Eight people, after recovery from serious illness, had higher levels of neutralizing antibodies against the virus than 10 patients with more mild symptoms, the data showed.
Neutralizing antibodies are proteins produced by the immune system to fight off infections.
The findings support the use of antibody-based therapy, perhaps with blood plasma donated by those who have recovered from COVID-19, to prevent and treat the disease, the researchers said.
"Even with a vaccine at our doorstep, arming clinicians with specific anti-[COVID-19] therapeutics is extremely important," wrote the authors, from Tel Aviv University in Israel.
"Combinations of neutralizing antibodies represent a promising approach towards effective and safe treatment of severe COVID-19 cases," they said.
Researchers have been exploring the use of antibody-based therapies to prevent and treat COVID-19 since the start of the pandemic, with several studies producing promising results.
Neutralizing antibodies that target the receptor-binding domain of the coronavirus' so-called spike protein are specifically thought to have the most potential.
These antibodies have been detected in convalescent patients, or those who have recovered from COVID-19, the researchers said.
Some of these recovered patients have robust and long-lasting immunity, while in others immunity wanes over time, for reasons that researchers say remain unclear.
For this study, the researchers compared antibody responses in eight patients with severe COVID-19 and 10 individuals with mild symptoms roughly six weeks after infection.
The very ill patients showed higher concentrations of antibodies targeting the virus' spike protein and other immune system cells that offer protection against the virus.
Among 22 antibodies cloned from two of these patients, six exhibited potent neutralization against the coronavirus.
The analysis suggests that most people would be capable of readily producing neutralizing antibodies against the virus, the researchers said.
And combinations of different types of neutralizing antibodies completely blocked the live virus from spreading, according to the study.
These antibody cocktails [should] be further tested in clinical settings as a useful means to prevent and treat COVID-19, "especially in the elderly population or chronically ill people, who will not be able to so easily produce these antibodies upon infection or vaccination," the researchers said.