The first study, published June 16 on the preprint server medRxiv, screened for antibodies in almost 1,500 coronavirus patients in Wuhan, China. The researchers compared their levels to three other groups: nearly 20,000 members of the general population; more than 1,600 patients hospitalized for reasons other than COVID-19; and more than 3,800 medical workers, whom the authors assumed had “inevitably” been exposed to the virus in its early days, meaning they should have developed antibodies.

They found that while almost 90 percent of COVID-19 patients had antibodies, roughly 1 percent to 5 percent of individuals in the others groups had them as well. The authors conclude in their paper that the remaining 10 percent of infected patients with no detectable antibodies, combined with the lack of antibodies in healthcare workers, suggest that “after SARS-CoV-2 infection, people are unlikely to produce long-lasting protective antibodies against this virus.”
See “What Do Antibody Tests For SARS-CoV-2 Tell Us About Immunity?”

In the second study, published June 18 in Nature Medicine, researchers compared the immune responses of 37 asymptomatic but positive patients to an equal number with severe symptoms living in the Wanzhou District in China. They found that asymptomatic individuals reacted less strongly to infection, with 40 percent having undetectable levels of protective antibodies in the two to three months after the infection compared to 13 percent of the symptomatic patients.