Why did Covid-19 emerge in the city of Wuhan, of all places? They say everyone loves a mystery, but this one comes at a global cost – millions of deaths, long-term health problems, and negative economic consequences. Currently, there are two dominant hypotheses about the origin of Covid.
The most likely hypothesis centers on the Huanan Seafood Market in Wuhan. It postulates that the SARS-CoV-2 virus arrived at the sprawling wet market in November 2019 through the animal trade. There, it transferred to humans and spread among the dense crowds. The joint report written by the WHO and China is consistent with this account. However, not all the relevant evidence was available to WHO investigators, casting some doubt on the findings.
The other hypothesis is the lab leak theory. It postulates that SARS-CoV-2 was kept in a laboratory at the Wuhan Institute of Virology, and an accidental breach in late 2019 caused it to spread among the local population. The lab leak theory has gained traction, but there is little to no direct evidence to support it. Although these hypotheses are quite different from each other, they both assume that human transmission first occurred in Wuhan.
Like many people, I too was curious about the origin of Covid. As a social scientist, I did the only thing I knew to do... look for data to analyse. This curiosity resulted in a paper written with co-author Zoey Xueqing Wang and
published in PLOS One.
We examined two nationally representative surveys conducted before 2019: The China Family Panel Studies (CFPS), which covers the general adult population, and the China Health and Retirement Longitudinal Study (CHARLS), which covers the older adult population. Using survey items on health, we calculated the incidence of lung and other diseases diagnosed among survey participants, by place and year.
In both datasets, participants living in the province of Hubei, whose capital city is Wuhan, experienced the highest growth in lung disease in 2018 relative to participants living in other provinces. That year, the incidence of lung disease in Hubei was roughly double the national average in the CFPS and roughly triple the national average in CHARLS. Only one other disease exhibited substantial growth in both datasets – psychiatric and nervous system disease. In 2018, Hubei had the highest incidence in the CFPS and the second highest incidence in CHARLS.
Besides comparing provinces, we also ranked smaller geographic areas according to the incidence of lung disease. We uncovered a dramatic shift in the burden of lung disease toward central China by 2018. Notably, a county in Hubei ranked first out of 153 total counties sampled by the CFPS, and a community in Hubei ranked first out of 446 total communities sampled by CHARLS. Figures 1 and 2, below, illustrate the trends.
Figure 1. Places with the highest incidence of lung disease and hospitalisation (CFPS)
Figure 2. Places with the highest incidence of lung disease (CHARLS)
It is important to recognise the limitations of our analysis. Sample sizes are relatively modest, and reported diagnoses are blurred by imperfect recall and understanding of diseases by participants. Nevertheless, the findings raise intriguing possibilities.
One of our conjectures is that there was an outbreak of influenza in central China. Influenza was more severe in the 2017-2018 season relative to previous seasons, especially in central China. Thus, it may be that China was struggling with unusually high rates of lung infection in the very places where Covid would emerge. The conditions that increased the susceptibility to influenza (e.g., air pollution) may have facilitated the later spread of Covid. This could explain why Hubei became the epicentre of Covid.
Another conjecture, though less probable, is that Covid was circulating at low levels in the population in central China during 2018. Covid can impact lung functioning and also neurological and mental health, which is congruent with our results. Therefore, it may be that human transmission had occurred prior to the super spreader event at the Huanan Seafood Market, and that the lab leak theory only arose when Covid came to Wuhan from other parts of Hubei or central China.
But our evidence is consistent with other explanations. Despite our best efforts, the mystery endures. To truly solve the mystery will most likely require China and the international community to practice trust and grace, both of which are lacking in our post-Covid world.
Visiting Associate Professor Andrew Francis-Tan’s research areas are: Demography and population; Education; Health; Labour; Race / Ethnicity