SARS repeatedly spilled over into humans during 2002-2003, until the markets were shut down and the infected animal populations were culled. A review on SARS [0] describes how many independent clusters of SARS popped up over a period of months, spread across different markets in the Pearl River Delta:
> Between November 2002 and February 2003, the first cases or clusters of SARS appeared in several independent geographic locations in the Pearl River Delta region in southern Guangdong, and suggested multiple introductions of a virus or similar viruses from a common source. Several of the early cases were reportedly associated with occupations that involved contact with wildlife, including handling, killing and selling wild animals as well as preparing and serving wildlife animal meat in restaurants (Xu et al. 2004). Moreover, a study of early SARS cases (i.e. those with disease onset prior to January 2003) compared to those identified later in the outbreak found that 39% of early-onset cases were food handlers, whereas only 2%–10% of cases between February and April 2003 were associated with this occupation.
The review goes on:
> It was observed that early cases of SARS occurred independently in at least five different well-separated municipalities in Guangdong Province. The study also found that early patients were more likely than later patients to report living near a produce market, but not near a farm, and nine of 23 (or 39%) early patients were food handlers with probable animal contact.
The review also discusses how many SARS spillover events were not recognized at the time:
> Several studies revealed a higher than normal seroprevalence of SARS-CoV antibodies among wild animal traders. Guan et al. (2003) found that eight of 20 (40%) wild animal traders sampled from a market in Shenzhen, Guandong, in 2004 had anti-SARS-CoV antibodies in comparison to 1 from 20 (5%) vegetable traders from the same market. Yu et al. (2003) analysed serum samples taken on May 4, 2003 from animal traders in three different live animal markets in Guangzhou. Out of 508 animal traders surveyed, 13% had antibodies to SARSCoV; 72% of traders of masked palm civets ( Paguma larvata ) were seropositive. Interestingly, none of the animal traders had SARS or atypical pneumonia diagnosed during the SARS outbreak in Guangdong, suggesting asymptomatic infection by SARS-CoV or a closely related SARS-like coronavirus.
SARS probably spilled over countless times into humans during 2002-2003, because there was a large population of farmed animals that had it, and very little was done to cut off the spillover source for months.
This is a key difference from SARS-CoV-2. This outbreak was detected much more quickly (because of China's experience with SARS), and the very first thing the authorities did was to close the Huanan market and crack down on farms that raise the types of animals that are most likely to be involved in the spillover.
0. https://link.springer.com/chapter/10.1007/978-3-540-70962-6_...