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After your immune system fights off a disease, conventional wisdom dictates that your body builds up resistance to it in the future.

Unfortunately, when it comes to COVID-19, the reality may be more complex.

The main way that the immune system builds up resistance is by producing antibodies, which are proteins coded to hunt down specific pathogens. Sometimes, these antibodies fend off disease for years. But The New York Times reports that the antibodies our bodies develop against COVID-19 can fade away in just two to three months — especially for those who had mild cases.

That poses a problem for governments that banked on developing herd immunity — resistance to future infections at a societal scale — in the absence of an effective vaccine or treatment for the coronavirus. It also suggests that doling out "immunity passports" will only complicate the situation as some survivors may gradually become vulnerable again.

"These data might indicate the risks of using COVID-19 'immunity passports' and support the prolongation of public health interventions, including social distancing, hygiene, isolation of high-risk groups and widespread testing," the scientists behind the study, published Thursday in the journal Nature Medicine, wrote in their paper.

It's important to note several caveats. For one, the research involved a small number of participants — only 37 survivors who showed symptoms and another 37 who didn't — so a larger study is necessary to gain more statistical certainty.

"Though this is quite a small sample size of patients," Imperial College London immunologist Danny Altmann, who didn't work on the study, told Agence France-Presse, "it is in line with some concerns that natural immunity to coronaviruses can be quite short-lived."