On November 6, China debuted a new travel testing policy. Before, incoming airline passengers had to submit results for a conventional PCR test that proved the absence of Covid-19 and, upon landing, head to a predetermined location for a mandatory 14-day quarantine. Now, another item has joined the list: a negative IgM antibody test result.
The policy is the first of its kind and has experts puzzled. One physician, in conversation with the New York Times, mused it might be “security theater.” Another called it a “huge inconvenience” for a measure of “minimal value.” But my best guess is that the Chinese government has information on the long-term consequences of infection that we don’t.
The onus of pandemic control in China, we must keep in mind, is drastically different from the United States and Europe. With only 925 new cases and three deaths reported in the past month, China—a country of nearly 1.4 billion people—has come so close to containing Covid-19 that not one infection can be overlooked. A single new case can be enough to trigger a barrage of testing (by the millions), rippling economic losses and disruption, and citywide shutdowns. Now that most of China’s outbreaks originate outside national borders, either from shipments of frozen foods or travelers returning from overseas, shoring up points of entry is the obvious move.
Another factor worth considering when analyzing the new policy is that China’s disease surveillance system is no longer oversaturated with ongoing outbreaks. Today, their health authorities have enough bandwidth and breathing room to connect the dots between rare yet scientifically proven clinical events. Long-haulers, long-term virus shedders, and Covid-19 patients with other mysterious symptoms or pathologies are now the primary targets of public health interventions—the outliers who typically slip through the cracks of more conventional control measures.
Let’s backtrack to basics. A standard PCR test, or “nucleic acid test,” searches the body for viral genetic material. IgM, on the other hand, is the first in a series of antibodies the body produces to defend against invading pathogens. They fade relatively rapidly, on average within five to seven weeks. Because IgM antibodies are a marker of recent infection, and because their duration is weeks long, an IgM test is more likely to screen out rare individuals who exhibit long-term virus shedding. One woman, it was found in a recent case study, had infectious virus more than 70 days after her initial diagnosis. Whether or not the Chinese government has evidence that new focal areas of infection are a consequence of long-term virus shedding, we don’t know. But it may well be the case, which is why they introduced the IgM test.
Another reason might be that travelers who initially tested negative by PCR later tested positive. If this is the case, the IgM test is again likely to screen out some, if not all, of those who test negative for viral RNA initially. The “re-positive” phenomenon was documented by Chinese researchers in a peer-reviewed Nature publication, as well as more recently in JAMA. Again, while examples of “re-positives” and long-term shedders are only available by the handful, it is precisely these outliers that a country as large and advanced in its pandemic control as China will have to prioritize to reach total elimination.
It will likely be many months before countries as beset by Covid-19 as the United States and India can prune at the edges as China now can. Even so, it is in our collective best interest to pay attention to how China attempts to propel itself from a handful of cases to none. Once we do find ourselves in a comparable position, we can follow suit—rather than reverting back to disaster.