While people across the world wait for news of treatments and vaccines against the virus that causes COVID-19, attention is also being given to antibody protection. Test results from a clinic in Queens, New York, showed 68% of patients had developed antibodies to COVID-19. There was community disparity, however; across the Bronx, which has had New York City's highest death rate from COVID-19, only about 37% of antibody tests were positive.

Antibody tests that look for proteins made in response to infections in a person’s blood such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are useful in identifying those who had a mild or asymptomatic infection or who never received a diagnostic test despite having symptoms. Although antibodies to the virus that causes COVID-19 may provide significant protection from getting infected with the virus again, researchers are still determining how much protection the antibodies may provide or how long this protection may last, if at all.

A major new study in Spain, one of the European countries most affected by the COVID-19 pandemic, found no evidence of widespread immunity to the virus, suggesting that people who experience mild symptoms do not have long-lasting protection. From April 27 to May 11, about 61,000 participants (75% of all contacted individuals within selected households) answered a questionnaire on history of symptoms compatible with COVID-19 and risk factors, received a point-of-care antibody test, and, if agreed, donated a blood sample for additional testing with a chemiluminescent microparticle immunoassay.

Just 5% of participants were detected to have antibodies to the virus by the point-of-care test and 4% to 6% by immunoassay. Of those who previously tested positive for antibodies, 14% tested negative just weeks later.

Another study based on the antibody responses of 90 patients and health workers at Guy's and St. Thomas' Hospitals in London showed that 60% of those tested had potent antibodies while battling COVID-19, but just 17% retained the same potency three months later. Antibody levels fell as much as 23-fold over the period, and in some cases, levels became undetectable.

In a recent Chinese study, which included 37 asymptomatic and 37 symptomatic patients, more than 90% of both groups showed steep declines in levels of SARS-CoV-2-specific immunoglobulin G (IgG) antibodies within two to three months after onset of infection. The patients included in the study were among 2,088 people who were tested for COVID-19 because they were close contacts of confirmed patients in China's Wanzhou district.

Of those who tested positive, 60 reported no symptoms in the preceding two weeks but were hospitalized for isolation. Of those, 23 people who had mild symptoms on admission or soon afterward were excluded from the study, leaving 37. Further, 40% of the asymptomatic group tested negative for IgG antibodies 8 weeks after isolation.

Antibody responses to SARS-CoV-2 can be detected in most infected individuals 10-15 days following the onset of COVID-19 symptoms. However, due to the recent emergence of this virus in the human population, it remains unclear how long these antibody responses will be maintained or whether they will provide protection from reinfection. In the meantime, researchers support prolonging public health interventions, including social distancing, hygiene, high-risk group isolation, and widespread testing.