM remains difficult to accurately detect when compared to other antibody types.
When asked whether China's new testing policy would address these concerns, the embassy spokesman said only that the IgM antibody test could give "stable" results. The combination of nucleic acid testing and IgM antibody testing "has been experimentally put into practice in some countries and is working well," said the spokesman, reiterating previous comments made at a press conference. The spokesman did not respond to repeated requests for clarification or comment on which countries had implemented such protocols or whether data was in place to support the effectiveness of the dual test tactic.
Another problem, said Dr. Theel, is that IgM tests can also be very difficult to find. The new requirements could create unnecessarily stressful barriers for people trying to get home, wasting time and money. "This is a major inconvenience for a test that I think adds a minimal amount of value," she said.