Few people who contract COVID-19 develop a "potent level" of antibodies against the disease, worrying new research has found.
And in some cases, the antibodies the body produces might actually help the virus infect you a second time.
Researchers in China examined blood serum from 26 COVID-19 patients, measuring the antibody response their immune systems had produced against SARS-CoV-2, the virus responsible for COVID-19.
While 21 of the recovered patients - 80.7 percent - had mounted a "strong response", "only a small portion of patients elicited a potent level of neutralisation activity".
Neutralising antibodies are the kind that can stop viruses from infecting cells.
"Three to four weeks after hospital discharge, the neutralising activity of antibodies from recovered patients declined significantly, suggesting that recovered COVID-19 patients might be susceptible to reinfection with SARS-CoV-2," the study, published in journal PLOS Pathogens, said.
Those who suffered particularly bad infections were less likely to be left with strong antibodies when they recovered.
"Severe COVID-19 patients had a large amount of non-neutralising antibodies, which may contribute to antibody-dependent enhancement of infection."
Non-neutralising antibodies can't stop a virus from infecting other cells. They're not useless though - they can still attach themselves to viruses, marking them as intruders so other types of immune cells know which to target.
Antibody-dependent enhancement of infection is when a virus ironically uses a weak antibody to gain access to a host cell - even immune cells, which are meant to kill them. Other coronaviruses have been seen doing this, as well as Dengue, influenza and Zika.
"It is unclear whether clinically recovered patients acquire the protective immunity from reinfection," the study, led by Yuxin Chen of Nanjing University Medical School, said.
"The rapidly declined neutralising activities in COVID-19 clinically recovered patients within 28 days after discharge, [suggests] that the circulating anti-SARS-CoV-2 neutralising antibodies might have a relatively short half-life."
The good news is that knowing how the immune system responds to an infection will help scientists develop a vaccine.
"Understanding the adaptive responses where the body makes antibodies that specifically bind to the SARS-CoV-2 among COVID-19 patients provides fundamental information for developing effective treatment and preventive vaccine."
Richard Malley, a paediatric infectious-disease specialist at Boston Children's Hospital in Massachusetts, told Nature last week diseases which can infect people more than once have traditionally been more difficult to develop vaccines for. He also said it raises the possibility a vaccine would only reduce symptoms, rather than prevent infection altogether, possibly turning people into unwitting asymptomatic carriers.
"They could still serve as an important reservoir of a future spread. We need to understand that better following natural infection and vaccination if we want to get out of this mess."
He noted some existing vaccines require booster shots, which might be the case for COVID-19.
There have only been a handful of recorded cases of reinfection to date, but scientists say that might only be because the pandemic is still in its early stages, and most of the world is yet to be infected. Some cases have been less severe than the first, but one patient - in Nevada - had much worse symptoms the second time around, evidence of antibody-dependent enhancement.